Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Microfluidic methods to study emulsion formation
    Muijlwijk, Kelly - \ 2017
    Wageningen University. Promotor(en): C.G.P.H. Schroën, co-promotor(en): C.C. Berton-Carabin. - Wageningen : Wageningen University - ISBN 9789463430715 - 169
    emulsions - microfluidics - food emulsions - droplets - adsorption - colloidal properties - emulsies - microfluidics - voedselemulsies - druppels - adsorptie - colloïdale eigenschappen

    Emulsions are dispersions of one liquid in another that are commonly used in various products, and methods such as high-pressure homogenisers and colloid mills are used to form emulsions. The size and size distribution of emulsion droplets are important for the final product properties and thus need to be controlled. Rapid coalescence of droplets during emulsification increases droplet size and widens the size distribution, and therefore needs to be prevented.

    To increase stability of emulsions, emulsifiers are added to adsorb at the oil-water interface before droplets collide. The time allowed for emulsifier adsorption is typically in the range of sub-milliseconds to seconds and to optimise emulsification processes, emulsifier adsorption and coalescence stability need to be measured in this time-scale, for which the microfluidic methods described in this thesis were developed.

    Chapter 2 provides an overview of existing literature on cross-flow microfluidic emulsification. The effects of various parameters such as microfluidic design, shear forces, and interfacial tension forces on droplet formation and the resulting droplet size are discussed, as well as the use of microfluidics to produce food-grade emulsions. Based on this evaluation, the methods to elucidate interfacial tension and coalescence stability are chosen, and these are presented in the next chapters.

    To measure emulsifier adsorption in the sub-millisecond time-scale, a tensiometric method was developed using a cross-flow microfluidic Y-junction, which is described in Chapter 3. This method is based on the relation between droplet size and interfacial tension at the moment of droplet formation, which is referred to as the acting interfacial tension. The acting interfacial tension of a system with hexadecane as the dispersed phase and sodium dodecylsulfate (SDS, a model surfactant) solutions as the continuous phase was successfully measured for droplet formation times ranging from 0.4 to 9.4 milliseconds and with high expansion rates (100-2000 s-1). Comparison of these results with data from a drop tensiometer (a conventional, static, and supra-second time-scale method) indicates that mass transport in the microfluidic Y-junction is fast and probably not limited by diffusion.

    Emulsifier mass transport conditions were further investigated in Chapter 4. The continuous phase viscosity and velocity were systematically varied and the effect on the acting interfacial tension in presence of water-soluble SDS was measured. We found that the acting interfacial tension was independent of the continuous phase viscosity, but was inversely dependent on continuous phase velocity. Both aspects led us to conclude that convective emulsifier transport in the continuous phase determines the acting interfacial tension in the Y-junction. When using oil-soluble surfactant Span 20 (dissolved in hexadecane), the acting interfacial tension also decreased with increasing continuous phase velocity, and we therefore concluded that convection also dominated mass transport of emulsifiers dissolved in the to-be-dispersed phase.

    The Y-junction method was used in Chapter 5 to elucidate the effect of the dispersed phase viscosity on adsorption of the food-grade emulsifiers Tween 20 (dissolved in the continuous water phase) and Span 20 (dissolved in the dispersed oil phase). A reduction in dispersed phase viscosity sped up adsorption of Tween 20, probably because the shorter hydrocarbon made intercalation of the hydrophobic surfactant tail at the interface easier. Dispersed phase viscosity had an even greater effect on adsorption of Span 20 because convective transport towards the interface was increased.

    Next to interfacial tension, also coalescence can be measured with microfluidics and a microfluidic collision channel was used in Chapter 6 to measure emulsion coalescence stability shortly after droplet formation under flow. Coalescence of emulsions stabilised with proteins was measured at various concentrations, pH values, and adsorption times. We found that protein concentrations just below the concentration needed for monolayer surface coverage may be used effectively. β-lactoglobulin-stabilised emulsions were most stable. Emulsions stabilised with whey protein isolate (with as main component β-lactoglobulin), were less stable and when these proteins were oxidised, this led to reduced stability, therewith indicating that also the oxidative state of proteins needs to be considered in emulsion formulation.

    The relevance of our work for microfluidic research and industrial emulsification processes is discussed in Chapter 7. Microfluidic devices can be used to study emulsion formation and stability under conditions relevant to industrial emulsification processes; at short time-scales and with convective mass transport. In this thesis we used various food-grade ingredients, and with that application in that field has come closer. We expect that the findings on emulsions can also be applied on foams. With the discussed microfluidic devices different aspects that are important for emulsion formation can be decoupled: for example interfacial tension during droplet formation and emulsion coalescence stability. Furthermore, microfluidic methods are available to for example gain insight in emulsion interface mobility and emulsion storage stability, and we envision that all these microfluidic methods will lead to faster ingredient screening, lower ingredient usage, and more energy efficient emulsion production.

    Core-shell particles at fluid interfaces : performance as interfacial stabilizers
    Buchcic, C. - \ 2016
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): R.H. Tromp; Marcel Meinders. - Wageningen : Wageningen University - ISBN 9789462578968 - 140
    stabilization - stabilizers - particles - colloidal properties - adsorption - interface - fluids - stabilisatie - stabiliseermiddelen - deeltjes - colloïdale eigenschappen - adsorptie - grensvlak - vloeistoffen (fluids)

    There is a growing interest in the use of particles as stabilizers for foams and emulsions. Applying hard particles for stabilization of fluid interface is referred to as Pickering stabilization. By using hard particles instead of surfactants and polymers, fluid interfaces can be effectively stabilized against Ostwald ripening and coalescence. A drawback of the use of hard particles as interfacial stabilizers is that they often experience a pronounced energy barrier for interfacial adsorption and that hard particles are very specific with regard to the type of fluid interface they can adsorb to. Soft particles, on the other hand, are known as good stabilizers against coalescence and they spontaneously adsorb to a variety of different fluid interfaces.

    The aim of this thesis was to investigate core-shell particles comprising a hard core and soft shell with regard to their interfacial behaviour and their ability to act as sole stabilizers for foams and emulsions. We hypothesised that the presence of the soft shell allows for easier interfacial adsorption of core-shell particles compared to the hard core particles only. To test this hypothesis, we prepared core-shell particles comprising a solid polystyrene (PS) core and a soft poly-N-isopropylacrylamide (PNIPAM) shell. To ascertain the effect of shell thickness, we prepared a range of core-shell particles with different shell thicknesses, containing identical core particles. We found that core-shell particles are intrinsically surface active and can generate high surface pressures at the air-water interface and oil-water interfaces, whereas core particles seemed to experience a large energy barrier for interfacial adsorption and did not lower the surface tension. We also confirmed by microscopy that core-shell particles are actually adsorbing to the fluid interface and form densely packed interfacial layers. Further, we found that a certain critical thickness of the soft shell is necessary in order to ensure facile interfacial adsorption. If the PNIPAM shell on top of the core particles is well above 100nm thick, particle adsorption at the air-water interface was found to be diffusion limited.

    By gentle hand-shaking we were able to produce dispersion of air bubbles and emulsion droplets solely stabilized by core-shell particles. The resulting bubbles still underwent Ostwald ripening, albeit slowly. For oil-in-water emulsions of hexane and toluene, both of which have a relatively high solubility in the continuous phase, we found that core-shell particles can stop Ostwald ripening. The resulting emulsion droplets adopted pronounced non-spherical shapes, indicating a high elasticity of the interface. The high stability and the remarkable non-spherical shape of the emulsion droplets stabilized by core-shell particles were features we also observed for fluid dispersion stabilized by hard particles. This shows that in terms of emulsion stability core-shell particles behave similar to hard particles as interfacial stabilizer.

    As to why the differences between the stability of bubble and oil dispersions arise could not be finally answered. Yet, microscopic analysis of the interfacial configuration of core-shell particles at the air-water interface reveals some peculiar insights which may suggest that core-shell particles adsorb in a polymer-like fashion with the soft PNIPAM shells adsorbing to the air-water interface only, while the hard PS cores reside in the continuous phase.

    In summary, we showed that core-shell particles with a hard core and a soft shell can indeed combine the advantageous properties of hard and soft particles. The soft shell enables spontaneous adsorption to a variety of fluid interfaces. Despite their spontaneous adsorption, core-shell particles strongly anchor and do not spontaneously desorb from the fluid interface again. Further, the hard core provides enough rigidity to the core-shell particles to allow the establishment of a stress bearing interfacial particle network. This network eventually stops Ostwald ripening in oil-in-water emulsions. Our results therefore show that in the case of oil-water interfaces, core-shell particles can perform better than solely hard particles as interfacial stabilizers.

    Transformation by photolysis in water in the pesticide model TOXSWA : implementation report
    Beltman, W.H.J. ; Mulder, H.M. ; Horst, M.M.S. ter; Wipfler, E.L. - \ 2015
    Wageningen : Alterra (Alterra report 2649) - 47
    waterverontreiniging - pesticiden - fotolyse - biochemische omzettingen - waterbodems - adsorptie - ecotoxicologie - modellen - water pollution - pesticides - photolysis - biochemical pathways - water bottoms - adsorption - ecotoxicology - models
    The TOXSWA model has been extended with the functionality to simulate photolysis in water. TOXSWA simulates the fate of substances in water bodies to calculate exposure concentrations for aquatic organisms or sediment-dwelling organisms as part of the risk assessment of plant protection products (PPP). Photolysis is modelled as a first-order process, where transformation occurs in the water phase only. The transformation rate is considered to be linearly proportional to global radiation. Studies in outdoor surface water systems can in principle be used to derive the PPP transformation rates due to photolysis.
    Phosphorus leaching from soils: process description, risk assessment and mitigation
    Schoumans, O.F. - \ 2015
    Wageningen : Alterra, Wageningen-UR (Alterra scientific contributions 46) - ISBN 9789462573666 - 261
    nutrient leaching - leaching - soil chemistry - adsorption - phosphorus - phosphate leaching - risk assessment - water quality - eutrophication - nutriëntenuitspoeling - uitspoelen - bodemchemie - adsorptie - fosfor - fosfaatuitspoeling - risicoschatting - waterkwaliteit - eutrofiëring
    Leaching of plant protection products and their transformation products : proposals for improving the assessment of leaching to groundwater in the Netherlands : version 2
    Boesten, J.J.T.I. ; Linden, A.M.A. van der; Beltman, W.H.J. ; Pol, J.W. - \ 2015
    Wageningen : Alterra, Wageningen-UR (Alterra report 2630) - 105
    bodemchemie - adsorptie - pesticiden - uitspoelen - chemische afbraak - schatting - modellen - soil chemistry - adsorption - pesticides - leaching - chemical degradation - estimation - models
    Assessment of leaching of plant protection products to groundwater is an important aspect of the environmental risk assessment of these substances. Analysis of available Dutch groundwater monitoring data for these substances triggered a critical review of the current Dutch leaching assessment. As a result, proposals were developed for improving this assessment. These include: (i) a procedure for correcting systematic errors in measured sorption coefficients, (ii) a preliminary procedure for a quality check of Freundlich exponents, (iii) a flow chart for obtaining parameters describing the relationship between the organic-matter/water distribution coefficient, Kom, and the pH for weak acids, (iv) a procedure for obtaining a Kom endpoint from a population of Kom values including lower and upper limits, (v) a procedure for estimating the total amount of substance in soil from a concentration profile (needed for assessment of degradation half-lives from field experiments). This report is an update of the proposals reported in 2011 by the same authors based on testing the feasibility of the proposals to a few dossiers
    Phosphorus leaching from soils: process description, risk assessment and mitigation
    Schoumans, O.F. - \ 2015
    Wageningen University. Promotor(en): Sjoerd van der Zee, co-promotor(en): Wim Chardon. - Wageningen : Wageningen University - ISBN 9789462572997 - 261
    nutriëntenuitspoeling - uitspoelen - bodemchemie - adsorptie - fosfor - fosfaatuitspoeling - risicoschatting - waterkwaliteit - eutrofiëring - nutrient leaching - leaching - soil chemistry - adsorption - phosphorus - phosphate leaching - risk assessment - water quality - eutrophication
    Er zijn succesvolle management strategieën voor P nodig om de waterkwaliteit te verbeteren en daarvoor is allereerst kwantitatieve informatieve nodig over de ruimtelijke verdeling van de fosfaatbelasting van het oppervlaktewater vanuit landbouwgronden. In Nederland is een protocol fosfaatverzadigde gronden ontwikkeld om het potentiële risico van verhoogde fosfaatconcentraties in het bovenste grondwater (op termijn) te voorspellen voor kalkarme zandgronden, omdat in deze gebieden veel intensieve veehouderij voorkomt. Voor deze grondsoort zijn de parameters voor het protocol vastgesteld. Echter, voor de overige grondsoorten is geen informatie verzameld, waardoor er voor Nederland als geheel geen ruimtelijke beeld bestaat van de mate van fosfaatverzadiging en van het potentiële risico van verhoogde fosfaatconcentraties in het bovenste grondwater. Daarnaast is er behoefte aan eenvoudige methoden om het (huidige) actuele risico van de fosfaatbelasting van het oppervlaktewater in kaart te brengen, zodat inzichtelijk gemaakt kan worden welke gebieden nu al substantieel de oppervlaktewaterkwaliteit en de eutrofiestatus beïnvloeden.
    Afvangen van fosfaat uit bloembollensector met ijzerzand : Test van maatregelen die fosforemissie verminderen
    Chardon, W.J. ; Groenenberg, J.E. ; Jansen, S. ; Buijert, A. ; Talens, R. ; Krol, A.F. - \ 2014
    Bodem 24 (2014)6. - ISSN 0925-1650 - p. 20 - 22.
    vollegrondsteelt - bloembollen - bodemchemie - emissiereductie - fosfaten - adsorptie - ijzer - bollenstreek - outdoor cropping - ornamental bulbs - soil chemistry - emission reduction - phosphates - adsorption - iron - bollenstreek
    In het oppervlaktewater van de Bollenstreek is de fosfaatconcentratie veel hoger dan de norm van de Europese Kaderrichtlijn Water. Met ijzerzand, een nevenproduct van drinkwaterproductie, kan fosfaat worden afgevangen. Het hoogheemraadschap van Rijnland heeft drie maatregelen op basis van ijzerzand laten onderzoeken op hun effectiviteit. Welke was de beste?
    Technologieën voor verwijdering gewasbeschermingsmiddelen
    Beerling, E.A.M. - \ 2014
    gewasbescherming - chemische bestrijding - milieueffect - cost effective analysis - afvalverwerking - oxidatie - scheidingsapparaten - bioremediëring - adsorptie - plant protection - chemical control - environmental impact - cost effectiveness analysis - waste treatment - oxidation - separators - bioremediation - adsorption
    Mogelijk geschikte technieken om chemische gewasbeschermingsmiddelen te verwijderen zodat ze niet het milieu verontreiningen worden geïnventariseerd met behulp van onafhankelijke experts uit de watersector (o.a. KWR, Wetsus, Mannen van de Wit en Stowa)
    Gedrag van chloorprofam en 3-chlooraniline in de bodem : bepaling van adsorptie, omzetting en uitloging met tarragrond van aardappelen
    Beltman, W.H.J. ; Matser, A.M. ; Linden, A.M.A. van der; Brand, E. - \ 2014
    Wageningen : Alterra, Wageningen-UR (Alterra-rapport 2523) - 65
    aardappelopslagplaatsen - aardappelen - opslag - kiemremmers - adsorptie - uitspoelen - grondwaterverontreiniging - potato stores - potatoes - storage - germination inhibitors - adsorption - leaching - groundwater pollution
    Bij de bewaring van aardappelen wordt als kiemremmer de werkzame stof chloorprofam gebruikt. Na verwerking van de aardappelen in de fabriek blijft chloorprofam en haar omzettingsprodukt 3-chlooraniline in de resterende tarragrond aanwezig. De tarragrond wordt hergebruikt in grootschalige bodemtoepassingen. Omzetting, adsorptie en uitloging van chloorprofam en haar metaboliet 3-chlooraniline zijn onderzocht met tarragrond van aardappelen (onbelast en belast met chloorprofam) en met bouwvoorgrond. De omzettingssnelheid van chloorprofam en 3-chlooraniline is bepaald in bouwvoorgrond, in onverzadigde onbelaste tarragrond en in waterverzadigde onbelaste tarragrond. De adsorptieparameters van de Freundlich sorptie isotherm zijn bepaald voor 3-chlooraniline in onbelaste tarragrond en in bouwvoorgrond. De uitloogproeven zijn gedaan met onverzadigde belaste tarragrond en met waterverzadigde belaste tarragrond. De omzettingssnelheden en adsorptieparameters uit deze studie worden gebruikt voor het bepalen van het risico van uitspoeling van chloorprofam en 3-chlooraniline uit grote bodemtoepassingen naar het grondwater.
    Implications of nanoparticles in the aquatic environment
    Velzeboer, I. - \ 2014
    Wageningen University. Promotor(en): Bart Koelmans. - Wageningen : Wageningen University - ISBN 9789461739506 - 253
    microplastics - polychloorbifenylen - nanotechnologie - adsorptie - ecotoxicologie - aquatisch milieu - verontreinigde sedimenten - aquatische ecologie - microplastics - polychlorinated biphenyls - nanotechnology - adsorption - ecotoxicology - aquatic environment - contaminated sediments - aquatic ecology
    De productie en het gebruik van synthetische nanodeeltjes (ENPs) nemen toe en veroorzaken toenemende emissies naar het milieu. Dit proefschrift richt zich op de implicaties van ENPs in het aquatisch milieu, met de nadruk op het sediment, omdat er wordt verwacht dat ENPs hoofdzakelijk in het aquatisch sediment terecht zullen komen. ENPs kunnen directe effecten veroorzaken op organismen in het aquatisch milieu, indirecte effecten op het levensgemeenschap niveau en/of voedselweb en kunnen effecten op het gedrag en de risico’s van andere contaminanten hebben. Om de risico’s van ENPs vast te stellen, is niet alleen informatie nodig over het gevaar, oftewel de kans op een effect, maar ook over de kans op blootstelling.
    Sterke vermindering van fosfaatuitspoeling uit landbouwgronden met de fosfaatbindende drain
    Groenenberg, J.E. ; Chardon, W.J. ; Koopmans, G.F. - \ 2013
    draineerbuizen - drainagewater - ijzer - adsorptie - waterkwaliteit - fosfaten - veldproeven - drain pipes - drainage water - iron - adsorption - water quality - phosphates - field tests
    Voor gedraineerde landbouwgronden (meer dan 50% in Nederland) heeft Alterra de fosfaatbindende drain ontwikkeld. Dit is een normale buisdrain die is ingebed in ijzerzand. De omhulling met ijzerzand bindt het fosfaat uit het fosfaatrijke water voor het de drainbuis instroomt. Het schone water wordt afgevoerd naar het oppervlaktewater.
    Goede waterkwaliteit en meer kansen voor natuur : met immobilisatie van fosfaat door restmaterialen
    Chardon, W.J. ; Groenenberg, J.E. - \ 2013
    landbouwgrond - fosfaatuitspoeling - adsorptie - kaderrichtlijn water - natuurgebieden - agricultural land - phosphate leaching - adsorption - water framework directive - natural areas
    Hoge fosfaatgehalten in landbouwgronden kunnen eutrofiëring van het oppervlaktewater veroorzaken, waardoor overmatige groei van waterplanten en algen optreedt. Met alleen het mestbeleid lukt het niet om de doelstellingen van de Kaderrichtlijn Water (KRW) te bereiken, onder andere als gevolg van de voorraden fosfaat die in de bodem aanwezig zijn. Daarom zijn aanvullende maatregelen noodzakelijk.. Een mogelijke oplossing is de verwijdering van fosfaat in het naar het oppervlaktewater uitstromende water.
    Manual of PEARLNEQ v5
    Boesten, J.J.T.I. ; Horst, M.M.S. ter - \ 2012
    Wageningen : Wettelijke Onderzoekstaken Natuur & Milieu (WOt-werkdocument 304)
    bodemchemie - pesticiden - adsorptie - degradatie - modellen - soil chemistry - pesticides - adsorption - degradation - models
    This manual describes the PEARLNEQ v5 software package . This package can estimate long-term sorption parameters using results of aged-sorption studies with soil, using a submodel for sorption and transformation that is identical to the submodel used for that purpose in the FOCUS_PEARL v3.3.3. The submodel assumes two types of sorption sites: equilibrium sites and non-equilibrium sites. The sorption isotherms for both sites are described with Freundlich equations. The mathematical equations describing the submodel are solved via a FORTRAN programme. An additional FORTRAN programme generates the necessary input files for the PEST optimisation package. Instructions are given how to obtain optimized parameters using an example dataset and how to obtain parameters using your own data.
    Verwijdering van fosfaat uit bodemwater met ijzerzand : De omhulde drain
    Koopmans, G.F. ; Chardon, W.J. ; Belder, P. ; Groenenberg, B.J. - \ 2011
    H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 44 (2011)20. - ISSN 0166-8439 - p. 35 - 38.
    grondwaterkwaliteit - fosfaatuitspoeling - eutrofiëring - drainagewater - draineerbuizen - zand - ijzer - adsorptie - waterzuivering - ijzeroxiden - oppervlaktewaterkwaliteit - veldproeven - groundwater quality - phosphate leaching - eutrophication - drainage water - drain pipes - sand - iron - adsorption - water treatment - iron oxides - surface water quality - field tests
    In het Nederlandse oppervlaktewater zijn de fosfaatconcentraties vaak te hoog en vormt eutrofiëring een groot probleem. Fosfaatuitspoeling uit landbouwgronden draagt flink bij aan de totale fosfaatbelasting van het oppervlaktewater. Met brongerichte maatregelen, zoals evenwichtsbemesting, zal in sommige delen van het landelijk gebied de hoofddoelstelling van de Kaderrichtlijn Water niet worden gerealiseerd. Hiervoor zijn aanvullende maatregelen nodig, die kunnen bijdragen aan het verbeteren van de chemische waterkwaliteit. We hebben de effectiviteit getest van een drainagebuis omhuld met ijzerzand om fosfaatuitspoeling te verminderen. Deze veldproef is uitgevoerd op een duinzandgrond in het bloembollengebied. Het gemiddelde zuiveringsrendement van de omhulde drainagebuis bedraagt 94 procent. Het omhullen van drainagebuizen met ijzerzand lijkt een veelbelovende maatregel voor het verminderen van fosfaatuitspoeling
    Brushes and proteins
    Bosker, W.T.E. - \ 2011
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Willem Norde. - [S.l.] : S.n. - ISBN 9789085859178 - 142
    biofilms - eiwitten - adsorptie - aangroeiwerende middelen - fabricage - biomaterialen - biofilms - proteins - adsorption - antifouling agents - manufacture - biomaterials

    Brushes and Proteins

    Wouter T. E. Bosker

    Protein adsorption at solid surfaces can be prevented by applying a polymer brush at the surface. A polymer brush consists of polymer chains end-grafted to the surface at such a grafting density that the polymer chains stretch out into the solution. This is schematically shown in figure 1.


    Fig. 1. Cartoons of a polymer brush. Two ways of preparation: (a) chemical grafting and (b) grafting through adsorption of block copolymers, for instance by Langmuir-Blodgett deposition (LB).

    The main parameters determining the protein resistance of a brush are the grafting density (σ), the chain length (N) and the solvent quality. The thickness of the brush is a function of these parameters: H ~ N σ1/3.

    This research is related to biofouling: 'the undesirable accumulation of proteins and cells at a surface', which starts by adsorption of proteins at the surface. Prevention of biofouling is of vital interest in medicine, where bacterial adhesion may cause severe infections on biomaterials used for implants. Treatment with antibiotics has hardly any effect. The only promising remedy against infections in this case is the prevention of a bacterial film. Because protein adsorption is the first step in this process, the research in this thesis is focused on prevention of protein adsorption by polymer brushes.

    Numerous studies over the past decades revealed that neutral polymer brushes, especially from poly(ethylene oxide) (PEO), can minimize protein adsorption. Mindful of the parameters determining the adsorbed amount mentioned above, the following three mechanisms can be identified, displayed in figure 2. Primary adsorption occurs when the diameter of the protein is (much) smaller than the distance between the polymer chains. In case of secondary adsorption, the protein is (much) bigger than the distance between the polymer chains. Ternary adsorption results from an attraction between the proteins and the polymer chains in the brush and was first discovered by Currie et al. In 1999. For a considerable time researchers have assumed a repulsion between the proteins and the polymer chains, thereby neglecting the possible ternary adsorption. However, there is increasing evidence that this attraction occurs, especially with PEO brushes. This is highlighted in this research, by adsorption studies at bimodal PEO brushes, consisting of a dense PEO brush of short chains with a varying PEO brush of long chains.

    Figure 2.
    Different mechanisms for protein adsorption at polymer brushes:

    primary, secondary and ternary adsorption.

    The main objective of this research was to investigate whether polysaccharide brushes, in particular dextran brushes, could be prepared at a solid surface and to study their protein repellency. It was suggested that brushes from these natural polymers would be more successful to prepare nonfouling surfaces with. Dextran brushes were prepared using Langmuir-Blodgett deposition (LB) and PS-dextran diblock copolymers, illustrated in figure 1. With the LB method it is possible to control both σ and N. The synthesis of the PS-dextran diblock copolymers is described in the thesis as well as the interfacial behavior. Quasi-2D aggregation occurred at the air-water interface during preparation (compression of the PS-dextran monolayer, see figure 1), resulting in inhomogeneous dextran layers at low grafting density. At higher grafting density these aggregates were pushed together to form a homogeneous dextran brush, as illustrated by AFM images. This transition from inhomogeneous to homogeneous results in non-continuous adsorption behavior at dextran brushes, in contrast to PEO brushes, as demonstrated in figure 3.


    Figure 3. Normalized adsorption of BSA (Γ/ Γ0) at dextran brushes (■) and PEO brushes (○).

    In case of dextran brushes the adsorption of BSA is constant up to a specific σ, followed by a drastic decrease, while PEO brushes show a gradual reduction.Figure 3 also demonstrates that dextran brushes are as efficient as PEO brushes in preventing protein adsorption, at high σ. This is the main conclusion of this research. It is expected that at even higher σ dextran brushes will completely suppress protein adsorption.

    Surface complexation at mineral interfaces: Multisite and Charge Distribution approach
    Hiemstra, T. - \ 2010
    Wageningen University. Promotor(en): Willem van Riemsdijk. - S.l. : s.n. - ISBN 9789085857174 - 383
    bodemchemie - geochemie - adsorptie - fosfaten - ionen - organische stof - kleimineralen - kationenwisseling - soil chemistry - geochemistry - adsorption - phosphates - ions - organic matter - clay minerals - cation exchange - cum laude
    cum laude graduation (with distinction)
    Why low powdered activated carbon addition reduces membrane fouling in MBRs
    Remy, M.J.J. ; Potier, V. ; Temmink, B.G. ; Rulkens, W.H. - \ 2010
    Water Research 44 (2010)3. - ISSN 0043-1354 - p. 861 - 867.
    afvalwaterbehandeling - waterzuivering - actieve kool - adsorptie - membranen - biologische filtratie - filtreerbaarheid - uitvlokking - biodegradatie - zuiveringsinstallaties - waste water treatment - water treatment - activated carbon - adsorption - membranes - biological filtration - filterability - flocculation - biodegradation - purification plants - waste-water treatment - bioreactor mbr - sludge - flux - performance - filtration - bioflocculation - operation
    Previous research had demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactor (MBRs). In this contribution several mechanisms to explain this beneficial effect of PAC were investigated, including enhanced scouring of the membrane surface by PAC particles, adsorption of membrane foulants by PAC and subsequent biodegradation and a positive effect of PAC on the strength of the sludge flocs. It was concluded that the latter mechanism best explains why low dosages of PAC significantly reduce membrane fouling. Cheaper alternatives for PAC may have a similar effect
    Karakterisatie van ijzerslib en -zand : een verkenning van de mogelijkheden van het gebruik van deze reststoffen om fosfaatverliezen vanuit landbouwgronden naar het oppervlaktewater te verminderen
    Koopmans, G.F. ; Chardon, W.J. ; Groenenberg, J.E. - \ 2010
    Wageningen : Alterra (Alterra-rapport 2047) - 52
    landbouwgronden - fosfaat - oppervlaktewater - ijzer - adsorptie - nederland - bodemchemie - intensieve veehouderij - fosfaatuitspoeling - maatregelen - agricultural soils - phosphate - surface water - iron - adsorption - netherlands - soil chemistry - intensive livestock farming - phosphate leaching - measures
    Het fosfaatgehalte van Nederlandse landbouwgronden in gebieden met (intensieve) veehouderij is vaak hoog. Dit leidt tot ongewenste fosfaatverliezen naar het oppervlaktewater. Perceelsgerichte maatregelen zijn noodzakelijk om deze verliezen te verminderen en de oppervlaktewaterkwaliteit te verbeteren. Een voorbeeld hiervan is het inwerken van fosfaatbindende materialen in ‘iron reactive barriers’ in de bodem of het toepassen van deze materialen in filters die aan het uiteinde van drainagebuizen kunnen worden gekoppeld. IJzerslib en ijzerzand kunnen worden gebruikt voor het binden van fosfaat.
    Reduction of protein adsorption on surfaces coated with Complex Coacercate Core Micelles
    Brzozowska, A.M. - \ 2010
    Wageningen University. Promotor(en): Martien Cohen Stuart; Willem Norde, co-promotor(en): Arie de Keizer. - [S.l. : S.n. - ISBN 9789085856832 - 246
    micellen - afdeklagen - eiwitten - adsorptie - oppervlaktechemie - micelles - coatings - proteins - adsorption - surface chemistry
    The structure and formation of Ionomer Complexes (ICs) consisting of linear polyelectrolytes
    (C3Ms) has been extensively studied in the past years. Recently, these structures were also
    considered for several applications. This thesis deals with the possible application of ICs as a
    surface coating suppressing protein adsorption independent of the properties of the native
    surface. To reach this goal it was crucial to understand the principles governing the adsorption
    and the stability of the IC layers on solid surfaces, as well as their interactions with proteins.
    Therefore, this research was limited to well defined model substrates: silica (model hydrophilic
    surface), polystyrene (model hydrophobic surface), and polysulfone (a surface mimicking
    polymeric membrane material), and model proteins: β-lactoglobulin, bovine serum albumin,
    fibrinogen, and lysozyme. The ultimate goal, however, was to apply the coating on surfaces of
    membranes used in water purification to suppress biofilm growth. We have observed that
    reduction of protein adsorption by coating formed by regular C3Ms is not satisfactory due to
    relatively low density of the polymer brushes formed on the coated surface. We increased the
    grafting density, and hence significantly improved the reduction of protein adsorption, by
    introducing grafted block and grafted copolymers into the micelles. In our work we discuss an
    influence of various factors, i.e. physical-chemical properties of the native surfaces, lengths of the
    charged blocks, distribution of the grafts along the backbone of the copolymer, salt concentration
    etc., on the performance of the formed coatings. In the final part we focus on the mechanical
    stability of the coatings formed with ICs, and discuss their applicability as a membrane surface
    coating.
    Adsorptive removal of manganese, arsenic and iron from groundwater
    Buamah, R. - \ 2009
    Wageningen University. Promotor(en): J.C. Schippers, co-promotor(en): B. Petrusevski. - [S.l.] : S.n. - ISBN 9789085855262 - 183
    grondwater - grondwaterverontreiniging - waterbeheer - watervoorziening - volksgezondheid - adsorptie - waterzuivering - mangaan - arsenicum - ijzer - ghana - grondwaterkwaliteit - groundwater - groundwater pollution - water management - water supply - public health - adsorption - water treatment - manganese - arsenic - iron - ghana - groundwater quality
    To determine the scale of the problem of arsenic, iron and manganese contamination of groundwater in Ghana a survey was performed in the first phase of the research to provide in depth information with respect to these contaminants. Presence of these mentioned contaminants in groundwater is not peculiar to Ghana alone. Many countries in the world have similar problems with their groundwater. Establishing the existence of the problem also calls for the need to find remedies. Over the years much studies have been done in the Unesco-ihe on the adsorptive mechanisms and techniques for iron and arsenic removal from groundwater. Therefore in this study much emphasis is placed on the adsorptive removal of manganese from groundwater. This research has the following as it objectives: 1. The conduction of a survey for the arsenic, iron and manganese content in selected groundwater aquifers in Ghana. 2. Determination of the arsenic, manganese and iron adsorption capacities of different locally available filter media. 3. Determination of the effect of pH on adsorption capacities of the selected media for arsenic, manganese and iron. To determine the effect of iron (II), manganese (II) presence on arsenic adsorption capacity of selected media – (competition). 4. Studies on the rate of oxidation of Fe (II) and Mn (II) adsorbed onto one or more selected media under different conditions (e.g. dissolved oxygen, pH etc.). 5. Determination of the rate of adsorption of Mn (II) onto one or more selected media under different oxic conditions. 6. To develop a conceptual model that describes the processes involved in adsorptive arsenic, manganese and iron removal. To investigate to what extent these processes can be described in a quantitative way with one or more mathematical models . Generally the study has the following relevance: 1. Provide a base line data on the arsenic and manganese content of selected groundwater aquifers in Ghana. 2. Provide information on the adsorption capacities of locally available filter media for manganese. 3. Contribute to the knowledge on the kinetics and mechanisms of catalytic adsorptive iron, manganese removal. 4. Provide an efficient backwashing procedure for regeneration of adsorptive sites in iron-oxide coated filters operating in the oxidative adsorption mode.
    Coupled Adsorption-Dryer Exchange
    Boxtel, A.J.B. van; Vos, R.M.H. ; Bartels, P.V. - \ 2009
    Wageningen : Wageningen Universiteit - 17
    kunstmatige droging - drogers - zeolieten - adsorptie - energiebesparing - artificial drying - driers - zeolites - adsorption - energy saving
    Voorafgaand aan dit project is in onderzoek door Wageningen Universiteit en Research Centrum het concept van meer-traps adsorptiedrogen met zeoliet ontwikkeld. Lucht nodig voor drogen wordt ontvochtigd met zeoliet, en lucht die een droger verlaat wordt na ontvochtiging met zeoliet opnieuw gebruikt voor drogen in een volgende droogstap. Door warmteterugwinning gebaseerd op procesintegratie kan bij meer-traps drogen energie op zeer efficiënte wijze worden hergebruikt. Op basis van berekeningen was voorspeld dat de energie efficiëntie bij drogen verbeterd kan worden van 55-65% voor conventionele droogtechnologie tot 85-90% in een meer-traps droger en in speciale gevallen zelfs tot 120%. Het doel van dit NEO-project betrof de validatie van het concept van meer-traps drogen door middel van experimenten en modelberekeningen die gebaseerd zijn op experimentele resultaten. Voor het onderzoek is door Ebbens Engineering Ingenieursbureau eerst een experimentele een-traps installatie en vervolgens een twee-traps drooginstallatie ontwikkeld. Door een PhDstudent van Wageningen Universiteit zijn diverse experimenten aan deze installaties in de laboratoria van WUR A&F uitgevoerd. Daarnaast zijn door dezelfde PhD-student diverse berekeningen uitgevoerd om inzicht te krijgen in het effect van de operationele condities op de energie efficiëntie en om het ontwerp te verbeteren. Het onderzoek toont op basis van de experimentele resultaten zonder meer aan dat met een meer-traps systeem de voorspelde energiebesparingen kunnen worden behaald. Verder blijkt dat door een goede keuze van de operationele condities in een twee-traps installatie een energie efficiëntie van 85% haalbaar is. Voor een conventionele droger is onder vergelijkbare condities de energie efficiëntie 52%. Deze stap in energie efficiëntie verbetering kan voortgezet worden naar het drogen in meerdere trappen waarbij een 30-50% lager energieverbruik kan worden gerealiseerd. De energiebesparingen die met een meer-traps adsorptiedroger met zeoliet in de voedingsen genotmiddelenindustrie behaald kunnen worden liggen in de range 2.4-4×1015J en de reductie van de CO2-uitstoot is 4-6% van de gehele sector. Het onderzoek heeft tot promotie aan Wageningen Universiteit geleid, diverse gepubliceerde artikelen, en is gepresenteerd op verschillende congressen. Contacten met potentiële gebruikers van de technologie verlopen positief en er is serieuze belangstelling. Door het onderzoeksteam is daarom een stappenplan naar de toepassing geformuleerd.
    Energy efficient multistage zeolite drying for heat sensitive products
    Djaeni, M. - \ 2008
    Wageningen University. Promotor(en): Gerrit van Straten; Johan Sanders, co-promotor(en): Ton van Boxtel; Paul Bartels. - [S.l.] : S.n. - ISBN 9789085852094 - 162
    drogers - drogen - droogmethoden - absorptie - adsorptie - adsorberende middelen - energiegebruik - ontvochtiging - drogende lucht - energiebesparing - computationele vloeistofdynamica - driers - drying - drying methods - absorption - adsorption - adsorbents - energy consumption - dehumidification - drying air - energy saving - computational fluid dynamics
    Although drying takes a significant part of the total energy usage in industry, currently available drying technology is often not efficient in terms of energy consumption. Generally, the energy efficiency for drying processes ranges between 20-60% depending on the dryer type and product to be dried. This research focuses on the development of single and multistage zeolite drying combined with heat recovery unit to enhance the energy efficiency. The evaluation using steady state and two dimensional model showed that for single stage system the energy efficiency can reach 70-75% and it increases corresponding to the stage number. For 2-3 stages, the energy efficiency 80-90% can be achieved. The experimental work using single stage zeolite dryer also showed that the energy efficiency is close to conceptual approach. Based on the impressive results, a number of potential options have been formulated to construct the zeolite dryer in continuous operating systems for industrial application.
    Polymer vs. surfactant : competitive adsorption at the solid-liquid interface
    Postmus, B.R. - \ 2008
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers. - [S.l. : S.n. - ISBN 9789085049289 - 143
    polymeren - oppervlaktespanningsverlagende stoffen - adsorptie - grensvlak - polymers - surfactants - adsorption - interface
    The research described in this thesis focuses on the competitive adsorption of nonionic polymer and nonionic surfactant on a silica surface. These type of systems are interesting from both an academical and a technological viewpoint. Our academic interest stems simply from the observation that we had a hard time predicting the (adsorption) behaviour of the system beforehand. The technological relevance of our study can be attributed to the observation that technological applications are often complex mixtures containing a large variety of additives. The interactions between all these different components, such as the formation of mixed aggregates or co-adsorption, are quite complex. For applications, these interactions are very important since the properties of a mixture on a microscopical scale can be used to manipulate the macroscopical behaviour. Or, in the case of undesirable macroscopic behaviour, a detailed knowledge about the microscopic interactions can be used to improve on the situation.

    We have restricted ourselves to relatively simple complex mixtures, i.e. we have chosen a well-defined model system consisting of homodisperse components. This model system is an aqueous mixture of the nonionic polymer PEO with the nonionic surfactant CnEm. To study the adsorption behaviour of this mixture, we have chosen to use a flat silica surface as a model surface. The CnEm surfactants adsorb (on a hydrophilic surface such as silica) with their head groups. Because the head groups consist entirely of EO segments, the binding mechanism of the surfactants to the silica is exactly the same as the PEO binding mechanism, namely H-bonding. By evaluating the competitive adsorption of the system, we are effectively investigating the subtle effects of layer structure. By making small changes to the choice of surfactant architecture, polymer length or solvent quality, large changes in layer structure can be induced.

    Reflectometry was used to look at the competitive adsorption from mixtures containing PEO and CnEm. There are several methods to test this competitive adsorption. In the case of simultaneous adsorption, the polymer and surfactant are allowed to adsorb from a mixture. It is also possible to study adsorption sequentially, i.e. first adsorb component A, and then sequentially try to displace component A with component B. We decided to start by doing sequential adsorption experiments, because these are easier to control. In such an experiment, the PEO is allowed to adsorb onto the surface from a solution with 5 mg PEO/L. Care was taken to insure that the layer was in its steady state. Next, the flow of PEO solution was replaced by the background solution, and subsequently by a solution containing only surfactants. The concentration of the surfactant solution was 110-4 mol/L for all surfactants except for C12E3, where solubility problems demanded the use of a lower concentration c = 610-5. Still, all surfactant solutions had a concentration higher than the CMC. The results of these experiments can basically be grouped in two categories. Upon changing to the surfactant solution, the adsorbed amount would either increase sharply, or the adsorbed amount would remain constant. In the first case where the adsorbed amount would increase until the amount that the surfactant would also reach from a single component solution. Furthermore, subsequent rinsing of this layer would result in a total dissolution of the layer, and hence, the adsorbed amount would go to zero. Since this is typical surfactant behaviour, we can conclude that the surfactant displaces the polymer as it adsorbs.

    To better understand the experimental observations, we have developed an SCF model. In this model, it is possible to calculate the charge on the silica surface as a function of the pH and the ionic strength. This yields titration curves that can be compared with experimental titration curves. Our calculated results correspond quite well with literature data.

    One can also use the model to make predictions about the adsorption of PEO on our silica surface. It is possible to go to concentrations much lower than those that are experimentally accessible. We have made predictions about the response of the adsorbed polymer layer upon changes in ionic strength and pH. The results show that PEO adsorption is relatively insensitive for the ionic strength at pH ≈ 7, but at pH ≈ 10, the ions can displace the polymer quite well. This type of behaviour is also found experimentally. Every time that we perform a calculation (and we do find a solution), we obtain the mean field free energy and the most likely conformation of the system. By looking at the profiles of the most likely conformation, i.e. plotting the volume fraction of a species versus the distance from the surface, we can see that the adsorbed polymer inhibits the adsorption of salt. Hence, the polymer and the salt are in competition for adsorption.

    The behaviour of CnEm surfactants can also be evaluated with the model. Here we use exactly the same parameters that we used for the PEO. Again, we started by evaluating the surfactant bulk behaviour. Instead of investigating the first occurrence of a micelle, we have defined a more experimentally relevant CMC. We have evaluated that concentration where the volume fraction of micelles is approximately equal to the volume fraction of unimers. Based upon this criterion we have calculated the CMC and the corresponding micellar size for a number of surfactant architectures and for a number of ionic strengths. We have also evaluated surfactant adsorption isotherms. These calculated adsorption isotherms feature a first order transition at the CSAC. By evaluating the behaviour of the CSAC, we have found that the CSAC shifts to a higher concentration when the pH or the ionic strength is increased. We identified conditions for which the CSAC > CMC, which effectively implies that the surfactant does not adsorb anymore. We compared these predicted results to data measured using a reflectometer, and we find that the model predicts the experimental results quite well.

    The next step is to use the model to try and reproduce the displacement results. We have defined systems that include both PEO and CnEm, at some pH and ionic strength. To determine which component adsorbs from a mixture, we evaluate the response of the CnEm to the competing polymer. The surfactant starts adsorbing at some concentration (CSAC). If the surfactant concentration is lower than the CSAC, then the PEO will adsorb (we assume that the pH and ionic strength are such that the PEO is capable of adsorbing). For surfactant concentrations higher than the CSAC but lower than the CMC, the surfactant will preferentially adsorb. In the case of CSAC > CMC, the surfactant will not adsorb. Typically, the polymer will adsorb in this case, however, one can think of situations (high pH and high ionic strength) where the polymer will also stay in solution.

    Using the method described above, we can model the competitive adsorption of PEO and CnEm. We can evaluate the response of the surfactant to competing species, such as PEO of length N. By identifying for every surfactant architecture that polymer length N where CSAC = CMC, we can make predictions about the adsorption from mixtures.


    De sorptie van orthofosfaat in veengronden en kalkrijke zandgronden en het belang van organisch gebonden fosfaat : kennislacunes met betrekking tot vastlegging en uitspoeling van fosfaat
    Chardon, W.J. ; Mol, G. ; Salm, C. van der; Sival, F.P. - \ 2007
    Wageningen : Alterra (Alterra-rapport 1480) - 36
    veengronden - zandgronden - fosfaten - kalkrijke gronden - fosfatering - adsorptie - monitoring - nederland - fosfaatuitspoeling - bodemmonitoring - peat soils - sandy soils - phosphates - calcareous soils - phosphatation - adsorption - monitoring - netherlands - phosphate leaching - soil monitoring
    Voor een goede voorspelling van de fosfaatuitspoeling uit landbouwgronden is inzicht in het gedrag van fosfaat in de bodem en bodemoplossing van groot belang. Kennis over het gedrag van anorganisch fosfaat in kalkloze zandgronden is ruim beschikbaar. Parameter waarden die de sorptie van fosfaat in andere grondsoorten beschrijven zijn relatief schaars. Daarnaast bestaat een gebrek aan kennis over het gedrag van organisch gebonden fosfaat. In deze studie is door het integreren van bestaande databestanden meer informatie verzameld over het voorkomen van organische P verbindingen en de bijdrage van deze verbindingen aan de uitspoeling. Met een aantal eenvoudige experimenten zijn parameters afgeleid over het sorptiegedrag van fosfaat in veengronden en kalkhoudende zandgronden.
    Vergroting van de fosfaatadsorptiecapaciteit en afname van de chemische beschikbaarheid van fosfaat in gronden door wisselvochtigheid : resultaten van desorptie en adsorptie-experimenten met zand-, klei-, en veengrond
    Kemmers, R.H. ; Nelemans, J.A. - \ 2007
    Wageningen : Alterra (Alterra-rapport 1546) - 60
    bodemchemie - fosfaat - adsorptie - bodemeigenschappen - bodemwater - experimenten - soil chemistry - phosphate - adsorption - soil properties - soil water - experiments
    Een zand-, klei- en veengrond werden in het laboratorium onderzocht op het desorptiegedrag van fosfaat na vernatting gevolgd door het adsorptiegedrag in een daarop volgende droge periode. Wij concluderen dat fosfaat tijdelijk vrijkomt, maar op iets langere termijn weer wordt vastgelegd. De bindingsterkte neemt bij inundatie af naarmate de grond rijker is aan organische stof. De resultaten bieden uitzicht op nieuwe vormen van waterbeheer waarmee fosfaatbeschikbaarheid op duurzame manier kan worden teruggedrongen
    Protein/polysaccharide complexes at air/water interfaces
    Ganzevles, R.A. - \ 2007
    Wageningen University. Promotor(en): Martien Cohen Stuart; Fons Voragen, co-promotor(en): Harmen de Jongh; Ton van Vliet. - [S.l.] : S.n. - ISBN 9789085046141 - 151
    polysacchariden - eiwitten - adsorptie - oppervlaktechemie - oppervlakteverschijnselen - polysaccharides - proteins - adsorption - surface chemistry - surface phenomena
    KEYWORDS:protein, polysaccharide,b‑lactoglobulin, pectin, electrostatic interaction, complex coacervation, adsorption, air/water interface, oil/water interface, surface pressure, surface rheology, spectroscopy

    Proteins are often used to create and stabilise foams and emulsions and therefore their adsorption behaviour to air/water and oil/water interfaces is extensively studied. Interaction of protein and polysaccharides in bulk solution can lead to the formation of soluble or insoluble complexes. The aim of this thesis was to understand the influence of (attractive and non-covalent) protein/polysaccharide interaction on adsorption behaviour at air/water interfaces (and oil/water interfaces) in terms of adsorption kinetics, and rheological and spectroscopic characterisation of the adsorbed layers. The approach was to first identify the relevant parameters (like charge density, charge distribution or molecular weight of the ingredients) in the mixed protein/polysaccharide adsorption process. Subsequently, for each parameter a range of ingredients was selected/prepared allowing variation of only this single parameter. After investigation of the phase behaviour in bulk solution of the different protein/polysaccharide mixtures to be used, the role of each parameter in mixed protein/polysaccharide adsorption was studied. The parameters most thoroughly assessed were: protein/polysaccharide mixing ratio, polysaccharide charge density and molecular weight and the sequence of adsorption. The majority of the measurements were performed withb‑lactoglobulin (in combination with various polysaccharides e.g. pectin or carboxylated pullulan) at air/water interfaces, at standard conditions of pH 4.5 and low ionic strength (< 10 mM). In addition, experiments were performed at higher ionic strengths, different pH's, with different proteins or at an oil/water interface, to extend the insight in mixed protein/polysaccharide adsorption. This results obtained lead to a generic mechanistic model of mixed protein/polysaccharide adsorption.

    In conclusion, protein/polysaccharide interaction can be exploited to control protein adsorption at air/water interfaces. Any parameter affecting protein/polysaccharide interaction (e.g. ingredient parameters like polysaccharide molecular weight, charge density and distribution or system parameters like charge ratio, pH and ionic strength) may be varied to obtain the desired adsorption kinetics, surface rheological behaviour, or net charge of the surface layer. The choice of simultaneous protein/polysaccharide adsorption (in the form of complexes) versus sequential adsorption (first the protein, than the polysaccharide) provides an extra control parameter regarding the functionality of mixed adsorbed layers.
    Supramolecular polymers in inhomogeneous systems
    Zweistra, H.J.A. - \ 2007
    Wageningen University. Promotor(en): Gerard Fleer, co-promotor(en): N.A.M. Besseling. - [S.l.] : S.n. - ISBN 9789085045526 - 78
    polymeren - adsorptie - desorptie - fractionering - zuiveren - fysicochemische eigenschappen - supramoleculaire chemie - polymers - adsorption - desorption - fractionation - purification - physicochemical properties - supramolecular chemistry
    This thesis describes theoretical results of supramolecular polymers in inhomogeneous systems. Supramolecular polymers are linear assemblies of which the monomers are joined by reversible bonds. Many types of supramolecular polymers have been synthesized in recent years. Moreover, there are numerous compounds in nature which exhibit similar behavior. Simulations of coarse-grained models of supramolecular polymers yielded new insights into the properties of supramolecular polymers in inhomogeneous systems.

    Self-consistent-field calculations on the quasi-chemical level of approximation were used to obtain information about adsorbed supramolecular polymers (chapters 2 and 3). In chapter 2, we describe the effect of adsorption on the mean chain length of supramolecular polymers. It is generally agreed that <N> always increases with concentration in homogeneous systems. Adsorbed supramolecular polymers exhibit qualitatively different if the adsorption energy per segment is strong enough.

    A very interesting non-monotonical concentration dependence of <N> of adsorbed supramolecular polymers was found. In other words: there exists a regime where <N> decreases with increasing concentration.

    This has never been shown before. The physical background is a change of the structure of the adsorbed layer: the adsorbed layer changes from flat to fluffy when the monomer concentration is increased.

    Chapter 3 also deals with adsorbing supramolecular polymers, but focuses on the adsorbed amount. This chapter describes how the model parameters influence the shape and position of the adsorption isotherms. Moreover a comparison is made with the adsorption isotherms of macromolecular polymers. It is found that supramolecular polymers adsorb at relatively high volume fractions and the filling of the surface occurs within a narrow range of concentrations. As a result, supramolecular polymers can be desorbed from the surface by diluting the surrounding solution. Macromolecular polymers usually cannot be desorbed in this manner. This has important implications for the use of supramolecular polymers as surface-active agents since the usefulness increases when they can be removed from the surface. Cleaning the surface requires little effort: diluting the surrounding solution is sufficient.

    Chapters 4 and 5 describe a different type of inhomogenous systems: phase-separated systems. The results of these chapters were obtained byMonte Carlosimulations. In chapter 4, we introduce the ``Helmholtz ensemble'', a formalism to calculate the compositions of two coexisting liquid phases by aMonte Carlosimulation. The general idea of this method is to use three simulation boxes (or more, if more than two coexisting phases are present). The only perturbations that are needed are molecule swaps and changes in the orientation of the molecules. Molecule displacements are only needed if a continuum model is used. Unlike the well-known Gibbs ensemble method, volume moves are unnecessary. As a consequence, an interface is formed in one of the simulation boxes. The compositions of the simulation boxes that contain homogeneous phases are used to obtain the compositions of the coexisting liquids.

    For a succesful simulation it is required that a flat interface is formed. Several tests are proposed to check the net curvature of the interface. If a curved interface is formed, then the simulation should be repeated with a different starting composition.

    The restraint that a flat interface should be formed therefore does not affect the range of applicability of the technique. The Helmholtz ensemble method is especially useful for liquids that are modeled on a lattice, since no volume moves are necessary as is the case in the (related) Gibbs ensemble method. It is shown that the Helmholtz ensemble reproduces the phase behavior of the 3D Ising problem very accurately.

    Supramolecular polymer systems are often polluted by monofunctional contaminants which are very difficult to remove. A new purification method aimed specifically at removing monofunctional contaminants is put forward in chapter 5. The idea is to decrease the solvent quality (e.g., by cooling) and to let the supramolecular polymer solution separate into two phases. It is to be expected that the phase that is poor in polymer has a relatively high concentration of monofunctional monomers. Therefore the solution can be purified by discarding the dilute phase.

    In chapter 5, the effectivity of the proposed purification method is investigated by means ofMonte Carlosimulations. The compositions of the concentrated and the dilute coexisting phases are calculated by means of the Helmholtz ensemble method. The entire phase diagram of bifunctional monomers, monofunctional molecules and solvent can be constructed. The efficiency of the several purification steps could be calculated directly from the phase diagram. Moreover, a parameterization of the phase diagram can be found. For purification purposes, the phase diagram can be described by three parameters. Only two simulations are needed to obtain these parameters. It therefore becomes feasible to predict the effectivity of the purification method for a wide range of linking energies. Extrapolations show that the vast majority of monofunctional contaminants can be removed by a single purification step if the conditions are well chosen. Several recommendations for experimental systems are also provided.
    Covalently bound organic monolayers on silicon surfaces : visible light attachment, charaterization, and electrical properties
    Smet, L.C.P.M. de - \ 2006
    Wageningen University. Promotor(en): Ernst Sudhölter, co-promotor(en): Han Zuilhof. - Wageningen : - ISBN 9789085043676 - 162
    unimoleculaire films - silicium - adsorptie - elektrische eigenschappen - unimolecular films - silicon - adsorption - electrical properties
    The full control over surface properties is a 'Holy Grail' in material science. A significant step forward in this area includes the modification of silicon surfaces, by the covalent attachment of organic monolayers. In this way receptors than can specifically bind with ions or molecules be attached to flat silicon surfaces.

    In this research, several facets have been studied that play a role in combining the world of organic chemistry (the monolayers) and the one of inorganic semiconductors (silicon). The development of a very mild method using visible light has been described. Via this method fragile sugar derivatives - which play an important role in cell recognition processes - have successfully been attached to silicon surfaces. Further the prevention of protein adsorption on modified silicon surfaces has been studied. In general, the adsorption of biomaterials adversely affects a diverse range of areas, e.g., the colonization of marine organisms on ship hulls or the fouling of filtration membranes. In addition, the electrical properties of organic monolayers have been described and finally the reaction mechanism of monolayer formation on porous silicon has been studied and discussed.

    With the combination of these studies a significant step forward has been made in the research on modified surfaces. We believe the work will stimulate the development of a variety of applications in the field of electrical devices and (medical) diagnostics.

    Bioflocculation of mesophilic and thermophilic activated sludge
    Vogelaar, J.C.T. ; Keizer, A. de; Spijker, S. ; Lettinga, G. - \ 2005
    Water Research 39 (2005)1. - ISSN 0043-1354 - p. 37 - 46.
    geactiveerd slib - afvalwaterbehandeling - biologische behandeling - temperatuur - uitvlokking - adsorptie - hydrofobiciteit - activated sludge - waste water treatment - biological treatment - temperature - flocculation - adsorption - hydrophobicity - paper-mill - water - effluent - temperatures - whitewater - reactors - flocs
    Thermophilic activated sludge treatment is often hampered by a turbid effluent. Reasons for this phenomenon are so far unknown. Here, the hypothesis of the temperature dependency of the hydrophobic interaction as a possible cause for diminished thermophilic activated sludge bioflocculation was tested. Adsorption of wastewater colloidal particles was monitored on different flat surfaces as a function of temperature. Adsorption on a hydrophobic surface varied with temperature between 20 and 60 °C and no upward or downward trend could be observed. This makes the hydrophobic interaction hypothesis unlikely in explaining the differences in mesophilic and thermophilic activated sludge bioflocculation. Both mesophilic and thermophilic biomass did not flocculate with wastewater colloidal particles under anaerobic conditions. Only in the presence of oxygen, with biologically active bacteria, the differences in bioflocculation behavior became evident. Bioflocculation was shown only to occur with the combination of wastewater and viable mesophilic biomass at 30 °C, in the presence of oxygen. Bioflocculation did not occur in case the biomass was inactivated or when oxygen was absent. Thermophilic activated sludge hardly showed any bioflocculation, also under mesophilic conditions. Despite the differences in bioflocculation behavior, sludge hydrophobicity and sludge zetapotentials were almost similar. Theoretical calculations using the DLVO (Derjaguin, Landau, Verweij and Overbeek) theory showed that flocculation is unlikely in all cases due to long-range electrostatic forces. These calculations, combined with the fact that bioflocculation actually did occur at 30 °C and the unlikelyness of the hydrophobic interaction, point in the direction of bacterial exo-polymers governing bridging flocculation. Polymer interactions are not included in the DLVO theory and may vary as a function of temperature.
    Ion adsorption modeling as a tool to characterize metal (hydr)oxide behavior in soil
    Rahnemaie, R. - \ 2005
    Wageningen University. Promotor(en): Willem van Riemsdijk. - Wageningen : WUR - ISBN 9789085041887 - 141
    ionen - goethiet - fosfaten - carbonaten - adsorptie - bodem - metaalionen - modellen - ions - goethite - phosphates - carbonates - adsorption - soil - metal ions - models
    This study aims to provide a better basis for application of adsorption models for metal (hydr)oxides to natural multicomponent systems. Adsorption of any ion in the environment will be potentially influenced by the effect of other ions present like calcium, phosphate, carbonate etc. The study starts with a detailed study of the binding of ions as outersphere complexes. The CD model has been extended to use the charge distribution for ions that bind as outersphere surface complex. This indicates that neither innersphere nor outersphere surface complexes are treated as point charges anymore. The new approach was applied to describe the adsorption of various electrolyte ions, phosphate, and carbonate. Batch experiments were performed using goethite as an adsorbent to determine the adsorption behavior of electrolyte ions (Li +1 , Na +1 , K +1 , Cs +1 , Ca +2 , Mg +2 , Cl -1 , NO 3-1 ), phosphate, and carbonate. The adsorption of phosphate and carbonate ions was studied in a 'single ion' system and their interaction in a competition system. The charge distribution value of innersphere surface complexes of phosphate and carbonate was calculated using the new approach. New is also the use of quantum chemical calculations to derive the CD value based on a calculated geometry of the surface complexes. The calculated geometries were interpreted with the Brown bond-valence model, resulting in a calculated CD of the surface complex. The calculated CD values were used as a constraint in the surface complexation modeling. The CD model for inner- and outersphere surface complexation successfully described the adsorption data of electrolyte ions, phosphate, and carbonate. For accommodation of adsorbed ions within the Stern layer, a Three Plane (TP) model was used as a framework. For outersphere surface complexes, it was shown that the minimum distance of approach of adsorbed ions depends on the finite size of ions and their degree of hydration, which determine their relative distances to the surface of minerals. It has been shown that the capacitance of the inner Stern layer is determined by the minimum distance of approach of the ion closest to the surface, while the capacitance of the outer layer is determined by the minimum distance of approach of the ion furthest away from the surface. Modeling of phosphate adsorption data revealed that phosphate adsorbed mainly as a bidentate surface complex. At low pH, protonated species of phosphate are a combination of monodentate and bidentate surface complexes. The new CD approach shows that phosphate interacts with sodium at the mineral surface, which could not be detected using previous approaches. Carbonate adsorption data were successfully described using a bidentate surface complex. This complex interacts with sodium at high pH and high salt level. The new approach not only predicts the shift in the isoelectric point as a function of phosphate loading, but also the measured zeta potential is in quite good agreement with predictions based on the assumption that the zeta potential coincides with the potential of the head end of the DDL. Furthermore, it has been shown that the parameterized CD model can be used to determine the effective reactive surface area of metal (hydr)oxides and the total reversibly adsorbed phosphate fraction in soils.
    Basics of macroscopic properties of adsorbed protein layers formed at air-water interfaces based on molecular parameters
    Wierenga, P.A. - \ 2005
    Wageningen University. Promotor(en): Fons Voragen; M.R. Egmond, co-promotor(en): Harmen de Jongh. - Wageningen : WUR - ISBN 9789085043270 - 171
    eiwitten - adsorptie - grensvlak - oppervlakte-interacties - fysicochemische eigenschappen - proteins - adsorption - interface - surface interactions - physicochemical properties
    The importance of protein characteristics on the role of proteins in forming and stabilising interfaces is studied. This study offers a new understanding of adsorbed protein layers.
    Natuurontwikkeling op fosfaatverzadigde gronden: fosfaatonttrekking door een gewas
    Sival, F.P. ; Chardon, W.J. - \ 2004
    Wageningen : Alterra (Alterra-rapport 1090) - 50
    fosfaten - gewassen - natuurbescherming - nederland - bodemchemie - marginaal land - adsorptie - natuurtechniek - soil chemistry - phosphates - adsorption - crops - marginal land - nature conservation - netherlands - ecological engineering
    Als gevolg van de realisering van de Ecologische Hoofdstructuur (EHS) komen landbouwgronden beschikbaar voor natuurontwikkeling. De kans is dan ook groot dat zonder verdere maatregelen alleen ruigtesoorten terug zullen komen. Naast stikstof heeft ook fosfaat zich opgehoopt in de bodem. In de bodem is het gedrag van fosfaat echter anders dan van stikstof. De maatregelen om stikstof te verlagen zijn niet altijd effectief om ook fosfaat te verlagen. Een literatuur- en gegevensonderzoek is uitgevoerd om de effectiviteit van gewasteelt met een aangepaste bemesting vast te stellen om fosfaat en stikstof in de bodem te verlagen. De economisch meest belangrijke gewassen in Noord-Brabant en Limburg waren in de jaren 2000 tot 2002: blijvend gras, snijmaïs, tijdelijk grasland, granen en aardappelen. De fosfaatonttrekking van die gewassen is in volgorde van potentiële P-afvoer (bij afvoer van gewasresten en bij voldoende voorziening met N): gras (40) > consumptieaardappel (35-40) > snijmaïs (25-30) > granen (20-25 kg P/ha). Een gewas heeft een optimale onttrekking als de productie hoog blijft, dus bij optimale bemesting; N-bemesting is daarvoor een bepalende factor. Akkerbouwgewassen kunnen de fosfaatbeschikbaarheid echter maar tot een Pw van 30 mg P2O5 L-1 grond verlagen, omdat beneden dat niveau de groei te sterk wordt geremd. Om lagere waarden lager te bereiken, en een niveau dat geschikt is voor vegetaties van voedselarme omstandig¬heden (Pw ca. 3-4 mg P2O5 L-1 grond), is gras het meest efficiënt
    Detailed characterization of adsorption-induced protein unfolding
    Engel, M.F.M. - \ 2004
    Wageningen University. Promotor(en): Ton Visser; Sacco de Vries, co-promotor(en): Carlo van Mierlo. - [S.l.] : S.n. - ISBN 9789085040019 - 125
    runderserumalbumine - alfa-lactalbumine - moleculaire structuur - grensvlak - fysische eigenschappen - adsorptie - spectroscopie - bovine serum albumin - alpha-lactalbumin - molecular conformation - interface - physical properties - adsorption - spectroscopy
    Modelling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs
    Noort, P.C.M. van; Jonker, M.T.O. ; Koelmans, A.A. - \ 2004
    Environmental Science and Technology 38 (2004)12. - ISSN 0013-936X - p. 3305 - 3309.
    aromatische koolwaterstoffen - polycyclische koolwaterstoffen - adsorptie - sorbaten - hydrofobiciteit - organische verbindingen - monitoring - waterkwaliteit - waterbodems - aromatic hydrocarbons - polycyclic hydrocarbons - adsorption - sorbates - hydrophobicity - organic compounds - monitoring - water quality - water bottoms - polycyclic aromatic-hydrocarbons - hydrophobic organic-chemicals - partition-coefficients - aqueous solubilities - black carbon - sorption - water - sediment - extraction - biphenyls
    Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van der Waals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions. In this modeling study, variation in maximum adsorption capacities is explained from sorbent and sorbate properties. Maximum adsorption capacities were calculated using (a) literature values for soot-water distribution coefficients for polycyclic aromatic hydrocarbons and polycholorobiphenyls on 10 different soot and soot-like materials and (b) Langmuir affinities for adsorption at a carbonaceous surface estimated using a recently reported method
    Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van der Waals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions. In this modeling study, variation in maximum adsorption capacities is explained from sorbent and sorbate properties. Maximum adsorption capacities were calculated using (a) literature values for soot-water distribution coefficients for polycyclic aromatic hydrocarbons and polychlorobiphenyls on 10 different soot and soot-like materials and (b) Langmuir affinities for adsorption at a carbonaceous surface estimated using a recently reported method. The variation in maximum adsorption capacities could be explained by the variation in sorbent specific surface area, sorbent organic carbon content, and the sorbent-sorbate contact area. Furthermore, increasing sorbate thickness was related to a decrease in maximum adsorption capacities, which points to adsorption in micropores. Maximum adsorption capacities decreased by 1-2 orders of magnitude as the contact area increased by 50%. This points to adsorption sites being hardly larger than sorbates.
    Adsorption of charged macromolecules on a gold electrode
    Barten, D. - \ 2003
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Mieke Kleijn. - Wageningen : S.n. - ISBN 9789058089281 - 104
    elektrodes - elektrolyten - adsorptie - macromoleculen - electrodes - electrolytes - adsorption - macromolecules
    In this thesis we have examined the role of electrostatic interactions in the adsorption of charged macromolecules from aqueous solution to a solid surface and the possibilitiesfor manipulatingthis process through the electric potential of the interface. Gold has been used as the adsorbent surface, and before studying theadsorption ofseveral types of macromolecules onto this substrate we have determined the dependence of the double layer potential of the gold on the pH and on the externally applied potential across the gold/electrolyte solution interface.

    Chapter 2 describes a colloidal probe atomic force microscopy study on the electric double layer of a gold electrode in aqueous solutions. The double layer potentials of the gold surface were obtained by fitting force-distance curves for the interaction with a spherical silica particle to the DLVO theory. It was found that the gold electrodeombinesthe features of reversible andpolarizableinterfaces, i.e., its charge and potential are determined by both the solution pH and the external potential. The pHlependenceis attributed to proton adsorption anddesorptionfromoxidicgroups at the gold surface. In the potential range studied, the double layer potentialyfvaries linearly with the applied potential; at a background electrolyte concentration of 1mM, the variation inyi* is roughly 10% of that in the applied potential. The potential of zero force (the external potential at which ^ - 0) varies with pH. The various features of the gold/electrolyte interface are described well by anamphifunctionaldouble layer model, which takes into account the simultaneous effects of the external potential across the interface and the association/dissociation of functional surface groups. The results of this study form the basis of the interpretation of adsorption studies on gold as a function of pH and externally applied potential, described in chapters 4, 5 and 6.

    Chapter 3 addresses the sensitivity of thereflectometerset-up for our adsorption studies as .well as the electro-optic effect. The electro-optic effect involves a change in the optical properties of the gold electrode as a result of applying a potential. This leads to a change in thereflectometersignal. Furthermore, calculations and measurements concerning the sensitivity ofreflectometermeasurements in studying the adsorption of charged polymers from aqueous solution onto gold are presented. This provides a measure for the reproducibility of the adsorption data obtained withreflectometry. The gold substrate is in fact a thin gold film (15 nm) on a silicon wafer. Between the gold film and the waferaS am layer of titanium has been deposited for better attachment. The sensitivity-factor/4,was calculated by modelling the system as a stack of layers of uniform refractive index. The influence of several parameters, such as the layer thickness of the gold and the angle of incidence of the laser beam, on the sensitivity of thereflectometerset-up was examined In the range of potentials applied (-0.2 to +0.6 V vs. Ag/AgClreference electrode) it was round thattrieelectro-optic effect is significant compared to the change in thereflectometeroutput signal •esultingfrom adsorption. The sensitivity factorAs, however, is much less affected by the electro-optic effect than by the effect of inaccuracies in the system parameters. Therefore, by recording a baseline at the same applied potential as at which adsorption is monitored, adsorption data can be obtained without interference of the electro-optic effect. The experimental error in the adsorption measurements is generally between 2-7% and is dominated by the uncertainty in the thickness of the gold film.

    In chapters 4, 5 and 6 the adsorption of charged macromolecules onto gold as measured usingreflectometry, is described. Generally, the adsorption kinetics anddsorbedamounts are determined as a function of the concentrationotthe macromolecule, the electrolyte concentration and the double layer potential of the gold. The double layer potential of the gold is varied through the pH as well as by applying an external potential. By comparing the results of adsorption measurements performed under pH control and under external potential control, effects of conformational changes and changes in the charge of the molecule in solution can beeparatedfrom effects of the variation electrostatic interactions between the molecule and the adsorbing surface.

    Chapter 5 describes the adsorption of a fifth generationdendrimer, 1,4-diaminobutanepoly(propyleneimine). The charge of this molecule is determined by the solution pH. It is a spherical molecule and conformational changes are limited. The electrolyte concentration and the pH of the solution do not have any significant effect on the total adsorbed amount. Neitherdothe initial adsorption rates as a function of pH and as a function of applied potential follow the electrostatic interactions, on the contrary. From this, it was concluded that electrostatics is not the dominant factor in the adsorption process. However, the total adsorbed amount shows linear decrease with increasing applied potential, similar to want was found for the adsorption of PVP* onto gold. We assume that the effect of the applied potential on the adsorbed amounts is an indirect one, e.g., stemming from an increase in binding strength betweendendrimerand metallic surface sites with decreasing applied potential.

    Protein adsorption onto gold was examined in chapter 6.Lysozymewas chosen as theadsorbatebecause the structure of this protein is relatively stable. For such a "hard" protein electrostatic interactions in the adsorption process are believed to be more important than for proteins with low internal stability, especially when thesorbentsurface is hydrophilic. The charge and structural stability of the protein varies with the solution pH. As for thedendrimersit was found thatlysozymeadsorption from aqueous solution onto gold is not controlled by electrostatic interactions. The initial adsorption rate is neither dependent on the electrolyte concentration and pH, nor on the externally applied potential. Quite unexpectedly, we found that the adsorbed amounts at constant pH exhibit a minimum around the potential of zero charge of the gold surface and increase more than linearly for decreasing as well as increasing potential. The curves of adsorbed amounts versus applied potential show a remarkable similarity with so-calledelectrocapillarycurves for metallic electrodes. This is a strong indication that the interfacial tension of the gold/solution interface is an important factor in the adsorption process. At the positive and negative sides of the double layer potential window that could be studied, the adsorbed amount approaches the value corresponding to a full monolayer of undisturbedlysozymemoleculesidsorbed'side-on'. Therefore, it is concluded that at the high-energy gold surface thelysozymemolecules undergo conformational changes, but as the interface becomes more polarized and the interfacial tension decreases, the extent of the surface-induced conformational changes decreases. This conclusion is corroborated by the dependence of the adsorbed amount on the pH, which reflects the combined effects of the change in interfacial tension with double layer potential and the decrease in structural stability with decreasing pH.

    Chapter 7 contains a number of general conclusions and suggestions for furtheresearch. It was concluded that our approach has been successful for determining the relative importance of electrostatic interactions in the adsorption process of charged macromolecules onto the gold substrate. The choice fix gold as thesorbentsurface has proven to be very suitable for our adsorption studies because of itsamphifunctionalcharacter. A disadvantage is die high surface energy of gold, winch makes that many kinds of molecules readily adsorb and that the role of electrostatics in adsorption processes is somewhat limited rather than dominantThismay be avoided by surface modification, for example attaching athiollayer with proton binding functional groups to the gold surface. By doing so, the surface is stillaraphifimctionaland the surface energy is lowered at the same time. Another possible advantage of surface modification is that the range in which the double layer potential can be varied is larger, because the density of proton binding sites can be made high andredoxreactions are somewhat suppressed. In this thesis only one type of protein was investigatedSincethe properties of proteins vary quite substantially from one protein to another, it is recommended to perform similar experiments with different types of proteins, possibly using a less high-energetic surface. Thepossibilities to control adsorption of macromolecules on metal electrodes seemsto be limited.Desorptionby changing the applied potential never occurred. Neither was it possible to affect the kinetics by raising an electrostatic barrier. For PVP" anddendrimerthe latter is presumably resulting from charge compensation by the presence ofcounterionswithin the molecules. Self-consistent field model calculations may help to get more insight in these aspects.

    Afspoeling van bestrijdingsmiddelen vanaf verhardingen
    Rooy, M. de; Beltman, W.H.J. - \ 2003
    H2O : tijdschrift voor watervoorziening en afvalwaterbehandeling 36 (2003)16. - ISSN 0166-8439 - p. 33 - 35.
    herbiciden - bestrating - uitspoelen - onkruidbestrijding - waterverontreiniging - oppervlakkige afvoer - adsorptie - pesticiden - stedelijke gebieden - wegen - herbicides - pesticides - pavements - roads - leaching - weed control - water pollution - runoff - adsorption - urban areas
    In een veldproef is de afspoeling van drie herbiciden vanaf een betonklinkerverharding gemeten. Een deel van de afspoelingsroutes is gekwantificeerd. Daardoor ontstond inzicht in het afspoelingsproces. Een belangrijk deel van de gedoseerde herbicide verdwijnt naar de ondergrond. De grootte van deze flux hangt af van de grootte van de voeg, de vochttoestand van de bodem en de intensiteit van de neerslag. De oplosbaarheid van een herbicide blijkt belangrijker te zijn voor de mate van afspoeling dan de adsorptiecoëfficiënt
    A self-consistent quantification of contributions to differential TGA results of volcanic soil samples
    Parisi, S. ; Meijer, E. ; Dazzi, C. ; Buurman, P. - \ 2002
    In: Volcanic soils : properties, processes and land use : abstracts and field guide - p. 32 - 34.
    vulkanische gronden - bodemwater - meting - adsorptie - volcanic soils - soil water - adsorption - measurement
    Aluminium speciation in natural waters: measurement using Donnan membrane technique and modeling using NICA-Donnan
    Weng, L.P. ; Temminghoff, E.J.M. ; Riemsdijk, W.H. van - \ 2002
    Water Research 36 (2002)17. - ISSN 0043-1354 - p. 4215 - 4226.
    metaalionen - aluminium - modellen - bodemoplossing - membranen - chemische speciatie - adsorptie - grondanalyse - bodemwater - metal ions - aluminium - models - soil solution - membranes - chemical speciation - adsorption - soil analysis - soil water - dissolved organic-matter - contaminated sandy soil - metal-ion binding - humic substances - ph - complexation - copper - solubility - protons - field
    The study of Al speciation is of interest for the assessment of soil and water quality. For the measurement of "free" aluminum (Al3+), a recently developed Donnan membrane technique was tested by measuring Al3+ in aluminum-fluoride solutions and gibbsite suspensions. It shows that the Donnan membrane technique can measure free Al3+ reliably up to 10¿9 M and the equilibration takes 3¿4 days. Next, Al binding to humic acid (HA) purified from a forest soil was measured using either the Donnan membrane technique or gibbsite suspension. Results were compared with those predicted with the non-ideal consistent competitive adsorption (NICA)-Donnan model. The predictions using the generic parameters without fitting were in reasonable agreement with the measured data. Finally, the Donnan membrane technique was used to determine Al binding to dissolved organic matter (DOM) in the solutions of 24 soil samples at pH interval of 3¿7. Measurements agree well with the predictions using the NICA-Donnan model assuming 30% of DOM is HA and 30% is fulvic acid. With this model, the effects of pH and DOM changes on the concentration of Al in 81 soil solutions were predicted reasonably without adjustment of model parameters. The comparison between the results of analysis and the modeling provides a mutual validation for the two methods
    Interactions between metal ions and biogeo-surfaces in soil and water : basis for quantitative risk assessment
    Weng, L.P. - \ 2002
    Wageningen University. Promotor(en): W.H. van Riemsdijk; E.J.M. Temminghoff. - S.l. : S.n. - ISBN 9789058086075 - 193
    metaalionen - zware metalen - risicoschatting - oppervlakten - bodem - bodemoplossing - water - chemische speciatie - adsorptie - bodemwater - milieu - metal ions - heavy metals - risk assessment - surfaces - soil - soil solution - water - chemical speciation - adsorption - soil water - environment

    To provide the basis for an improved quantitative risk assessment of heavy metals in the environment, the interactions between the metal ions and the biogeo-surfaces in soil and water were studied using both experimental and modelling approaches.

    The Donnan membrane technique was developed and optimised for the measurement of metal speciation in soil solutions, in which a soil column was linked to the Donnan cell to provide buffering capacities. In acid natural waters and soils, aluminium is potentially a relatively strong competitor for the heavy metals to the binding on colloidal particles such as organic matter. Using the Donnan membrane technique, aluminium binding to purified humic acid and to the dissolved organic matter in soil solutions was measured. The ability of the NICA-Donnan model for the prediction of Al binding to humic substances was tested.

    A multi-surface model was developed for the prediction of metal speciation in soil and water. This model can also be used to estimate the relative contribution of the individual sorbents to the control of metal distribution. This multi-surface model considers soil as a set of independent sorption surfaces. Metal binding to soil organic matter (solid and dissolved), to clay mineral and to iron hydroxides was modelled using the NICA-Donnan, Donnan, DDL and CD-MUSIC models. This model approach was validated against the concentrations of total dissolved metal concentration and the activities of free metals in sandy soil samples measured using the Donnan membrane technique.

    The interactions between the metal ions and the natural organic matter are not only important for the metal distribution, but also for the solubility and mobility of organic matters. Using the NICA-Donnan model, the effects of the binding of protons and metal ions on the physico-chemical behaviour of humic acid was studied. It was found that the coagulation of humic acid started when the calculated electrostatic potential was less negative. The electrostatic potential of the dissolved organic matter in the soil solution of six forest soil profiles was calculated using the same model. The measured concentration of the dissolved organic matter also decreased in a soil profile, as the calculated potential became less negative. Humic and fulvic acids are the major components in the dissolved organic matter. The mobility and transport of humic and fulvic acids were studied in a column experiment. By comparing the breakthrough curves and the modelled physico-chemical behaviour, it was found that coagulation might be important in the control of the solubility and mobility of humic acid, while adsorption was more likely the mechanism that could explain the immobilisation of fulvic acid in this soil.

    The experimental and the modelling approaches developed are helpful in improving the quantitative risk assessment of the heavy metals. Pot experiments using three different soils and a solution culture experiment have been conducted to study the effects of pH and soil characteristics on the phytotoxicity of nickel. Using the model predictions the differences in the bioavailability of Ni in the three different soils could be explained. By comparing the toxicity of the free Ni 2+ion in the nutrient solution and in the soil solution, the toxicity of Ni in the soils could be predicted reasonably using the quantitative approach.

    The developedDonnan membrane technique is proved to be a useful analytical technique for the determination of metal speciation in soil and water samples. Further research is needed to improve the method to make it capable to measure lower concentrations. The multi-surface model approach proposed in this research is validated against the measurements of the samples from several different soils. Further validation and improvement of this model approach can be achieved after comparing the prediction with the measurement for more different soils, and by better understanding of the interactions between metal ions and the biogeo-surfaces.

    Soil phosphorus quantity-intensity relationships to predict increased soil phosphorus loss to overland and subsurface flow
    Koopmans, G.F. ; McDowell, R.W. ; Chardon, W.J. ; Oenema, O. ; Dolfing, J. - \ 2002
    Chemosphere 48 (2002)7. - ISSN 0045-6535 - p. 679 - 687.
    bodemchemie - fosfaten - kwaliteitsnormen - uitspoelen - waterverontreiniging - adsorptie - fosfaat - mestproblematiek - milieu - nutriënten - soil chemistry - adsorption - phosphates - quality standards - leaching - water pollution
    Soil phosphorus (P) quantity-intensity (q-i) relationships, based on common extraction methods, may potentially be used to estimate the risk of P loss in overland flow and subsurface drainage water. Some workers have used nonlinear q-i relationships to derive thresholds in soil test P (STP; a quantity factor) above which the risk of P loss increases, while others find linear relationships and no threshold. We present here a simple modelling exercise (based on Langmuir adsorption theory) along with data from literature to explain the behaviour of q-i relationships, and to give an explanation for this apparent discrepancy.The data indicate that q-i relationships are dependent upon the soil to solution ratio of the P intensity parameter, adsorption capacity (Qmax) and strength (K) of the soil, and the total range in STP. In turn, this affects the calculation of a threshold in STP. The q-i relationship tends towards linearity under conditions of a narrow total range of STP and/or when using a wide soil to solution ratio for estimating the P intensity parameter. Under such conditions, a threshold is difficult to detect, and uncertain.We conclude that the sensitivity of thresholds to experimental conditions and soils needs to be considered if thresholds are to be successful in environmental management to decrease P loss to surface waters.
    Sorption (kinetic and capacity) and desorption of trace elements in volcanic soils of Italy and the Azores
    Tanneberg, H. ; Jahn, R. ; Meijer, E.L. - \ 2001
    In: Volcanic soils : properties, processes and land use, international workshop (abstracts) - p. 58 - 59.
    vulkanische gronden - bodemchemie - adsorptie - sporenelementen - italië - volcanic soils - soil chemistry - adsorption - trace elements - italy
    Wetting of brushes by polymer melts
    Maas, J. - \ 2001
    Wageningen University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789058084422 - 135
    polymeren - polyvidon - polystyrenen - adsorptie - kinetica - borstels - smelts - deformatie - polymers - polyvidone - polystyrenes - adsorption - kinetics - brushes - melts - deformation

    The scientific and practical importance of thin polymer films is evident and in many applications polymer films are required. Hence, studying properties of polymer films is relevant. Adsorption of polymer at liquid/solid interfaces can stabilise particles in a matrix. Homopolymers are often used for steric stabilisation of particles although these polymers can also destabilise the system by bridging. Diblock copolymers with one adsorbing (anchor) block and one non-adsorbing (buoy) block may be a much better choice. Bridging between particles is then avoided because the protruding buoy blocks do not adsorb on the opposite surface. The use of polymers as stabilisers can also improve the wettability of the pigment particles. The wetting behavior of a liquid onto a surface can be modified by grafting suitable polymers to the substrate. However, grafting polymers very densely onto a surface may have the opposite effect. This thesis deals with the wetting behavior of brushes by polymer melts. These brushes are obtained either by adsorbing block copolymers or by a chemical grafting procedure.

    In chapter 2 we describe the adsorption of PVP/PS block copolymers consisting of a poly-4-vinylpyridine anchor block and a polystyrene buoy block onto silicon oxide surfaces. We investigated the adsorption kinetics and the adsorbed amounts as function of the composition of the block copolymer. Adsorption from solution is very fast and the surface is covered with the polymer within ten seconds. Adsorption from the melt generates denser brushes due to the fact that the chains are less swollen in the melt than in a good solvent. The properties of brushes obtained by both preparation techniques are compared to theoretical scaling laws. Good agreement for both data sets is found, indicating that with both preparation methods brushes are formed which are not too far from equilibrium.

    Chemical grafting of brushes is described in chapter 3. We used a polystyrene with a vinyl end group which can react both with hydrogen-terminated silicon and with silicon oxide. Grafting performed directly from the melt results in very dense brushes. Reaction of the vinyl-terminated polystyrene with passivated silicon forms a silicon-carbon bond, whereas reaction of the same polymer with a silicon oxide surface gives a silicon-oxygen-carbon bond. The latter bond can be hydrolysed by boiling water or basic solutions. The chemical grafting technique allows us to prepare mixed brushes. Also, a reaction of functionalised polymer with a hydrogen-terminated silicon surface which is oxidised selectively at UV-illuminated patches (thus giving silicon oxide patches) can be performed. After hydrolysis, a brush pattern remains on the silicon parts, whereas the silicon oxide parts are free of polymer. Repeating the reaction with a polymer with a different chain length enables patterns long and short chains in a polymeric brush to be prepared.

    Chapter 4 describes the wetting behavior of a polystyrene melt on a polystyrene brush as a function of the grafting density and the chain length of the brush and the melt. Dewetting was observed for low grafting densities (allophobic region). Increasing the grafting density resulted in complete wetting. For high molar mass of the melt dewetting was again observed at very high grafting densities (autophobic regime). Low molar mass polymers showed complete wetting at high grafting densities. Numerical self-consistent-field calculations applied to this system shows that wetting can be controlled by changing the grafting density or by changing the length of the polymer melt. These calculations support our experimental observations.

    The wetting and dewetting transitions are characterised by two intermediate regimes present at both the allophobic and the autophobic transition. In chapter 5 we focus on these intermediate regimes. The first intermediate regime consist of droplets in equilibrium with a mesoscopic thin film and a nearly dry surface. This equilibrium situation can occur if the free energy curve as function of the film thickness has a special shape with a double minimum. This was confirmed by numerical self-consistent-field calculations. The second intermediate regime is polymer droplets in equilibrium with a mesoscopic thin film. In this situation no dry regions are present.

    Chapter 6 describes the temperature effect of a polymer droplet on top of a brush. At a grafting density of 0.55 nm -2 , the droplet shows a decreased contact area at increasing temperatures. A brush consisting of chains with a grafting density of 1 nm -2 showed no effect of temperature. The former brush can be considered as a 'soft' brush in which the melt chains can partially mix with the brush. At elevated temperatures the chains are expelled from the brush and the droplet reduces its contact area. A 'hard' brush does not show this behavior. In this chapter, also the wetting behavior of a bimodal brush by a melt is descibed. Incorporating very few long chains into a soft brush shifts the wetting behavior from dewetting to complete wetting. However, at high amounts of long chains (~60%) autophobic dewetting is again observed.

    The deformation of the adsorbed PVP/PS polymer films by the low tip-to-sample forces in the AFM is described in chapter 7. The contact mode AFM experiments showed that the AFM tip produces rims oriented perpendicularly to the scanning direction. A wide range of molar masses of both blocks was investigated to check if there is a dependence of the rim distance of the block copolymers on the size of the PVP block copolymer. A linear relation between the composition of the polymer at a constant length of the polymer was found. A tentative explanationis proposed.

    In chapter 8 we present some observations using three different systems for adsorption and wetting experiments. The first is a polystyrene layer on a brush of a triblock copolymer of vinylpyridine (end blocks) and styrene (middle block). The triblock copolymer shows complete wetting behavior when the triblock copolymer layer is annealed after its deposition, whereas there is partial wetting on a not annealed film. In the former case, peculiar needle like structures became visable when the sample with its overlayer of free chains, was annealed.

    The second system investigated in this chapter is film formation and wettability of PS on top of a layer of a polypropylene imine dendrimer with a attached chain of PS. In all cases dewetting was observed, probably due to the relatively low amounts of hairs onto the surface. Finally, the adsorption of poly-2-vinylpyridine-polycaprolactone (PVP/PCL) block-copolymers onto silicon oxide surfaces is described, as well as the wettability of the homopolymer polycaprolactone onto such adsorbed layers. In this case the partial crystallinity of the polymer is a complicating factor.

    Immunoglobulin adsorption on modified surfaces
    Bremer, M.G.E.G. - \ 2001
    Wageningen University. Promotor(en): J. Lyklema; W. Norde; T.A.M. Beumer. - S.l. : S.n. - ISBN 9789058084408 - 147
    immunoglobulinen - polymeren - adsorptie - immunoglobulins - polymers - adsorption

    Preservation of biological functioning of proteins during immobilisation is of special interest in various biomedical and biotechnical applications. In industry physical adsorption of immunoglobulins (IgGs) onto solid surfaces is still the predominant immobilisation procedure because it is relatively easy to perform. Physical adsorption, however, often results in an undesired loss of biological activity. This loss of activity may be caused by changes in the specific folding (the conformation) of the IgG or by a reduced accessibility of the antigen binding sites by blocking, for instance when IgG molecules are adsorbed with their antigen binding sites oriented towards the sorbent surface.

    To control the orientation and conformation of adsorbed IgG molecules we studied the interactions involved in physical adsorption of IgG molecules on solid surfaces. Our goal was to optimize the biological activity of adsorbing IgG molecules. For this, we introduced a new method to achieve oriented physical adsorption of IgG. This concept is based on the anisodimensionality of IgG molecules and resembles 'molecular sieving' on the sorbent surface. The 'sieve' is created by preadsorbed molecules that form a steric barrier preventing adsorption at some sites, but leaving patches of uncovered surface area. The open areas in the preadsorbed layer can be tuned in such a way that only the smaller part of anisodimensional molecules can enter and adsorb. In the case of 19G this means that only the Fc part can adsorb to the surface and thereby forcing the larger antigen binding parts (F(ab) 2 ), directed towards the solution and, hence, accessible to bind antigens. A 'sieve' formed either with preadsorbed IgG molecules or with triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, of the type PEOPPO-PEO was proven to yield a higher specific biological activity of subsequently adsorbing IgG.

    In brief, the effect of a 'molecular sieve' on IgG adsorption is essentially threefold. Firstly, it induces oriented IgG adsorption. Secondly, it prevents extended undesirable structural changes in adsorbed IgG and thirdly, it prevents undesirable reorientation of the adsorbed IgG.

    In chapter 1 it is explained that in many applications there is a need to control the biological activity of adsorbed IgG. The physical properties and characteristics of

    IgG molecules and the interactions involved in physical adsorption of proteins are described and, more importantly, our variant of 'molecular sieving' is introduced. Finally, an outline of this thesis is given.

    The influence of electrostatic interactions on the adsorption of IgG is examined both theoretically and experimentally in chapter 2. The long range interaction between IgG and the sorbent surface is treated in terms of the DLVO theory. We attempted to make use of the dipolar character of the IgG molecules to control their orientation upon adsorption. It is concluded that electrostatic interactions have a strong influence on the adsorption behaviour of IgG molecules on hydrophilic charged surfaces. Due to extensive desorption of IgG from both positively and negatively charged surfaces, electric field-induced orientation of IgG could not be established unambiguously.

    Chapter 3 is mainly dedicated to the orientational aspects of IgG adsorption. In this chapter the phenomenon of 'molecular sieving' is demonstrated first theoretically using a Random Sequential Adsorption (RSA) model and second experimentally by a set of reflectometry experiments on surfaces partially covered with preadsorbed layers of either IgG or triblock copolymers of PEO-PPO-PEO. The rate of IgG adsorption and the maximum adsorbed amount decreases with increasing adsorbed amount of triblock copolymer. On the precoated layers, IgG is indeed adsorbed in a preferential orientation which yielded a higher specific biological activity of the IgG molecules. Furthermore, we observed that the preadsorbed layers prevent undesirable reorientation of adsorbed IgG.

    The mass flux towards the surface also has a profound effect on the adsorbed amount and, consequently, on the orientation of IgG.

    In chapters 4, 5 and 6 conformational changes in IgG are studied. In chapter 4 a structural analysis of a monoclonal lgG adsorbed on different silica surfaces (hydrophilic, hydrophobic, hydrophobic with preadsorbed triblock copolymers) using ATR-FTIR spectroscopy is given. The secondary structure of adsorbed IgG on a hydrophilic silica surface resembles that of native 19G in solution. The presence of preadsorbed triblock copolymers on the hydrophobic silica surface cause a decrease in the adsorbed amount of IgG and, more importantly prevent substantial structural rearrangements in the adsorbed IgG molecules.

    In chapters 5 and 6, Circular Dichroism (CD) is used as a spectroscopic technique for studying protein structure in the adsorbed state. Chapter 5 gives information on the structural changes of IgG molecules induced by adsorption on a hydrophobic

    surface and compares these changes with those induced by heat treatment. Neither heatinduced nor adsorption-induced structural changes lead to complete unfolding into an extended polypeptide chain, but leave a significant part of the IgG molecule in a globular or corpuscular form. The structural changes induced by heating and by adsorption are different.

    The effect of preadsorbed IgG and triblock copolymer molecules on the secondary structure of subsequently adsorbing IgG molecules is studied in chapter 6. Structural rearrangements were less extensive with increasing surface coverage of the polymer. It was found that preadsorbed IgG molecules have comparable effects on the secondary structure of subsequent adsorbing IgG; a more native-like structure is retained for the higher adsorbed amounts. Hence, partial pre-coating of a surface is an effective way to control the secondary structure of later adsorbed IgG molecules.

    To examine whether the model results apply to industrially manufactured diagnostic methods we implemented in chapter 7 the triblock copolymer preadsorption procedure in a microplate assay. Radio-actively labelled IgG and hCG molecules allowed us to monitor the adsorption of IgG and the subsequent specific binding of hCG. The data obtained are in agreement with our earlier model studies and demonstrate that sieving of the IgG by the polymer does take place, resulting in the creation of a more favourably oriented IgG layer. Our studies indicate that the amount of precoated material is critical in the formation of an operational 'sieve'.

    Adsorptive iron removal from groundwater
    Sharma, S.K. - \ 2001
    Wageningen University. Promotor(en): J.C. Schippers. - S.l. : S.n. - ISBN 9789054104308 - 202
    grondwaterverontreiniging - adsorptie - verwijdering - ijzer - drinkwater - groundwater pollution - adsorption - removal - iron - drinking water

    Iron is commonly present in groundwater worldwide. The presence of iron in the water supply is not harmful to human health, however it is undesirable. Bad taste, discoloration, staining, deposition in the distribution system leading to aftergrowth, and incidences of high turbidity are some of the aesthetic and operational problems associated with iron in water supplies. Iron removal from groundwater is, therefore, a major concern for water supply companies using groundwater sources. The WHO recommended guideline value of iron in drinking water is 0.3 mg/l and the EC directive has set a parametric value of 0.2 mg/1. In the Netherlands, the guideline value of iron in drinking water is≤0.05mg/1 and several Dutch water supply companies are aiming at iron concentrations below 0.03 mg/1 to minimise distribution network maintenance costs. Of the different methods available to control iron in water supplies (oxidation-filtration, ion exchange, lime softening, sub-surface iron removal, and sequestration), aeration followed by rapid sand filtration is the most commonly used.

     

    Different mechanisms (physical, chemical, and biological) may contribute to the removal of iron in filters and the dominant mechanism depends on water quality and process conditions applied. Fig. 8. 1 summarises the different mechanisms of iron removal in filters and the steps involved. Under anoxic conditions, adsorption is the only mechanism of iron removal from groundwater. In the presence of oxygen, iron removal can take place via three different mechanisms, namely i) oxidation-floe formation (floe filtration), ii) biological oxidation, and iii) adsorption-oxidation (adsorptive filtration). Under the commonly applied treatment conditions in iron removal plants, the oxidation-floe formation mechanism is commonly believed to be dominant. The adsorptionoxidation mechanism (adsorptive iron removal), however, has several potential advantages over the oxidation-floe formation mechanism, namely longer filter run, shorter filter ripening time, and less backwash water use and sludge production.

    The relationship between the molecular structure and ion adsorption on goethite
    Rietra, R.P.J.J. - \ 2001
    Wageningen University. Promotor(en): W.H. van Riemsdijk. - S.l. : S.n. - ISBN 9789058085030 - 117
    goethiet - adsorptie - ionen - moleculaire structuur - goethite - adsorption - ions - molecular conformation

    Keywords:

    Ion adsorption modeling, goethite, iron oxide, CD-MUSIC, phosphate, arsenate, vanadate, molybdate, tungstate, sulfate, selenate.

    A study is presented on the adsorption of inorganic ions on goethite with emphasis on the adsorption of oxyanions. Experimental results for a range of oxyanions (PO4, AsO4, VO4, WO4, MoO4, CrO4, SeO3, SeO4, SO4, Cl, NO3, ClO4) and Ca are presented and interpreted using the CD-MUSIC model. For some of these ions the coordination and structure of the adsorbed ions on goethite are known from spectroscopy (SO4, SeO4, PO4, AsO4, SeO3). Ideally, surface complexes derived from spectroscopy correspond with those resulting from the modeling of macroscopic adsorption data. This would assure that the mechanistic description of ion binding scales from the microscopic molecular structure to the macroscopic adsorption behavior. In the CD-MUSIC model it is assumed that the charge of the adsorbed ions is distributed at the interface as a function of the coordination and structure of the adsorbed ions and that this distribution of charge can be estimated using the bond valence concept of Pauling. In this study it is found that the macroscopic proton-ion adsorption stoichiometry is almost solely determined by the interfacial charge distribution of adsorbed complexes. It is shown that the experimentally determined proton-ion adsorption stoichiometry can be predicted on the basis of the spectroscopically identified structures of sulfate, selenite, phosphate and arsenate on goethite. By doing so a direct relationship is demonstrated between the molecular structure of adsorbed ions and macroscopic adsorption phenomena. By using this knowledge it is in principle possible to identify the structure and coordination of adsorbed complexes from the macroscopic adsorption data and vice versa. It is found that the spectroscopically suggested differentiation between inner- and outersphere complexes of sulfate and selenate, and the differentiation between bidenate and monodentate phosphate can be modeled satisfactory with the CD-MUSIC approach although the differentiation cannot be established solely from the available adsorption data. It is also found that the proton adsorption on goethite decreases in electrolyte solutions of NaCl, NaNO3 and NaClO4 (below the PZC) in the order Cl>NO3>ClO4 while sulfate and phosphate adsorption is lower in the order Cl<NO3<ClO4. These results can be explained well by assuming outersphere complexes of the electrolyte anions on the goethite surface with different intrinsic affinities.

    Modeling the adsorption of weak organic acids on goethite : the ligand and charge distribution model
    Filius, J.D. - \ 2001
    Wageningen University. Promotor(en): W.H. Van Riemsdijk; J.C.L. Meeussen. - S.l. : S.n. - ISBN 9789058084545 - 183
    organische zuren - goethiet - adsorptie - biologische beschikbaarheid - transportprocessen - organic acids - goethite - adsorption - bioavailability - transport processes

    A detailed study is presented in which the CD-MUSIC modeling approach is used in a new modeling approach that can describe the binding of large organic molecules by metal (hydr)oxides taking the full speciation of the adsorbed molecule into account. Batch equilibration experiments were performed using the iron (hydr)oxide goethite to determine the adsorption of a series of weak organic acids (e.g. lactic acid, oxalic acid, malonic acid, phthalic acid, citric acid, and fulvic acid). In order to develop the new modeling approach, the binding of weak organic acids with a well-defined structure and charging behavior is first described using the classical CD-MUSIC model approach. The adsorption can be described accurately with a limited number of surface species, which differ in the degree of protonation and in the number of inner sphere and outer sphere complexes formed between the reactive groups of the organic molecule and the oxide surface. For the formation of inner and outer sphere complexes, a constant distribution of charge over the solid/water interface is assumed. With the same charge distribution for inner and outer sphere complexes, the adsorption of FA is described with the CD-MUSIC model using a small set of discrete surface species. This approach is not fully satisfactory since it does not take the full speciation of the adsorbed weak organic acids into account as can be inferred from spectroscopic data. In order to take the full possible speciation of the adsorbed organic molecule into account, a new model concept was developed. In the ligand and charge distribution (LCD) model concept, the number of inner sphere, outer sphere and proton complexes of the reactive groups of one adsorbed organic molecule is calculated using the NICCA equation. From the resulting speciation of the adsorbed molecule, the main input parameters of the CD-MUSIC model are obtained. The new model concept is successfully tested on the adsorption and infrared data of benzenecarboxylic acid adsorption on goethite by Boily et al. (2000a,b). The LCD model was extended for the surface complexation of the phenolic groups in order to describe the previously determined data of FA adsorption by goethite. Simultaneously, the concentration, pH, and salt dependency of the FA adsorption are described well. Furthermore, the co-adsorption of protons upon the adsorption of FA by goethite is predicted accurately. The developed model approach offers new insights in the fundamental understanding of ion adsorption under natural conditions.

    Release rates of Al from inorganic and organic compounds in a sandy podzol, during laboratory experiments
    Salm, C. van der; Westerveld, J.W. ; Verstraten, J.M. - \ 2000
    Geoderma 96 (2000)3. - ISSN 0016-7061 - p. 173 - 198.
    bodemoplossing - bodem - aluminium - zure gronden - adsorptie - soil solution - soil - aluminium - acid soils - adsorption
    Research with respect to release rates of Al and Al concentrations in the soil solution has led to two contrasting hypotheses, stressing either the importance of kinetically constrained dissolution reactions or the role of complexation of Al to soil organic matter. The existence of two contrasting hypotheses is partly due to the fact that the interpretation of existing results is hampered by the lack of specificity of the extractants used to quantify the organic and inorganic pools of Al and by the fact that simultaneous measurements on the release of Al and the change in Al pools are missing. This study was conducted to obtain more information on the release rates of Al from various amorphous pools and to evaluate the validity of the above hypotheses. To overcome the above-mentioned shortcomings both release rates of Al and changes in the various,Al pools were measured during the experiment. Experiments were conducted for Ap, Bs and C horizons from a sandy podzol. Results indicated that Al was released from both the inorganic Al pool (oxalate minus pyrophosphate extractable Al) and the strongly bound organic Al pool (pyrophosphate minus CuCl2 extractable Al) according to a first-order reaction. The decline in the weakly bound organic Al pool (CuCl2 extractable minus adsorbed Al) could be described by a rapid dissolution of part of the pool, combined with an equilibrium reaction describing the complexation of Al to organic sites. Rate constants were highest for the non-complexed part of the weakly bound organic Al pool, intermediate for the inorganic Al pool and lowest for the strongly bound organic Al pool. Large differences exist in rate constants of the organic pools between the three horizons. Differences between the rate constants of the inorganic Al pool for the three horizons were not significant.
    Polymers and surfactants in solution and at interfaces : a model study on detergency
    Torn, B. - \ 2000
    Agricultural University. Promotor(en): J. Lyklema; L.K. Koopal; A. de Keizer. - S.l. : S.n. - ISBN 9789058082640 - 140
    polymeren - oppervlaktespanningsverlagende stoffen - adsorptie - polymers - surfactants - adsorption

    This thesis deals with detergency-related adsorption phenomena of (mixtures of) polymers and surfactants. Both types of molecules play an important role in the removal and subsequent stabilization of soil from a substrate. Starting with a model detergency system consisting of polymers, surfactants, soil and a substrate, a division is made into a set of sub-systems, each focusing on the interactions of two or more of these model components.

    The first chapter gives a short introduction on the typical behavior of polymers and surfactants in solution and at interfaces, and touches upon the physicochemical principles of detergency. In a washing process it is important to prevent the redeposition of soil, which in an earlier stage has been removed from a substrate. A way to keep particles dispersed in solution is to cover them with a thick polymer layer providing electrostatic and/or steric stabilization. The adsorption of the uncharged polymer poly(vinylpyrrolidone) (PVP) on Na-kaolinite has been studied in chapter 2. The surface of this clay mineral is patchwise heterogeneous with respect to its charge and chemical composition. In order to reveal these charge characteristics, potentiometric acid-base titrations were performed on samples at different concentrations of sodium chloride. An interpretation of the proton adsorption/desorption in terms of the contributions of the individual surface types, i.e. edges and plates, has been given. At the latter type, protons are strongly favored over sodium ions. Striking similarities were observed between the proton adsorption and the PVP adsorption experiments. PVP readily adsorbs high affinity on at least part of the kaolinite surface. Studying the effect of the pH, the electrolyte concentration, and the presence of multivalent ions on the amount adsorbed at the plateau has given further insight into the adsorption mechanisms. Increasing the pH or the electrolyte concentration leads to a decrease in PVP adsorption. A model is proposed in which PVP adsorbs on edges and basal planes by different mechanisms. The adsorption of PVP on the edges is strongly pH dependent, but that on the plates only weakly. Specifically adsorbed protons at the plates act as anchor sites for PVP segments. Multivalent ions do not influence the proposed adsorption mechanism directly, but primarily change the surface area accessible for PVP.

    Before studying adsorption of a polymer-surfactant mixture, information on the interaction between the polymer and the surfactant in solution is indispensable. Chapter 3 covers the interaction between the anionic surfactant sodium dodecylbenzenesulphonate (SDBS) and the uncharged polymer poly(vinylpyrrolidone) (PVP) by titration microcalorimetry. Since hydrophobic attractions are typically dependent on temperature, which is in general not the case for other types of interaction, measurements carried out at different temperatures have yielded information on the nature of the associations. The interaction enthalpy of mixed PVP/SDBS systems clearly showed a consecutive endothermic and exothermic region with increasing surfactant concentration. The endothermic part can be looked upon as an incremental binding isotherm and reflects the number of surfactant molecules involved in the process. The exothermic region features the inverse of hydrophobic bonding behaviour. In our opinion, this is due to conformational changes of the polyelectrolyte complexes. With increasing amount of surfactants bound to the chain, electrostatic repulsion of neighbouring surfactant head groups tends to expand the complexes, whereas hydrophobic interactions do the opposite. Beyond a certain coverage, the coulombic repulsion forces the chains to swell. This is accompanied by losing hydrophobic inter- and intrachain linking. Additional surfactants continue to adsorb on the vacant hydrophobic adsorption sites. The influence of the initial amount of polymer and the electrolyte concentration support our proposals.

    The results and the knowledge obtained with this study has helped to understand the mixed adsorption of PVP and SDBS on kaolinite, which is the subject of chapter 4. Both components adsorb from their mixture on the clay. This process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. In the absence of PVP, SDBS adsorbs on the clay by electrostatic and hydrophobic interactions. When polymers are present, the adsorbed amount of SDBS is at 10 -2 M NaCl mainly determined by charge compensation on the edges.

    Under different conditions PVP shows similar behaviour as a function of the surfactant concentration. With increasing SDBS concentration three subsequent regions in the PVP adsorption can be distinguished: initially a small increase, followed by a strong decrease, which finally flattens off to a plateau. These regions are related to the surface affinity of the species actually present in solution. They reflect the changing character of the charge of the polymer-surfactant complexes with increasing surfactant concentration. At low surfactant content, the polymer chains are not or hardly charged, and they adsorb on the clay by hydrogen bonding and hydrophobic interactions. At high surfactant concentrations, the adsorption of polymer-surfactant complexes is dominated by coulombic attraction. There is experimental evidence for the presence of mixed surface aggregates at the edges. The composition of these complexes differs from that in solution and is controlled by the surface charge. With increasing electrolyte concentration, this difference becomes smaller.

    After a detailed look at the solution side of the washing process, we have to focus on the substrate. In order to carry out fundamental studies, a flat and well-defined surface was needed which was a good mimic for cotton. To that end, a cellulose surface was developed which was able to function as a model for cotton. Chapter 5 describes the preparation of thick cellulose films. The method is based on the attachment of hydrophobized cellulose on a wafer and subsequent chemical regeneration to cellulose. With the spincoating technique, reproducible, rapidly prepared, and flat cellulose surfaces can be obtained. These are characterized by their thickness, roughness, swelling behaviour, stability, charge, and wetting and adsorption properties.

    So far, all studies concerned equilibrium aspects. However, in a washing process, the dynamics of processes, such as adsorption, removal, and stabilization, are very important. Kinetic and equilibrium aspects of nonionic surfactant adsorption on cellulose surfaces just described, are studied in a stagnation point flow cell (chapter 6). Nonionic surfactants readily adsorb on cellulose, thereby showing three distinct regions. At low surface coverages, molecules adsorb more or less in a flat state, with a contribution from both the head group and the tail. At increased concentrations, lateral attraction between surfactant molecules is dominant, leading to the formation of half-micelles at the surface. In line with the results of chapter 5, the adsorption features of cellulose are in between those for a hydrophilic and a hydrophobic surface.

    The kinetics of nonionic surfactant adsorption depends on the composition of the surfactant. Below the CMC, the initial adsorption rate is determined by monomer diffusion. Above the CMC, the magnitudes of the micellar dissociation rate and the micellar diffusion coefficient, should be compared to that of the monomer diffusion coefficient. If the micellar properties are sufficiently large, micelles acts as monomer-suppliers. This was observed for the most hydrophilic surfactant under study. The desorption rate depends on the surface coverage. Initially, it is controlled by monomer detachment. The desorption rate coefficients of different surfactants scaled with the CMC, suggesting an analogy between the surface aggregates to those formed in solution.

    The set-up of the cellulose surfaces in a stagnation point flow cell can be used for a variety of adsorbates and serve as a model for (re)deposition studies.

    Transport of reactive carriers and contaminants in groundwater systems : a dynamic competitive happening
    Weerd, H. van de - \ 2000
    Agricultural University. Promotor(en): W.H. van Riemsdijk; A. Leijnse. - S.l. : S.n. - ISBN 9789058082909 - 141
    grondwater - waterverplaatsing - organische stof - grondwaterverontreiniging - modellen - adsorptie - groundwater - groundwater pollution - water transfer - organic matter - adsorption - models

    Transport of contaminants constitutes a potential threat for public health and ecosystems. One of the potential pathways for contaminant transport in groundwater systems is transport adsorbed to carriers (colloidal particles, large molecules). Figure 1 shows a detail of a groundwater system with carriers and contaminants. In this thesis adsorption and transport models for carriers and contaminants are developed and applied in order to increase the understanding of the behavior of carriers and the behavior of contaminants in a system with carriers. Emphasis is put on mobile natural organic matter (NOM), which is a major carrier for contaminants in soil and groundwater systems.

    Inline: Figure 1 shows a detail of a groundwater system with carriers and contaminants
    Fig. 1

    The first part of this thesis considers the behavior of contaminants in a system with a constant concentration of carriers. Using a model for coupled carrier and contaminants transport (COLTRAP) the effect of reaction rates and non-linearity on contaminant transport is studied. Varying the reaction rates of carrier-solid matrix and contaminant-carrier reactions, it is shown that a range exists in which time dependent reactions determine the transport of contaminants. Transport processes within this range can only be described and understood using models considering dynamic sorption, like COLTRAP. Transport at and beyond the limits of this range can be calculated with equilibrium models. Simulation of a column experiment under natural conditions shows that time dependent contaminant-carrier and carrier-solid matrix interactions are important under these conditions. In this experiment, the mobility of a radioactive contaminant (americium) is dramatically increased due to the presence of mobile NOM. Both desorption of Am from NOM and interaction between mobile and immobile NOM are found to be time dependent (slow).

    The second part of this thesis considers the behavior of NOM in more detail. As mobile NOM is a mixture of molecules varying from simple small molecules like citric acid to complicated large molecules like humic acid, its adsorption behavior cannot be fully understood from mono-component adsorption models. The NOMADS model (NOM adsorption) is developed in order to describe the dynamic competitive adsorption/desorption of NOM subcomponents on a homogeneous surface. Furthermore, a method to minimize the number of adaptable parameters is developed. The model allows illustrating and quantifying important features of sorption and transport of a mixture of NOM molecules, like apparent adsorption/desorption hysteresis, slow increasing sorption maxima and large tailing of NOM breakthrough curves. It is shown that experimental results over a large range of temporal and spatial scales and surface to volume ratio's can be successfully described using the same set of model parameters. The behavior of NOM is described in laboratory batch experiments with a few grams of soil and some tens of milliliters of water and also in a field experiment with large travel times, distances of some meters and large volumes of soil.

    The two main mechanisms found in this research are time dependent sorption reactions and dynamic competitive sorption of a carrier mixture. It is shown that these mechanisms are relevant for contaminant and carrier transport under natural conditions. They may determine main features of contaminant and carrier transport in groundwater systems and therefore also determine potential risks for public health and ecosystems.

    Keywords : adsorption, desorption, adsorption kinetics, transport processes, contaminant transport, facilitated transport, contaminants, radionuclide, competitive adsorption, polydispersity, natural organic matter, NOM, dissolved organic matter, DOM, DOC, humic substances, colloids, colloidal particles, modeling, simulation, groundwater, porous media, soil

    Static and dynamic properties of proteins adsorbed at liquid interfaces
    Benjamins, J. - \ 2000
    Agricultural University. Promotor(en): J. Lyklema; E.H. Lucassen-Reynders. - S.l. : S.n. - ISBN 9789058083173 - 212
    colloïdale eigenschappen - eiwitten - adsorptie - schuim - emulsies - colloidal properties - proteins - adsorption - foams - emulsions

    The aim of the investigation described in this thesis was to increase the level of understanding of the role that proteins play in the preparation and subsequent stabilisation of foams and emulsions. One aspect of this role is facilitation of break-up, due to surface tension lowering. A second aspect is the formation of a viscoelastic interfacial layer, which affects both the short-term and long-term stability of the dispersion. Therefore, a systematic study of the changes in static and dynamic interfacial properties induced by proteins was carried out.

    For part of this study, dealing with the interfacial rheology, several experimental techniques were used. These techniques were either properly modified existing techniques (Chapter 3, modified longitudinal wave set-up) or newly developed (Chapter 4, Dynamic Drop Tensiometer; Chapter 5, Concentric Ring Surface Shear Rheometer) to meet the requirements for measuring the rheology of adsorbed protein layers at liquid/liquid interfaces. These requirements are (i) isotropic deformation, without leakage of the interfacial layer, for the dilational modulus measurements at air/water and oil/water interfaces and (ii) shear modulus measurements at small oscillatory deformation.

    The proteins chosen for this study wereβ-casein,β-lactoglobulin (BLG), bovine serum albumin (BSA), ovalbumin and lysozyme. This set of proteins was chosen, because they differ considerably in relevant aspects, such as molecular weight, molecular structure and iso-electric point.

    In Chapter 1 the scope and context of this study are given including a brief introduction into (i) the molecular properties of these proteins, that are relevant to the adsorption, (ii) protein adsorption and interfacial rheology, and (iii) the relation between interfacial properties and the properties of emulsions and foams.

    Chapter 2 deals with the adsorption of proteins at the air/water interface. The adsorption was determined by ellipsometry, a method by which not only the adsorbed amount but also the layer thickness and protein concentration in the adsorbed layer could be determined. The ellipsometric studies were combined with surface tension measurements at the same surface.

    All proteins examined show high affinity adsorption, i.e. strong adsorption at low concentration in solution. The initial rate of adsorption of all proteins is well described by a simple diffusion equation. For all proteins examined, the value of the surface pressure (Π) are protein-specific, but otherwise unique, time-independent functions of the adsorption (Γ). Time independence of theΠ(Γ) curve was concluded from the finding thatΠandΓpairs measured at different bulk concentrations and at different stages of adsorption, all collapse into one single curve. In other words, each protein has a unique surface equation of state indicated by its measuredΠ(Γ) curve. This curve reflects the relative rigidity of the protein molecule. For flexible molecules likeβ-casein and PVA ,Γ min (=ΓwhereΠstarts to deviate measurably from zero) is low and from this point onward the surface pressure increases gradually with increasingΓ. For rigid globular proteins (BSA, ovalbumin and lysozyme)Γ min is higher and with further increase of the surface concentration the surface pressure increases steeply. At high protein concentration and long adsorption times, for most proteins multilayer adsorption takes place.

    For ovalbumin, in the pH range 4-8 the effect of pH on theΠ-Γcurve is small, which indicates that electrostatic intermolecular forces do not contribute much to the surface pressure.

    In Chapter 3 a longitudinal wave technique, modified to ensure isotropic surface deformation, was used to determine the dilational modulus,ε, of adsorbed protein layers, at the air/water interface. This modification fully eliminated the complicating shear effects that became apparent in dilational modulus measurements with adsorbed layers of proteins in a conventional set-up.

    For all proteins examined at frequencies in the range from 0.01 to 1 rad/s, the initial part of theε(Π) plot is a straight line through the origin. The slope of this initial part ranges between +4 and +12 . No clear relationship between the slope and the rigidity of the protein molecule was found. However, the extent of this linear range is smaller for the flexible molecules (β-casein and PVA). From the fact that this slope significantly exceeds the ideal value of +1, it must be concluded that the behaviour of the adsorbed layer is far from ideal. In the linear range, the measured moduli coincide with the limiting moduli,ε 0 , calculated from theΠ(Γ) curve. This indicates that the surface pressure adjusts "instantaneously" to the changing adsorption during a compression-expansion cycle in time-scales ranging from 1 to 100 s. This also means that the modulus is purely elastic, i.e. the effect of relaxation processes is negligible. In this elastic range, differences between individual proteins are related to different degrees of non-ideality, reflected in the surface equation of state.

    At higher surface concentrations a relaxation mechanism becomes operative, which is most probably not caused by diffusional exchange between surface and solution. This conclusion is based on calculations of the diffusional transport rate and the theoretical frequency spectrum of the modulus. Relaxation due to conformational changes is plausible. In the visco-elastic regionε≥ε 0 for all proteins examined. This is an extra argument against diffusional exchange.

    The modulus increases in the order: PVA <β-casein

    Chapter 4 describes a new method, the Dynamic Drop Tensiometer, especially suitable for determining the dynamic properties of proteins adsorbed at oil/water interfaces. According to this method, a small drop is subjected to sinusoidal oscillations of its volume. The corresponding area changes produce interfacial tension changes, which are evaluated from measurements of the fluctuating shape of the drop, using the Young-Laplace equation. Compared to the conventional Langmuir trough set-up, this method is particularly suited for liquid/liquid interfaces, because (i) interfacial leakage is fully eliminated and (ii) uniform deformation is ensured even if one of the liquids is a viscous oil. An additional advantage of the method is its short response time. The dynamic properties of adsorbed protein layers at three interfaces (TAG (triacylglycerol)-oil/water, tetradecane/water and air/water) were compared. At the three interfaces, at low protein concentration, the conformation change upon adsorption is fairly fast, occurring within 1 min.. However, at high protein concentration (> 1g/l), during the first minutes after adsorption a situation exists that differs from the equilibriumΠ(Γ) curve. At low interfacial pressures, during a modulus measurement, the adaptation of the conformation is faster (< 1 s.). Non-ideality of the adsorbed layer increases in the sequence TAG-oil < tetradecane < air, which is probably related to a decrease of solution quality for the more hydrophobic amino acids, which decreases in the same sequence. At each of the different interfaces non-ideality increases with increasing rigidity of the protein molecule (β-casein<β-lactoglobulin

    The surface shear properties of adsorbed protein layers are described in Chapter 5. These properties were determined with a newly developed concentric ring surface shear rheometer. The technique allows measurements over a wide range of frequencies and deformations. As the magnitude of the shear deformation markedly affects the shear modulus,μ s , an extrapolation to zero deformation is required to asses the shear properties of the undisturbed surface. Because the surface dilational modulus and the surface shear modulus both increase in the sequence PVA< Na-caseinate s ≥3 indicates that the adsorbed protein layer can be modelled as a thin homogeneous gel layer. Such a model points to a significant ideal monolayer contribution toεat low to medium surface concentrations.

    In Chapter 6 models describing the surface equation of state of adsorbed macromolecules were applied to the experimentalΠ(Γ) curves. These models were also applied to understand the dynamic behaviour of these layers. Statistical models, in which it is assumed that the macromolecules adsorb with all segments in direct contact with the surface, e.g. Singer equation, only explain the very low pressure part of the experimental curves of PVA andβ-casein. To explain the higher pressure part, progressive loop formation and molecular interaction must be accounted for. For rigid globular proteins, simple statistical models are unable to fit any part of the experimental curves, because such molecules only slightly change their conformation upon adsorption and consequently, will adsorb with only a small fraction of the segments at the surface, even at very low pressures.

    A 2-D solution model, which accounts to first order for both entropy and enthalpy, is used to describe the non-ideal behaviour of adsorbed protein layers. This non-ideality was deduced from the highΓneeded to produce a measurableΠand the steep initial slopes of theε(Π) curves.

    All above models need modification to describe the S-shaped part of theΠ(Γ) curves at high surface concentrations. This part of the curve can be described by the Soft Particle concept, which is a modification of the surface equation of state of a 2-D hard sphere fluid. The S-shape is attributed to a decrease of the molecular cross-sectional area with increasing surface concentration. This effect appears to be more pronounced for flexible molecules like PVA andβ-casein than for globular rigid molecules like BSA, ovalbumin and lysozyme. Experimentalε(Π) curves are within the limits that are predicted by this concept. A promising option is combining a molecular compressibility as used in the Soft Particle concept with the 2-D solution model.

    In Chapter 7 it is shown that interfacial properties typical for proteins predict a larger drop size and a lower stability against recoalescence during production compared to low molecular weight (LMW) surfactants.

    In the presence of both types of surfactant, concentrations and conditions can be chosen such that the LMW surfactant determines the dispersion efficiency, while the protein determines the long-term stability. A comparison between the different proteins reveals that, in the production stage, a higher dilational modulus at short times correlates with a faster build-up of stability against recoalescence. For a good long term stability a high dilational modulus of adsorbed protein layers at longer times is more important. In foams, retardation of Ostwald ripening, i.e. the growth of large bubbles at the expense of small ones, is probably the major factor. This mechanism depends on the ratio of the modulus to the surface tension, which ratio is considerably higher for proteins than for LMW surfactants in relevant cases.

    For a measurable shear modulus a high surface concentration is required. Therefore, shear properties may only affect long term stability of emulsions and foams, but not break-up and stability against recoalescence during production.

    Ontwikkeling van een methode voor de scheiding van fosfaatpools uit zand-, klei- en veengronden
    Lepelaar, P. ; Schoumans, O.F. - \ 1999
    Wageningen : Staring Centrum (Rapport / DLO-Staring Centrum 575) - 73
    grondanalyse - kwantitatieve technieken - fosfor - adsorptie - soil analysis - quantitative techniques - phosphorus - adsorption
    Om vast te stellen in welke vormen het fosfaat in verschillende grondsoorten wordt opgehoopt, is nagegaan welke extractiemethodiek in de praktijk goed hanteerbaar blijkt te zijn en daarnaast ook goed aansluit bij de huidige visie omtrent fosfaatvastlegging in de bodem. Met behulp van vier verschillende extracties (infinite-sink methode, oxalaatextractie, zoutzuurdestructie met en zonder oxydatie) worden de belangrijkste fosfaatvormen op relatief eenvoudige wijze uit de bodem geëxtraheerd. De geëxtraheerde bodemfosfaten vertegenwoordigen: makkelijk desorbeerbaar fosfaat, gesorbeerd fosfaat, mineraal fosfaat en totaal fosfaat (mineraal fosfaat + organisch gebonden fosfaat).Vergelijking van het gebruik van een sequentiële methodiek met die van een parallelle methodiek toont aan dat de parallelle methodiek nauwkeuriger en betrouwbaarder is.
    Beschrijving van het gedrag van anorganisch fosfaat in veengronden
    Schoumans, O.F. - \ 1999
    Wageningen : Staring Centrum, Instituut voor Onderzoek van het Landelijk Gebied (Rapport / DLO-Staring Centrum 522) - 53
    bodem - fosfor - adsorptie - veengronden - nederland - soil - phosphorus - adsorption - peat soils - netherlands
    Mede voor de onderbouwing van de fosfaatverliesnormen in het mestbeleid dient vastgesteld te worden in welke mate fosfaat daadwerkelijk chemisch gefixeerd wordt in veengronden. In opdracht van het ministerie van LNV is daarom nagegaan hoe zowel de binding als het weer vrijkomen van fosfaat beschreven kan worden en van welke bodemkenmerken dit gedrag van fosfaat afhangt. Uit de resultaten van het laboratoriumonderzoek blijkt dat de totale hoeveelheid fosfaat die veenmonsters kunnen vastleggen, onderling niet verschilt van die van zandmonsters. De grootste verschillen worden waargenomen in de mate waarin het fosfaat weer vrij kan komen. De fractie desorbeerbaar fosfaat is in veengronden groter en sterk afhankelijk van het organische stofgehalte. Naarmate het organische stofgehalte toeneemt, neemt ook de fractie desorbeerbaar fosfaat toe.
    Behaviour of metamitron and hydroxy-chlorothalonil in low-humic sandy soils
    Pas, L.J.T. van der; Matser, A.M. ; Boesten, J.J.T.I. ; Leistra, M. - \ 1999
    Pesticide science : a journal of international research and technology on crop protection and pest control 55 (1999)9. - ISSN 0031-613X - p. 923 - 934.
    zandgronden - bodemwater - herbiciden - metamitron - pesticiden - chloorthalonil - adsorptie - transformatie - sandy soils - soil water - herbicides - metamitron - pesticides - chlorothalonil - adsorption - transformation
    The relationship between molecular structure and ion adsorption on variable charge minerals
    Rietra, R.P.J.J. ; Hiemstra, T. ; Riemsdijk, W.H. van - \ 1999
    Geochimica et Cosmochimica Acta 63 (1999)19-20. - ISSN 0016-7037 - p. 3009 - 3015.
    mineralen - structuur - adsorptie - minerals - structure - adsorption
    Ion adsorption modeling is influenced by the presumed binding structure of surface complexes. Ideally, surface complexes determined by modeling should correspond with those derived from spectroscopy, thereby assuring that the mechanistic description of ion binding scales from the nanoscopic molecular structure to the macroscopic adsorption behavior. Here we show that the structure of adsorbed species is a major factor controlling the pH dependency of adsorption. An important aspect of the pH dependency is the macroscopic proton-ion adsorption stoichiometry. A simple and accurate experimental method was developed to determine this stoichiometry. With this method, proton-ion stoichiometry ratios for vanadate, phosphate, arsenate, chromate, molybdate, tungstate, selenate and sulfate have been characterized at 1 or 2 pH values. Modeling of these data shows that the macroscopic proton-ion adsorption stoichiometry is almost solely determined by the interfacial charge distribution of adsorbed complexes. The bond valence concept of Pauling can be used to estimate this charge distribution from spectroscopic data. Conversely, the experimentally determined proton-ion adsorption stoichiometry allows us to successfully predict the spectroscopically identified structures of, for example, selenite and arsenate on goethite. Consequently, we have demonstrated a direct relationship between molecular surface structure and macroscopic adsorption phenomena.
    Kinetic Freundlich equation applied to soils with a high residual phosphorus content
    Chardon, W.J. ; Blaauw, D. - \ 1998
    Soil Science 163 (1998)1. - ISSN 0038-075X - p. 30 - 35.
    bodem - fosfor - absorptie - adsorptie - soil - phosphorus - absorption - adsorption
    Effects of dissolved organic matter on the mobility of copper in a contaminated sandy soil.
    Temminghoff, E.J.M. ; Zee, S.E.A.T.M. van der; Haan, F.A.M. de - \ 1998
    European Journal of Soil Science 49 (1998). - ISSN 1351-0754 - p. 617 - 628.
    bodemchemie - adsorptie - zandgronden - koper - bodemverontreiniging - organische stof - soil chemistry - adsorption - sandy soils - copper - soil pollution - organic matter
    Semi-flexible polymers near interfaces : equilibrium aspects and adsorption kinetics
    Eijk, M. van - \ 1998
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789054858546 - 99
    oppervlakte-interacties - polymeren - xanthan - adsorptie - oppervlaktechemie - surface interactions - polymers - xanthan - adsorption - surface chemistry

    The first chapter is about semi-flexible polymers at a liquid-liquid interface: self-consistent-field calculations. The adsorption of semi-flexible polymers at a liquid-liquid interface largely differs from that at a solid surface. The width of the interface is an additional length scale in the problem, making the system behaviour particularly rich. We consider two phase-separating monomeric liquids, C and D, and a polymer A N which dissolves equally well in both liquids. We study this system in a self-consistent-field model in the dilute regime. The stiffness of the polymer is controlled by the use of a rotational isomeric state approach. We show that the interfacial widthxi(determined by the interaction parameter between the two solvents), the persistence length q, and the chain length N are relevant parameters in the adsorption behaviour.

    A key observation is that, while keeping N 1/2/xiconstant, the adsorbed amount goes through a minimum with increasing q/xi. An initial increase of q/xi(q/xikleiner of gelijk aan1) effectively leads to a larger coil size, leading to a decrease of the adsorbed amount. However, when q/xiveel groter dan1, alignment of parts of the polymer within the interfacial region occurs due to the lack of entropic penalties. This alignment process induces an increase of the adsorbed amount. These observations also have implications for the ongoing discussion about the preferential adsorption in a mixture of flexible and stiff polymers. In this discussion one should consider the effects of the finite size of the interfacial region.

    The second chapter is about wetting by polymers of a liquid-liquid interface: effects of short-range interactions and of chain stiffness. The behaviour of both flexible and semi-flexible polymers near a liquid-liquid interface is investigated with the aid of the self-consistent-field theory as developed by Scheutjens and Fleer. Aternary system (A/B N C) is studied near the wetting transition. In a symmetric system, i.e.χ AB = χ BC = χ, a change in the interaction parameterχintroduces a wetting transition. The ratio of the interfacial widthxiof the binary A/C system and the coil size of the polymer determines the order of this transition. Beyond a certain chain length N C (at fixedxi) the wetting transition is of first order, whereas it is of second order for NC . The characteristics of the prewetting line, including the prewetting critical point, are discussed in some detail. The non-trivial N-dependence of the position of this critical point is analysed in terms of a crude thermodynamic model. For a semi-flexible polymer an increase of the chain stiffness at a certain value ofχis sufficient to introduce a wetting transition.

    Chapter 3 is about adsorption kinetics of the polysaccharide xanthan on ZrO 2 . The adsorption kinetics of the polysaccharide xanthan from aqueous solution on zirconium oxide were studied as a function of pH and ionic strength. The adsorption was monitored by reflectometry in astagnation-point flow setup. At intermediate pH and ionic strength, xanthan is present in a helical form and it can be viewed as a semi-flexible polymer under these conditions. By lowering the salt concentration or increasing the pH a helix-coil transition takes place. This transition is caused by the mutual electrostatic repulsion of the short side chains of xanthan. The so-formed coil can be considered as a Gaussian chain, with a large radius of gyration. The conformation of the polysaccharide is roughly reflected in its adsorption behaviour.

    It is, however, deduced that the electrostatic interaction between polymer and surface influences the stability ofthe helix. The adsorption process can be divided in two regimes. At low surface coverage the rate of adsorption is transport-limited, which in a stagnation-point flow leads to a linear time dependence of the adsorbed amount. The adsorption rate in this regime hardly changes with ionic strength or pH. The time range over which it holds, however, does, which can be understood in terms of electrostatic effects. At higher surface coverage two types of behaviour are observed. At low ionic strength and on a highly charged surface the adsorbed amount saturates abruptly. This kind of kinetics resemble those of flexible polymers. In this case the xanthan presumably adsorbs in a coil-like conformation, because the helix becomes unstable in the vicinity of the surface. At higher ionic strength and on a weakly charged surface, the adsorbed amount increases gradually over very long times. Under these conditions, the helix conformation is more stable so that we ascribe this slow process to tentatively rearrangement and alignment processes of the stiff chains on the surface.

    Chapter 4 is about the competition between transport and spreading in protein adsorption kinetics. The saturation adsorbed amount of polymers on solid surfaces is mostlyfound to be independent of the polymer transport rate, or flux J, to the surface. In most cases this is because the experimental rate of transport strongly deviates from the relaxation rate in the polymer layer. We studied the adsorption of both immunoglobulin G and savinase on SiO 2 from aqueous solution and found that the transport rate is an important parameter in the adsorption kinetics. The adsorption process can be viewed as an attachment to, followed by the spreading over the surface of a polymer molecule. In this way the adsorbed amount strongly depends on J if the time for transport to the surface is in the same range as the spreading time. Using an analytical "growing disk" model for the polymer adsorption, we are able to, at least qualitatively, describe the adsorption kinetics.

    Chapter 5 is about adsorption and spreading of polymers at plane interfaces; theory and molecular dynamics simulations. Nonequilibrium processes play a key role in the adsorption kinetics of macromolecules. It is expected that the competition between transport of polymer towards an interface and its subsequent spreading has a significant influence on the adsorbed amount. An increase of the transport rate can lead to an increase of the adsorbed amount, especially when the polymer has too little time to spread at the interface. In this study we present both molecular dynamics simulations and analytical calculations to describe some aspects of the adsorption kinetics. From MD simulations on a poly(ethylene oxide) chain in vacuum near a graphite surface, we conclude that the spreading process can, in first approximation, be described by either a simple exponential function or by first-order reaction kinetics. Combining these spreading models with the transport equations for two different geometries (stagnation-point flow and overflowing cylinder)we are able to derive analytical equations for the adsorption kinetics of polymers at solid-liquid and at liquid-fluid interfaces.

    Copolymer adsorption and the effect on colloidal stability
    Bijsterbosch, H.D. - \ 1998
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : S.n. - ISBN 9789054857907 - 139
    adsorptie - membranen - colloïden - oppervlakteverschijnselen - adsorption - membranes - colloids - surface phenomena
    The main aim of the work described in this thesis is to study the effect of different types of copolymers on the stability of aqueous oxide dispersions. Such dispersions are a major component in water-borne paints. In order to obtain a better insight in steric stabilisation we first investigated the relation between the adsorbed amount and layer thickness, and paid attention to the effect of the type of copolymer on the adsorbed amount. We also studied the adsorption kinetics as these are relevant for industrial purposes.

    An introduction on steric stabilisation is given in Chapter 1. For block copolymers the solvent may be non-selective or selective. In a non-selective solvent both blocks are solvated and the polymer molecules are likely to be in a non-aggregated conformation. However, in a selective solvent the molecules form micelles in which the non-soluble blocks are clustered together, surrounded by a layer of solubilised chains. The adsorption kinetics are expected to be affected by the existence of such micelles. Another important feature for the adsorption of block copolymers is the selectivity of the surface. When only one of the blocks has affinity for the surface this will give rise to selective adsorption. On the other hand, the adsorption of a block copolymer in which both blocks have affinity for the surface is non-selective. The resultant polymer layer will differ for both cases. In thesis we studied selective and non-selective adsorption from a selective and a non-selective solvent. As the architecture of the copolymers is also relevant we paid attention to the adsorption of both block copolymers and graft copolymers.

    In Chapter 2 we describe the properties of spread monolayers of polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymers at the air-water interface. The surface pressure and the thickness of the layer were measured as a function of the adsorbed amount. The thickness was determined with neutron reflectivity measurements.

    Upon compression of the polymer monolayer the surface pressure increases over the entire experimental range of compression. At low coverage the adsorbing PEO block forms a flat "pancake" structure at the surface. When the surface area per molecule is decreased the PEO is pushed out of the surface layer into the solution to form a "cigar" or "brush" structure, which is firmly anchored by the PS block. Some scaling analysis have suggested that this desorption occurs as a first-order surface phase transition. When the polymer layer is compressed further, so that the surface density σincreases, the chains stretch and the thickness H of the layer increases too. Theories predict that H scales as Nσ 1/3, where N is the number of monomers per polymer chain. This is confirmed by our results. However, our experimental data do not show the first-order surface phase transition between pancake and brush. Numerical self-consistent-field calculations also show a gradual transition rather than a first-order phase transition.

    In Chapter 3 we present a study on the non-selective adsorption of two series of diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2oxazoline)- poly(ethylene oxide), from aqueous solution on a macroscopically flat silicium oxide surface. The adsorbed amounts in this study, and in that of Chapters 4 and 5, were measured with an optical reflectometer in an impinging jet flow cell. The hydrodynamic layer thickness was determined by dynamic light scattering.

    The different blocks in the copolymers all have affinity for the silica surface. In all cases there is a small difference between the segmental adsorption energies of the two blocks, giving rise to non-selective adsorption of the block copolymers. For the two types of block copolymers used in this study, the adsorbed amount as a function of block copolymer composition shows a shallow maximum; at this maximum the longest block is also the more strongly adsorbing block. The same trend is found for the hydrodynamid layer thickness. These findings differ from theoretical predictions concerning selective adsorption, where a pronounced maximum is found for a short anchor block. With numerical self-consistent field calculations we demonstrate that the same trends as in our experimental findings can be predicted by theory. In non-selective adsorption of diblock copolymers, with a small difference between the adsorption energies of the blocks, both blocks compete for the same adsorption sites on the surface. When the blocks are incompatible they try to avoid each other, which promotes an anchor-buoy structure. These factors then give rise to a maximum in the adsorbed amount as a function of the block copolymer composition. At this maximum the longest block is also the more strongly adsorbing block. The adsorbed layer has the typical anchor-buoy structure which is necessary for an effective steric stabilisation, but this structure is less pronounced than for selective adsorption.

    The kinetics of adsorption of diblock copolymers can be very slow if the polymers form micelles in solution. In Chapter 4 we compare the experimental adsorption rates on silica and titania with the theoretical flux of copolymer molecules towards the surface for four poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) diblock copolymers with the same block length ratio but different molar masses. In aqueous solution these block copolymers form large polydisperse micelles with a very low critica l micellisation concentration (lower than 2 mg 1-1).

    On both surfaces the adsorption behaviour is governed by the anchoring of the hydrophobic siloxane blocks The adsorption kinetics are affected by the exchange rate of free polymer molecules between micelles and solution. For the three smallest molar masses the exchange rate is fast compared to the time a micelle needs to diffuse across the diffusive layer. Before the micelles arrive at the surface they have already broken up into free polymers. Because the cmc is very low, the experimental adsorption rate is determined by the diffusion of micelles towards the surface. For the longest polymer this is not the case: the exchange of polymer molecules between micelles and solution is now relatively slow. As the micelles do not adsorb directly, the adsorption rate is retarded by the slow exchange process. We were able to make an estimate of the micellar relaxation time, i.e., the time a micelle needs to break up. For the largest polymer the relaxation time is of the order of a few tens of seconds. The other polymers have a micellar relaxation time that is shorter than roughly one second.

    The adsorption increases linearly as a function of time, up to very high adsorbed amounts where it reaches a plateau. Such high adsorbed amount is expected for strongly (and selectively) adsorbing diblock copolymers with a relatively short anchor block. The adsorbed amount on silica is considerably higher than on titania. The reason is probably that the hydrophobic block is more strongly anchored to a silica surface than to titania, so that the density of the adsorbed layer can become higher on silica.

    In Chapter 5 we investigate the interfacial behaviour of graft or comb copolymers. We compare the adsorption of graft copolymers with an adsorbing backbone and nonadsorbing side chains to the reverse situation of adsorbing side chains and a nonadsorbing backbone. Two high- molar-mass poly(acryl amide)-graft-poly(ethylene oxide) copolymers with different side chain densities were used in this study.

    On titania only the backbone of these polymers adsorbs and the side chains do not. The adsorbed amount is then about the same as that found for the homopolymer without side chains. On the other hand, on silica the side chains adsorb and the backbone does have no affinity for the surface. For both polymer samples we observe a maximum in the adsorbed amount as a function of time ("overshoot"), after which the adsorbed amount decreases and a plateau is reached. The plateau adsorbed amount on silica is much higher than on titania and also much higher than for both types of homopolymers. Upon adsorption the graft copolymers initially adopt a conformation in which only part of the side chains are adsorbed. Following the overshoot, the graft copolymers show a decrease in the total adsorbed amount. The overshoot depends on the polymer concentration, which suggests that it is not caused by conformational changes in the adsorbed layer but by an exchange process between surface and solution.

    Differences in graft distribution and graft density in the polymer sample are probably responsible for the displacement of adsorbed chains by polymer molecules from solution. The average number of grafts per molecule is rather low in our polymer samples. On statistical grounds there is probably an appreciable polydispersity in graft distribution and in graft density. Molecules in which the grafts are clustered to some extend can displace molecules with more regularly separated grafts, and molecules with a high graft density can displace those with a lower number of side chains. The newly arriving molecules can then adsorb in a flatter conformation with a lower adsorbed amount as the extra loss in conformational entropy is compensated by the gain in adsorption energy.

    The effect of the polymers used in Chapters 3 to 5 on the stability of an aqueous silicium oxide dispersion is described in Chapter 6. The time-dependent increase of the average hydrodynamic radius of silicium oxide aggregates in the presence of electrolyte was measured. The increase of this radius with time is a measure of the aggregation rate of the dispersion. The effect of polymers on the stability of a dispersion was studied by adding polymer to the dispersion and recording the effect in the aggregation rate

    Comparison of the aggregation rate of this "protected" silica with that of uncovered silica particles gives then an indication of the steric stabilisation by the adsorbing polymers.

    Four different series of diblock and graft copolymers were used in these stability measurements. For two series of non-selectively adsorbing diblock copolymers, poly(vinyl methyl ether)-poly(2-ethyl-2-oxazoline) and poly(2-methyl-2-oxazoline)poly(ethyiene oxide), we find a good correlation between the adsorbed amount and the stabilising effect. A higher adsorbed amount provides a better steric stabilisation. Nevertheless, for these polymers the adsorbed amounts are not high enough (up to about 1.2 mg M -2) to protect the dispersion completely against aggregation. A series of amphiphilic diblock copolymers of poly(dimethyl siloxane)-poly(2-ethyl-2-oxazoline) with very high adsorbed amounts (between 3.5 and 8 mg M -2) give excellent steric stabilisation of the dispersion. Adsorbed layers of the two graft copolymers of poly(acryl amide)-poly(ethylene oxide), with a non-adsorbing backbone and adsorbing side chains, are also effective in preventing the silica from aggregating. Even though the adsorbed amount of these graft copolymers is only around 1.3 mg M -2, which is much lower than that of the amphiphilic polymers, aggregation is completely prevented.

    The best steric stabilisation is found for those systems in which either the surface or the solvent is selective. In practical aqueous systems, however, it is difficult to synthesise diblock copolymers in which both blocks are soluble and where only one of the blocks has affinity for the surface. We have shown that copolymers with a different architecture, graft copolymers, also can provide good steric stabilisation and may be a good alternative to diblock copolymers. Very good steric stabilisers are amphiphilic diblock copolymers in a selective solvent. However, it is important that the hydrophobic blocks are flexible enough for fast adsorption kinetics and that they completely wet the surface. Which copolymer should be chosen for the steric stabilisation of a practical colloidal system depends largely on the nature of the particles and the solvent, and on the availability of suitable copolymers.

    Electrodynamics of colloids
    Minor, M. - \ 1998
    Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Minor - ISBN 9789054858010 - 145
    colloïden - adsorptie - oppervlakten - oppervlaktechemie - elektrodynamica - colloids - adsorption - surfaces - surface chemistry - electrodynamics - cum laude

    The goal of the present study is to deepen the insight into the non-equilibrium properties of the electric double layer of colloidal systems. Of basic interest are the ionic mobilities in the different regions of the electric double layer as well as the potential at the plane of shear, i.e., the electrokinetic potential (ζ-potential). These parameters determine the colloidal behaviour under non-equilibrium conditions when the double layer is perturbed, for instance if external fields are applied and in particle-particle interaction during coagulation.

    One of the experimental methods utilized in this study is the measurement of the conductivity and the streaming potential of close-packed plugs of particles. From the resulting data we retrieved the dzeta.gif -potential, the surface conductivity, and the mobility of the counterions behind the plane of shear. The results are well comparable to those from the experimental low-frequency (LF) dielectric response of dilute dispersions of latex particles.

    The electrodynamic parameters can be influenced by adsorbing neutral polymer onto the surface

    It is shown that the ζ-potential as well as the mobilities of the ions behind the plane of shear are decreased by the polymer film.

    The data in the above studies were successfully interpreted under the assumption of local equilibrium between the (complete) electric double layer and the adjacent electrolyte. However, there are double-layer conditions where this assumption is violated. In order to study these, we theoretically investigated the influence of relaxation of the compact part of the double layer (occupied inner-Helmholtz Stern layer) on the LF dielectric response and electrophoretic mobility. Possible relaxation mechanisms are retarded adsorption/desorption and ion migration along the surface. Along the same lines, the stability of the sol against coagulation was expressed in terms of the relaxation characteristics of the Stem layer.

    Chapter 2 dealt with the determination of plug conductivities and streaming potentials of a close-packed porous plug of latex particles for a number of indifferent electrolytes and ionic strengths. From these, the dzeta.gif -potentials and surface conductivities were computed. Monodisperse sulphate latex is an ideal model system since the surface charge consists of strong acidic groups so that a constant surface charge density is maintained throughout all the experiments. It was shown that the surface conductivity is insensitive to the ionic strength and that a large part of the countercharge is situated behind the shear plane. Furthermore, it was demonstrated that the ions in the double layer have a mobility close to the bulk mobility.<

    In chapter 3 practical expressions were developed for the low-frequency (LF) dielectric response of dilute dispersions of spherical particles suspended in a binary electrolyte. The LF dielectric response of dilute sulphate latex dispersions was experimentally determined in the frequency range of 500 Hz to 500 kHz as a function of the ionic strength of suspending KCI. The resulting surface conductivities are insensitive to the ionic strength and practically identical to the values obtained by steady state methods (chapter 2). It was proposed that counterion motion can be retarded by specific interaction with the surface and by neutral polymer hairs present on the surface. In order to test the latter effect, the influence of the adsorption of uncharged polymer poly(ethylene) oxide onto the latex surface was investigated by means of LF spectroscopy, plug conductivities and streaming potentials of plugs in chapter 4. It was found that the polymer film on the surface reduces the surface conductivity. The drag on the ions in the polymer film can be described by considering the polymer layer as an inhomogeneous Brinkman fluid, characterised by a Darcy permeability which depends on the local polymer volume fraction. The polymer and counterion distributions were calculated from statistical self-consistent field lattice models.<

    In order to investigate the influence of the surface charge density on the streaming potential and static conductivity, plugs of monodisperse spherical Stöber-silica particles were studied in chapter 5. Contrary to the latex, the surface charge density of silica can be controlled by pH. The high-charge silica plug showed more surface conduction than the low-charge plug since more mobile counterions are present in the double layer of the former. Stöber-silica particles are highly porous. For the relatively large particles under consideration, the major part of the countercharge is situated in the micropores of the particles. It was shown that these counterions do not contribute to the plug conductivity because of their low mobility.

    Chapter 6 analysed the dynamic aspects of particle electrophoresis. It was shown theoretically as well as experimentally that colloidal particles respond to an applied electric field much faster than does the liquid inside a measuring capillary. Therefore, it is possible to apply an alternating electric field with such a frequency that unwanted electroosmosis, induced by charge on the capillary wall, is suppressed, whereas the particles are still able to follow the field according to their dc mobility. This study illustrates that knowledge of the dynamics and the corresponding relaxation times is not only of purely scientific interest, but that it also offers solutions to very practical problems.

    In chapter 7 the influence of polarization of surface charge (or charge in an inner-Helmholtz layer) on the particle mobility, static conductivity, and low-frequency dielectric response was studied within the framework of the thin double-layer theory. It was shown that the characteristic times of relaxation processes in the Stern layer are accessible from dielectric spectroscopy. The relaxation phenomena under consideration are Stern-layer polarization via retarded adsorption/desorption and polarization via lateral transport in the Stem layer. The two processes may occur simultaneously. Since these relaxation processes are also relevant for particle-particle interaction, chapter 8 considered the implications for colloidal stability. In the situation of small transient disequilibrations of the surface charge, the stability could be expressed in terms of the characteristic times of surface charge relaxation. This allows the use of electrodynamic data obtained by dielectric spectroscopy in the interpretation of colloidal stability. On an even more rigorous level, the free energy of particle-particle interaction was also considered in the space of the two variables surface charge and separation. This formalism opens the way to investigate coagulation far from equilibrium.

    Effect of oxygen status on pesticide transformation and sorption in undisturbed soil and lake sediment.
    Vink, J.P.M. ; Zee, S.E.A.T.M. van der - \ 1997
    Environmental Toxicology and Chemistry 16 (1997). - ISSN 0730-7268 - p. 608 - 616.
    adsorptie - sorptie - gewasbescherming - pesticiden - pesticidenresiduen - persistentie - microbiële afbraak - waterbodems - adsorption - sorption - plant protection - pesticides - pesticide residues - persistence - microbial degradation - water bottoms
    Charging and ion adsorption behaviour of different iron (hydr)oxides
    Venema, P. - \ 1997
    Agricultural University. Promotor(en): W.H. van Riemsdijk. - S.l. : Venema - ISBN 9789054856580 - 209
    bodemchemie - anorganische verbindingen - mineralen - adsorptie - sorptie - ijzer - soil chemistry - inorganic compounds - minerals - adsorption - sorption - iron

    Metal (hydr)oxides are of importance for many soil systems. All metal (hydr)oxides have a surface charge that varies with the pH. The variation in this surface charge is caused by adsorption and desorption of protons. The adsorption of cat- and anions on the metal (hydr)oxide surface is strongly influenced by their variable surface charge. The description of the charging behaviour of metal (hydr)oxides should therefore always be the starting point for modeling.

    For the modeling of the chemical adsorption behaviour of metal (hydr)oxide surfaces, many different models are available. Five of these models were compared in their ability to describe an extended data set of cadmium adsorption on goethite (iron oxyhydroxide). None of the models could describe all data simultaneously. The best results were obtained with a surface complexation model, based on the MUlti SIte Complexation MUSIC approach of Hiemstra and van Riemsdijk.

    A combination of a new interface model and the MUSIC model, the charge distribution (CD) MUSIC model is used for the description of cadmium adsorption on goethite. The CD- MUSIC mode I could give a good simultaneous description of an extended data set for cadmium adsorption on goethite. The same model, with adapted parameters, could describe extended adsorption data sets for adsorption on goethite in a system with only cadmium, phosphate and a mixture of these two ions.

    The MUSIC model predicts proton affinities for individual surface groups of metal- (hydr)oxides. A refinement of the MUSIC model shows that the prediction of the proton affinities of both dissolved and surface groups can be understood in one theoretical framework. The application of the refined MUSIC model to different iron (hydr)oxides shows that the model can predict the charging behaviour very well. For a good prediction of the charging behaviour, the crystal structure and morphology of the oxide must be well known.

    Finaly, a comparison is made of the charging behaviour and the cadmium and phosphate adsorption for three different goethites, lepidocrocite and hematite. The differences between the chemical behaviour of the different goethites could be modelled well with parameters which were consistent with the refined MUSIC model. The modelling of the behaviour of lepidocrocite and hematite was more problematic because the crystal morphologies were less well known.

    Phosphate and sulfate adsorption on goethite: single anion and competitive adsorption.
    Geelhoed, J.S. ; Hiemstra, T. ; Riemsdijk, W.H. van - \ 1997
    Geochimica et Cosmochimica Acta 61 (1997). - ISSN 0016-7037 - p. 2389 - 2396.
    absorptie - adsorptie - sorptie - fosfaten - fosforpentoxide - derivaten - waterbodems - absorption - adsorption - sorption - phosphates - phosphorus pentoxide - derivatives - water bottoms
    Early diagenesis of phosphorus in continental margin sediments
    Slomp, C.P. - \ 1997
    Agricultural University. Promotor(en): L. Lijklema; W. van Raaphorst. - S.l. : Slomp - ISBN 9789054856849 - 176
    fosfor - gesteenten - sedimentmaterialen - adsorptie - sorptie - noordzee - waterbodems - phosphorus - rocks - sedimentary materials - adsorption - sorption - north sea - water bottoms

    Most of the organic material in the oceans that reaches the sea floor is deposited on continental margins and not in the deep sea. This organic matter is the principal carrier of phosphorus (P) to sediments. A part of the organic material is buried definitely. The other part decomposes, resulting in a release of dissolved HP0 42-to the pore water. This HP0 42-either returns to the overlying water and becomes available for uptake by phytoplankton, or is retained in the sediment in an organic or inorganic form.

    Quantification of the P release from and P retention in sediments on relatively short time scales of days to years is necessary for a correct understanding of the nutrient dynamics in regional seas such as, for example, the North Sea. An accurate assessment of the modem global ocean burial flux of reactive P (i.e potentially bioavailable P) and the burial flux in the geological past is important for understanding the global oceanic P cycle. This, in turn, can provide insight in possible controls on organic C burial and atmospheric concentrations of C0 2 and 0 2 , because P may limit oceanic primary production and thus determine the amount of organic material in the oceans on geological time scales.

    The research presented in this thesis concentrates on the short-term processes controlling sediment P release and retention in temperate, non-upwelling, continental margin environments. The research commenced with a laboratory study on the effect of organic matter deposition and macrofauna on sediment-water exchange and retention of P in Fe oxide-poor, sandy sediments (Chapter 2). A suspension of dead algal cells (Phaeocystis sp.) was applied to sediment in experimental systems (boxcosms), either once or every week during 19 weeks. The results demonstrate that deposition of organic matter on this type of sediment enhances pore water concentrations and sediment-water exchange of HP0 42-. The enhanced HP0 42-release was due to microbially mediated mineralization of the organic material and due to direct release of HP0 42-from the algal cells (lysis). A major portion of the algal material remained at the sediment-water interface and this organic layer probably regulated the sediment-water exchange of HP0 42-directly. The activity of the macrofauna was mainly limited to reworking of the sediment. The effect of the macrofauna on the sediment-water exchange Of HP0 42-was negligible. In the boxcosms to which organic material was added only once, the concentration of NaOH-extractable sediment P increased following the addition, especially in the presence of macrofauna.

    Sorption of P to Fe oxides is the most important short-term process responsible for the retention of P in sediments. Using a combination of differential X-ray diffraction (DXRD) and extraction procedures, the character of the Fe oxides that bind P in 4 North Sea sediments was studied (Chapter 3). The results indicate that poorly crystalline ferrihydrite and akageneite were present in the fine sediment fraction (< 10 μm) of surface samples from all locations. Combination of these results with bulk sediment extractions of Fe and P and sorption characteristics for P provides evidence for the dominant role of poorly crystalline Fe oxides for the binding of P in these North Sea sediments. These poorly crystalline Fe oxides are suggested to act as both a temporary and permanent sink for P.

    The redox conditions in continental margin sediments can vary both seasonally and spatially. To obtain more insight in the redox conditions in North Sea sediments, the Mn and Fe cycle at 15 locations in 4 different sedimentary environments was studied in 2 contrasting seasons (Chapter 4). The quality and quantity of the organic matter deposited in each environment was found to determine whether sediments become sufficiently depleted of 0 2 and NO 3-to allow for (1) Fe and Mn reduction and (2) escape of dissolved Fe 2+and Mn 2+to the overlying water. A steady- state diagenetic model describing solid phase and pore water metal profiles was developed and applied to Mn and Fe data for 11 and 3 stations, respectively. The model results demonstrate that (1) reversible sorption in combination with sediment mixing can enhance diffusive transport of dissolved metals; (2) precipitation of Fe 2+and Mn 2+in the form of reduced authigenic minerals can explain the reversal of the pore water Fe 2+and Mn 2+gradients at depth at many stations, and (3) in most North Sea sediments, Fe and Mn oxides do not play an important role as redox intermediates in organic C oxidation (accounting for < 4 %); only in the depositional environment of the Skagerrak, metal oxide reduction may contribute substantially to organic C oxidation (-20%).

    Reversible sorptive reactions can both constrain and enhance the flux of HP0 42-from the sediment to the overlying water. The role of sorption in sediment-water exchange of HP0 42-in North Sea sediments was investigated for 15 locations in 2 seasons (Chapter 5). P sorption data, pore water HP0 42-profiles, solid phase results and measured and calculated rates of sediment- water exchange of HP0 42-were combined. Sorption was found to play an important role in controlling sediment-water exchange of HP0 42-during at least part of the year in 3 of the 4 North Sea environments. At most stations, adsorption limits the flux of HP0 42-to the overlying water. At one station in the Skagerrak, however, desorption is responsible for the maintenance of a flux of HP0 42-to the overlying water. A onedimensional reaction-diffusion model describing the sedimentary P cycle was developed and applied to the results for 2 stations, The model results show that both enhanced retention and enhanced release due to sorption can be adequately described when simultaneous equilibrium and first-order reversible sorptive reactions are assumed.

    P bound in authigenic minerals may not be solubilized again, whereas Fe-bound and organic P can still be released upon deep burial. Therefore, more insight in the extent of authigenic P mineral formation in continental margin sediments is important. A combination of pore water and solid phase analysis was used to determine whether authigenic carbonate fluorapatite (CFA) is currently forming at two locations on a North Atlantic continental platform (Chapter 6). Results of selective extractions suggest that an authigenic P phase is forming at the expense of Fe-bound P at both stations. A steady-state diagenetic model for the P cycle was developed and applied to the data of I station. The model results indicate that CFA formation can account for the observed increase of authigenic P with depth at this station. Furthermore, the results show that an intense cycling of P between Fe-bound P and pore water HP0 42-at the redox interface can create conditions beneficial for CFA formation. This internal P cycle is driven by downward, bioturbational transport of mainly in-situ formed Fe-bound P into the reduced sediment zone. Losses from the internal P cycle due to CFA formation and HP0 42+diffusion are compensated for by sorption of HP0 42-released from organic matter to Fe oxides. Fe bound P thus acts as an intermediate between organic P and CFA. Burial of CFA can account for between 25 and 70% of the total burial flux of reactive P and thus may act as an important sink for P in this low sedimentation, continental margin environment.

    Zacht, groen, nat & mooi: kolloidkunde in Wagenings perspectief.
    Cohen Stuart, M.A. - \ 1997
    Wageningen : Landbouwuniversiteit Wageningen - 19
    chemie - colloïden - adsorptie - oppervlakten - fysica - colleges (hoorcolleges) - oppervlaktechemie - macromoleculen - chemistry - colloids - adsorption - surfaces - physics - lectures - surface chemistry - macromolecules
    Immobilisatie van zware metalen en arseen in situ.
    Lexmond, Th.M. ; Vangronsveld, J. - \ 1996
    Bodem 6 (1996). - ISSN 0925-1650 - p. 142 - 145.
    bodem - zware metalen - bodemchemie - anorganische verbindingen - mineralen - arsenicum - absorptie - adsorptie - volksgezondheidsbevordering - decontaminatie - schoonmaken - soil - heavy metals - soil chemistry - inorganic compounds - minerals - arsenic - absorption - adsorption - sanitation - decontamination - cleaning
    Some physicochemical and environmental factors affecting transformation rates and sorption of the herbicide Metamitron in soil.
    Vink, J.P.M. ; Zee, S.E.A.T.M. van der - \ 1996
    Pesticide science : a journal of international research and technology on crop protection and pest control 46 (1996). - ISSN 0031-613X - p. 113 - 119.
    absorptie - adsorptie - gewasbescherming - pesticiden - pesticidenresiduen - persistentie - microbiële afbraak - bodemchemie - absorption - adsorption - plant protection - pesticides - pesticide residues - persistence - microbial degradation - soil chemistry
    Phosphorus fixation in lake sediments; the case of Lake Veluwe, The Netherlands.
    Danen-Louwerse, H.J. ; Portielje, R. ; Lijklema, L. - \ 1996
    Wageningen : Agricultural University - 115
    meren - reservoirs - plassen - adsorptie - sorptie - fosfor - bibliografieën - nederland - waterbodems - lakes - reservoirs - ponds - adsorption - sorption - phosphorus - bibliographies - netherlands - water bottoms
    Adsorption of polyelectrolytes and charged block copolymers on oxides consequences for colloidal stability
    Hoogeveen, N.G. - \ 1996
    Agricultural University. Promotor(en): G.J. Fleer; M.A. Cohen Stuart. - S.l. : Hoogeveen - ISBN 9789054854883 - 148
    elektrolyten - adsorptie - polymeren - colloïdale eigenschappen - stabiliteit - electrolytes - adsorption - polymers - colloidal properties - stability

    The aim of the study described in this thesis was to examine the adsorption properties of polyelectrolytes and charged block copolymers on oxides, and the effect of these polymers on the colloidal stability of oxidic dispersions. For this purpose the interaction of some well-characterised polyelectrolytes and block copolymers with oxidic substrates has been systematically studied. A set of block copolymers with one charged block and one neutral water-soluble block had to be synthesised because this type of block copolymers was not commercially available. These block copolymers were prepared by anionic polymerisation by Dr. Arnold's group in Halle (Germany).

    In order to measure the amount of polymer adsorbed as a function of several experimental parameters (pH, ionic strength, type of polymer, type of substrate) we used a reflectometer equipped with a stagnation-point flow-cell. With this optical technique the adsorbed amount on an (optically flat) solid substrate is measured. This technique is also suited to follow the kinetics of the adsorption process. Information about the amount of charge in the adsorbed layer was obtained from streaming potential measurements (on flat surfaces, Ch. 5) and electrophoresis (on particles, Ch. 6). The effect of polymer on the colloidal stability of oxidic dispersions was probed by measuring the changes in the optical transmission with time (Ch. 5).

    In Chapter 1 it is explained that in many applications there is a need to control the colloidal stability of oxidic dispersions, and it is described how in general the stability can be affected by polymer addition. Also, the aim, scope and outline of this study are given.

    In the following three chapters we present the adsorption properties of homopolyelectrolytes (Chs. 2 and 3) and of charged block copolymers (Ch. 4) on oxides. The homopolyelectrolytes used were two polymers with a constant charge (quaternised polyvinyl pyridine, PVP +, and quaternised polydimethylaminoethyl methacrylate, AMA +) and one with a pH-dependent charge (polydimethylaminoethyl methacrylate, AMA). The block copolymers consisted of a charged block (AMA) and a water-soluble neutral one (dihydroxypropyl methacrylate, HMA). As the substrates we used both silicon oxide (SiO 2 .), which is acidic in nature and the amphoteric titanium dioxide (TiO 2 ) .

    In Chapter 2 the rate of adsorption and the final adsorbed amount of the homopolyelectrolytes are studied as a function of pH and ionic strength. The initial adsorption rate is found to be equal to the rate with which the polymer molecules arrive to the surface; hence, the transport of molecules from the bulk of the solution to the surface is rate-limiting. Above a certain coverage the adsorption rate goes down, indicating that the already adsorbed molecules form a kinetic barrier for further adsorption.

    The adsorbed amount at saturation depends on pH and ionic strength. For both PVP +and AMA +a monotonic increase in the adsorbed amount is observed with increasing pH, since the surface gets more negatively charged. For AMA, which has a pH-dependent charge, a maximum in the adsorbed amount is found when the pH, and, thus the polymer charge, is varied.

    When the adsorbed charge of polyelectrolyte is compared to the bare surface charge, as determined from titration experiments, it is found that the net adsorbed charge exceeds by far the net bare surface charge (overcompensation). Part of the surplus of charge is neutralised by adjustment of the pH-dependent charges on the surface and on the polymer.

    In Chapter 3 the reversibility of the adsorption of PVP and AMA is studied by taking the system out of equilibrium, e.g., by a change of the pH or the free polymer concentration. Then, the subsequent relaxation is followed. In case of complete reversibility, the adsorbed amount of the relaxed system should be equal to that for direct adsorption on a bare surface under these conditions. Also, the reversibility was investigated by examining the exchange of adsorbed polyelectrolyte molecules by molecules from the solution.

    The experiments indicate that the adsorbed layer was never fully relaxed. Therefore, we conclude that the experimental systems are only partly reversible on the timescale of the experiments (30 min.). Presumably, the reorganisation of molecules in the adsorbed layer is rather slow, because of the strong (electrostatic) bond between polyelectrolyte and surface. A model for the structure of the adsorbed layer of strongly charged polyelectrolytes, allowing little reconformation, is proposed. In this model the molecules adsorb as isolated chains on the surface. These islands repel mutually, thereby forming a heterogeneous layer. An indication for the existence of empty spaces between the molecules comes from the observation that after adsorption of the polyelectrolyte neutral polymer molecules can attach to the surface.

    In Chapter 4 the synthesis and characterisation of the AMA-HMA block copolymers are described. Next, the adsorption properties of the block copolymers are studied as a function of pH, ionic strength and block length ratio. A maximum in the adsorbed amount is observed when the composition of the block copolymer is varied, similar to the maximum found for homopolyelectrolytes upon variation of the segment charge (see Ch. 2). Also, the adsorbed amount follows the same trends with pH and ionic strength as does the homopolyelectrolyte AMA; almost no effect of the presence of the neutral block on the adsorption data can be detected. From these facts we Infer that the charged block in the block copolymer anchors on the surface. Probably, the neutral block can also adsorb, since charged molecules usually do not cover the surface completely, because they repel each other (see Ch. 3). Only at high adsorbed amounts the surface becomes so densely populated that the neutral block is forced into the solution, thereby forming an extended layer which may provide steric stabilisation.

    In Chapter 5 we discuss the effect of addition of charged (block co)polymers on the stability of oxidic dispersions. We note that the same type of polymer can both stabilise and destabilise a dispersion. For example, a low dosage of strongly charged (say, cationic) polymer molecules to a dispersion of oppositely charged (anionic) particles may cause mosaic flocculation, whereby bare (negatively charged) and covered (positively charged) parts on neighbouring particles attract each other. When the polymer is very long bridging flocculation could also occur whereby one polymer molecule adsorbs on two particles at the same time. When, however, the dosage Is high enough to saturate the particles, the dispersion may be stabilised sterically or electrostatically, depending on the thickness of the steric layer and on the amount of charge on the particles.

    As illustrated above, the effect of the polymers depends critically on the polymer charge, on the dosage and on the molar mass of the polymer. In Ch. 5 a set of requirements could be formulated to optimise the performance of the polymers for stabilisation or flocculation.

    Finally, in Chapter 6 we describe the formation and stability of multilayers of polyelectrolytes. Since charge reversal occurs upon adsorption of a strongly charged (say, cationic) polyelectrolyte (see Ch. 2), an oppositely charged (anionic) polymer molecule is attracted to such a covered surface. Therefore, when cationic and anionic polymers are supplied in alternating order to a solid substrate, multilayers can be formed.

    The multilayer build-up is characterised by a step-wise Increase of the adsorbed amount and the layer thickness, and by alternatingly highly positive and highly negative values for the ζ-potential. The stability of the multilayer is shown to depend strongly on the polymer charges and the ionic strength and, hence, on the electrostatic interaction between the polymers involved. When this interaction Is weak no stable multilayers form, but polycations and polyanions form complexes at the surface which then may desorb. For pairs of strongly interacting polymers, which formed very stable multilayers, the charge stoichiometry could be studied. This charge stoichiometry, which was not always 1 : 1, was found to be independent of the substrate, the pH or the ionic strength, but rather sensitive to the monomer structure.

    Beschrijving en validatie van de procesformulering van de abiotische fosfaatreacties in kalkloze zandgronden
    Schoumans, O.F. - \ 1995
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 381) - 52
    bodem - fosfor - absorptie - adsorptie - zandgronden - soil - phosphorus - absorption - adsorption - sandy soils
    Wil men het verloop van de fosfaatuitspoeling naar het grond- en oppervlaktewater kunnen voorspellen, dan moeten de fosfaatreactiemechanismen in de bodem nauwkeurig worden beschreven. Dit rapport geeft een overzicht van de huidige inzichten in het reactiemechanisme van fosfaat in de bodem, met de bijbehorende procesformuleringen. Uit gepubliceerde laboratoriumexperimenten zijn de procesparameters afgeleid, waarna met drie kolomexperimenten de procesformulering is gevalideerd. De resultaten geven aan dat de fosfaatdoorbraak (tijdens sorptie en desorptie) goed gemodelleerd wordt. Op grond van deze goede voorspelling wordt verwacht dat het model een goede indruk geeft van het fosfaattransport door de bodem naar grond- en oppervlaktewater.
    Bioavailability and effects of non-ionic organic pesticides in soil
    Ronday, R. - \ 1995
    Wageningen : SC-DLO (Report / DLO-Winand Staring Centre 110) - 43
    absorptie - adsorptie - bodemchemie - biologische beschikbaarheid - pesticiden - ecotoxicologie - bioaccumulatie - absorption - adsorption - soil chemistry - bioavailability - pesticides - ecotoxicology - bioaccumulation
    In soil contamination studies the extent of contamination is usually described in terms of the content of the chemical on a dry soil mass basis. However, it has been found that a particular content of a chemical in soil exhibits divergent bio-availability, and thus toxicity, in different soils. Measuring the chemical concentration in the soil pore water is a better method. For evaluating the biological risk of pesticides in soil, concentrations measured in pore water should be compared with soil quality criteria based on the saline-water effect concentrations for soil organisms derived in the laboratory.
    Vermindering fosfaatuitspoeling; mogelijkheden bij fosfaatverzadigde gronden
    Schoumans, O.F. ; Kruijne, R. ; Molen, D.T. van der - \ 1995
    Landschap : tijdschrift voor landschapsecologie en milieukunde 12 (1995)6. - ISSN 0169-6300 - p. 63 - 73.
    bodem - fosfor - fosfaten - fosforpentoxide - derivaten - uitspoelen - absorptie - adsorptie - waterkwaliteit - waterbeheer - veluwe - gelderland - soil - phosphorus - phosphates - phosphorus pentoxide - derivatives - leaching - absorption - adsorption - water quality - water management - veluwe - gelderland
    Om de fosfaatuitspoeling uit landbouwgronden naar het oppervlaktewater te verminderen is de effectiviteit onderzocht van drie maatregelen in aanvulling op het huidige bemestingsbeleid, namelijk: achterwege blijven van fosfaatbemesting (bemestingsmaatregel), toediening van fosfaatbindende bestanddelen aan de bodem (bodemchemische maatregel), en verandering van de ondiepe grondwaterstroming naar het oppervlaktewater (hydrologische maatregel). Het blijkt dat vooral de laatste twee maatregelen de fosfaatbelasting van het oppervlaktewater verminderen. Elke maatregel heeft specifieke voor- en nadelen. De bodemchemische maatregel is het duurst; de bemestings- en bodemchemische maatregel zijn het flexibelst toepasbaar.
    Voorspelling van de fosfaatuitspoeling naar het oppervlaktewater in het stroomgebied van de Schuitenbeek
    Schoumans, O.F. ; Kruijne, R. - \ 1995
    Wageningen : DLO-SC - 79
    bodem - fosfor - absorptie - adsorptie - fosfaten - fosforpentoxide - derivaten - uitspoelen - rivieren - waterlopen - kanalen - water - oppervlaktewater - waterverontreiniging - waterkwaliteit - informatie - informatieverspreiding - nederland - veluwe - gelderland - soil - phosphorus - absorption - adsorption - phosphates - phosphorus pentoxide - derivatives - leaching - rivers - streams - canals - water - surface water - water pollution - water quality - information - diffusion of information - netherlands - veluwe - gelderland
    De bruikbaarheid van de fosfaatprocesformulering om op regionale schaal de fosfaatbelasting van het oppervlaktewater te voorspellen is onderzocht door een validatie uit te voeren voor het stroomgebied van de Schuitenbeek. De gemeten fosfaatvrachten van de beek zijn vergeleken met de fosfaatbelasting van het oppervlaktewater berekend met het model ANIMO. Het blijkt dat met de huidige fosfaatprocesformulering en bijbehorende procesparameters de fosfaatbelasting van het oppervlaktewater goed wordt gemodelleerd. Daarom wordt verwacht dat de effecten van bemestingsscenario's op de verandering in de fosfaatbelasting van het oppervlaktewater goed gemodelleerd kunnen worden.
    Critical loads and excess loads of cadmium, copper and lead for European forest soils
    Reinds, G.J. ; Bril, J. ; Vries, W. de; Groenenberg, J.E. ; Breeuwsma, A. - \ 1995
    Wageningen [etc.] : SC-DLO [etc.] - 91
    bodem - zware metalen - absorptie - adsorptie - bosbouw - bodemchemie - anorganische verbindingen - europa - soil - heavy metals - absorption - adsorption - forestry - soil chemistry - inorganic compounds - europe
    Recently, concern has arisen about the impact of the dispersion of heavy metals in Europe. Therefore, a study (ESQUAD) was initiated to assess critical loads and steady-state concentrations of cadmium, copper and lead for European forest soils. The calculation methods used strongly resemble those used to compute critical loads for acidifying compounds. Results show that the computed critical loads and associated excess loads for forest soils strongly depend on the threshold values chosen and on the soil phase (adsorbed/dissolved) considered. Further research is needed to improve input data and the modelling of heavy metal adsorption.
    Emissies van bestrijdingsmiddelen en nutriënten in de bloembollenteelt; adsorptie en omzetting van bestrijdingsmiddelen in bloembollengronden
    Matser, A.M. ; Pas, L.J.T. van der; Boesten, J.J.T.I. ; Leistra, M. - \ 1995
    Wageningen : SC-DLO (Rapport / DLO-Staring Centrum 387.2) - 47
    bodem - uitspoelen - gewasbescherming - pesticiden - bloembollen - adsorptie - modellen - waterlopen - oppervlaktewater - waterverontreiniging - waterkwaliteit - kalkrijke gronden - zandgronden - zuid-holland - soil - leaching - plant protection - pesticides - ornamental bulbs - adsorption - models - streams - surface water - water pollution - water quality - calcareous soils - sandy soils - zuid-holland
    Voor uitspoelingsberekeningen zijn in het laboratorium de adsorptie en omzetting bestudeerd van drie bestrijdingsmiddelen in twee zandige bloembollengronden. Metamitron werd in bouwvoormateriaal matig geadsorbeerd en had korte halfwaardetijden (drie dagen) door microbiële adaptatie. In met water verzadigd ondergrondmateriaal was de halfwaardetijd van metamitron veertig dagen. Het omzettingsprodukt HTI van chloorthalonil werd sterk geadsorbeerd en de omzetting verliep zeer traag. Het omzettingsprodukt3-chloorallylalcohol van 1,3-dichloorpropeen werd nauwelijks geadsorbeerd in ondergrondmateriaal en de halveringstijd was enkele dagen. De gemeten adsorptie en omzettingssnelheid wijken af van de verwachtingen gebaseerd op literatuurgegevens.
    Correction of experimental sorption coefficients using DOC measurements and apparent solubility enhancements.
    Koelmans, A.A. - \ 1995
    Environmental Toxicology and Chemistry 14 (1995). - ISSN 0730-7268 - p. 2015 - 2016.
    adsorptie - sorptie - ddt - organische chloorverbindingen - gechloreerde koolwaterstoffen - waterbodems - ecotoxicologie - adsorption - sorption - ddt - organochlorine compounds - chlorinated hydrocarbons - water bottoms - ecotoxicology
    Modelling phosphate sorption kinetics in acid soils.
    Freese, D. ; Riemsdijk, W.H. van; Zee, S.E.A.T.M. van der - \ 1995
    European Journal of Soil Science 46 (1995). - ISSN 1351-0754 - p. 239 - 245.
    bodem - fosfor - absorptie - adsorptie - zure gronden - kattekleigronden - modellen - onderzoek - soil - phosphorus - absorption - adsorption - acid soils - acid sulfate soils - models - research
    Reversibility and mechanism of bacterial adhesion.
    Rijnaarts, H.H.M. ; Norde, W. ; Bouwer, E.J. ; Lyklema, J. ; Zehnder, A.J.B. - \ 1995
    Colloids and Surfaces. B: Biointerfaces 4 (1995). - ISSN 0927-7765 - p. 5 - 22.
    corynebacteriaceae - pseudomonas - microbiële afbraak - chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - corynebacteriaceae - pseudomonas - microbial degradation - chemistry - colloids - adsorption - surfaces - surface chemistry
    The isoelectric point of bacteria as an indicator for the presence of cell surface polymers that inhibit adhesion.
    Rijnaarts, H.H.M. ; Norde, W. ; Lyklema, J. ; Zehnder, A.J.B. - \ 1995
    Colloids and Surfaces. B: Biointerfaces 4 (1995). - ISSN 0927-7765 - p. 191 - 197.
    micro-organismen - biochemie - fysiologie - microbiële fysiologie - microbiële afbraak - chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - microorganisms - biochemistry - physiology - microbial physiology - microbial degradation - chemistry - colloids - adsorption - surfaces - surface chemistry
    Reduction of artificial iron(III) phosphate complexes by hydroxylammonium chloride.
    Best, J. de; Danen-Louwerse, H.J. ; Portielje, R. ; Lijklema, L. - \ 1995
    Water Research 29 (1995). - ISSN 0043-1354 - p. 1885 - 1894.
    fosfaten - fosforpentoxide - derivaten - ijzer - absorptie - adsorptie - waterbodems - phosphates - phosphorus pentoxide - derivatives - iron - absorption - adsorption - water bottoms
    Speciation of heavy metal ions as influenced by interactions with montmorillonite, Al hydroxide polymers and citrate
    Janssen, R.P.T. - \ 1995
    Agricultural University. Promotor(en): W.H. van Riemsdijk; M.G.M. Bruggenwert. - S.l. : Janssen - ISBN 9789054853992 - 127
    bodem - zware metalen - absorptie - adsorptie - kleimineralen - chemische speciatie - metaalionen - montmorilloniet - aluminiumhydroxide - polymeren - citroenzuur - soil - heavy metals - absorption - adsorption - clay minerals - chemical speciation - metal ions - montmorillonite - aluminium hydroxide - polymers - citric acid

    Clay minerals, metal-hydroxides and organic matter can bind metal ions; moreover they also interact with each other. These mutual interactions influence the metal binding to a significant extent. In this study, the speciation of the heavy metal ions Zn and Ph was investigated in model systems consisting of various combinations of the clay mineral montmorillonite (Na saturated), Al hydroxide polymers and citric acid at pH 5.0, 6.0 and 6.6. A speciation model was developed to determine the distribution of Zn and Pb in these systems. pH had a pronounced effect on Zn and Pb binding. Zn and Pb binding to the clay/Al hydroxide polymer systems decreased with decreasing pH. The decrease of the pH led to a decrease of the cation exchange capacity and to a decrease of the affinity of the Al hydroxide polymers for metal ions. The decrease in metal ion binding was also influenced by increasing competition of Al 3+. The effect of the Al:clay ratio on metal binding was influenced by the pH. This is because the nature of the Al hydroxide is dependent on the Al:clay ratio at pH 5.0 and independent at pH 6.6. The available binding sites on the Al hydroxide polymers have a high affinity for Zn and Pb ions. Zn and Pb binding on the siloxane surface became relatively more important when the binding sites on the Al hydroxide polymers were nearly all occupied. Citrate sorption to clay/Al hydroxide polymer systems was strongly dependent on the conditions of formation of the systems. Citrate adsorbed to the edge faces of the clay platelets of aged clay/Al hydroxide systems. When citrate is present during the formation of clay/Al hydroxide systems, it was found that citrate was incorporated in the Al hydroxide polymers. At increased citrate:Al ratios, citric acid can perturb the formation or dissolve the Al hydroxide polymers. The distribution of Zn over the various species in a clay/Al hydroxide polymer/citrate system was calculated by using the speciation model. Only slightly less Zn was sorbed to the Al hydroxide polymers compared to the same systems without citrate. For high molar fractions of Zn bound to the exchange sites, binding of Zn to the Al hydroxide increased strongly in the presence of citrate, suggesting the formation of a coprecipitate or solid solution between Al- and Zn hydroxide and citrate. At increasing citrate:Al ratios, Zn sorption decreased strongly, due to Zn and Al complexation by citrate.

    Toepassingsmogelijkheden van TRISOPLAST voor de afdichting van afval- en reststofbergingen
    Weitz, A.M. ; Boels, D. ; Wiegers, H.J.J. - \ 1994
    Wageningen : DLO-Staring Centrum (Rapport / DLO - Staring Centrum 300) - 59
    bodembescherming - verzegelen - bodem - uitspoelen - afvalverwijdering - huisvuilverwijdering - afvalstortplaatsen - stortterreinen - vuilnisbelten - adsorptie - sorptie - Nederland - bekledingen - soil conservation - sealing - soil - leaching - waste disposal - municipal refuse disposal - waste disposal sites - landfills - refuse tips - adsorption - sorption - Netherlands - linings
    TRISOPLAST is de handelsnaam van een mengsel van bentoniet, zand en een polymeer. Het polymeer gaat hierbij een binding aan met bentoniet.
    Adsorption of charged diblock copolymers : effect on colloidal stability
    Israels, R. - \ 1994
    Agricultural University. Promotor(en): G.J. Fleer, co-promotor(en): F.A.M. Leermakers. - S.l. : Israels - ISBN 9789054852315 - 100
    adsorptie - sorptie - kunststoffen - industrie - colloïden - dispersie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - colloids - dispersion - macromolecular materials

    In this thesis we present Scheutjens-Fleer (SF) calculations on the adsorption of diblock copolymers. More specifically, we restrict ourselves to adsorption at uncharged surfaces, while the specific type of block copolymers we consider have one uncharged adsorbing "anchor" block and one non-adsorbing charged "buoy" block. We compare these systems with a more simple one, that of the charged brushes. A polymer brush is the structure that is formed when polymer molecules are attached by one end to a surface, with a density high enough so that the chains are obliged to stretch away from the interface. Complementary to the numerical computations, the scaling behaviour of these systems is discussed. We study the structure of the adsorbed layer, and try to answer ultimately the question what the effect of the adsorption is on colloidal stability.

    In the introductory Chapter 1 we explain the most important terms and discuss the relevance of this study. Furthermore, we introduce the SF model and compare it to two other approaches: Monte Carlo and Scaling. Finally, we briefly present the available information on the two systems under consideration, and compare them to a number of related systems.

    The body of this work is divided in two parts. In Chapters 2 and 3 we discuss charged brushes, systems that are simpler than diblock copolymer adsorption, but still exhibit similar characteristics. In the subsequent two chapters we then proceed to the adsorption of diblock copolymers (Chapter 4) and its effect on colloidal stability (Chapter 5).

    In Chapter 2 we present numerical results from the SF model for the structure and sealing behaviour of charged brushes and compare these with predictions of an analytical model on the same system. The relevant parameters are the chain length N , the average anchoring density σ, the average segmental charge αon the chains, and the salt concentration φ S .

    At high anchoring densities, three regimes of brush behaviour may be distinguished. In the salt-free case, the behaviour of the brush is dominated by electrostatic interactions if the charges are high (the so-called Osmotic Brush) or by non-electrostatic excluded volume interactions if the charges are low (the quasi-Neutral Brush regime). Upon adding salt a third regime can be found: the Salted Brush. The behaviour in this regime, although resulting from electrostatic interactions, is very similar to that in a neutral brush and can effectively be described using an electrostatic excluded volume parameter vel ≈ φ S-1α2. We find excellent agreement regarding structure as well as scaling relations between the two theories in these three (high anchoring density ) regimes. At extremely low anchoring densities, the agreement with the analytical theory is less good. This is due to the breakdown at low densities of the mean-field approximationpresently used in the numerical model.

    In between, at intermediate anchoring densities, the analytical theory predicts a very peculiar regime, where the thickness H scales as H ≈N-1α2. This so-called " Pincus Brush ", named after the author who originally described it, is not recovered with the numerical theory. For the wide range of parameters used, we find the Pincus regime is too small to be detected. This is probably true for any reasonable set of parameters.

    In Chapter 3 we consider the acid-base equilibrium of the charged brush segments, so that grafted weak polyacids may be studied. For these systems the charge of a brush segment depends on its local environment and on the pH in the solution. The scaling dependence of the thickness H on the salt concentration φ S for such a brush is very different from that for a conventional charged brush with constant charge density.

    In Chapter 4 we proceed to the adsorption of ionic diblock copolymers. One block, the "anchor", consists of N A uncharged adsorbing A segments, whereas the "buoy" block has N B segments which carry a fixed charge and are non-adsorbing. Upon adsorption these sorbed amount and layer thickness as a function of the block lengths N A and N B , the charge αe on the B segments, and the salt concentration φ S in each of the four regimes. The scaling relations axe checked using SF calculations.

    The existence of two regimes for uncharged diblock copolymer adsorption has been reported previously. We argue that those HU and LU regimes are closely related to the two regimes HC and LC we find for charged molecules. Scaling relations can be translated from the uncharged to the corresponding charged regimes by replacing the excluded volume parameter v of the buoy segments by an effective electrostatic excluded volume parameter ve = α 2S .

    In the LC regime the chain density σscales as σ α( N A /N B ) 3/2ve-1and the layer thickness H as H α ( N A /N B ) 1/2. The latter scaling is independent of ve . Using the SF model, these relations axe found to be valid for an adsorbed amount of A segments below 10% of monolayer coverage.

    In the HC regime the adsorption is dominated by the anchoring block and the scaling relation σ α1/ N A for the chain density is identical to that for uncharged molecules. The SF calculations show that this regime will not be reached in practical situations.

    Finally, we address in Chapter 5 the effect of the adsorption of charged diblock copolymers on colloidal stability. Using again a scaling as well as the SF approach, we focus on the LC regime and find that the adsorbed layer may cause a significant repulsive interaction between two surfaces, despite the very low adsorbed amounts. The magnitude of this repulsion is well within the range that could be mea, sured using a surface force apparatus. Moreover, we estimate that the repulsive interaction may be strong enough to induce kinetic stability, provided the particle radius is large enough. Upon lowering the salt concentration, however, a critical concentration φ S * is reached eventually, below which the repulsion is no longer strong enough to effect colloidal stability. The scaling analysis predicts that this critical concentration scales as:
    φ S * ≈ N 2/ RN A3

    where R is the radius of the particles and the other parameters have been defined above. Thus the repulsive interaction decreases when the relative importance of charge effects increases, i.e., with decreasing salt concentration, and increasing buoy block length or buoy block charge. This counterintuitive behaviour can be explained from the effect that electrostatic interactions have on the adsorbed amount: stronger interactions lead to a lower adsorbed amount, which, in turn, leads to a weaker repulsion. The SF calculations confirm these scaling predictions.

    TIRF and its application to protein adsorption : electrostatics and orientation
    Bos, M.A. - \ 1994
    Agricultural University. Promotor(en): J. Lyklema; J.M. Kleijn. - S.l. : Bos - ISBN 9789054852551 - 151
    adsorptie - sorptie - eiwitten - effecten - vaste stoffen - elektrostatica - adsorption - sorption - proteins - effects - solids - electrostatics

    The aim of the study in this thesis was to develop a method for determining the orientation of adsorbed protein molecules and to study the influence of the electrical potential of the interface on the interfacial properties of proteins, including their orientation.

    In the adsorption of proteins on solid surfaces many factors play a role. The most important are electrostatic and hydrophobic interactions between the protein molecules and the sorbent surface, and structural rearrangements in the protein molecules. From earlier studies it was concluded that proteins with regard to their adsorption behaviour can be roughly divided into "hard" and "soft" proteins. For the "hard" proteins structural changes upon adsorption are negligible and these proteins do not adsorb on hydrophilic surfaces unless there is electrostatic attraction. "Soft" proteins have a tendency to unfold partially upon adsorption and they adsorb on all kind of surfaces, irrespective of any electrostatic repulsion, partly caused by a gain of conformational entropy during adsorption.

    The orientation of adsorbed protein molecules plays an important role in the effectivity and the development of immunoassays and diagnostic tests. One can imagine that if the orientation of an antibody (or enzyme) is not the right one, no recognition of the antigen (or substrate) occurs. Therefore, either much research is done to develop methods to adsorb antibodies and enzymes in the proper orientation or in ways to steer the adsorption process. In the development of biosensors knowledge of and insight into the adsorption process and the orientation of the adsorbed proteins molecules (used as the selector molecules) on inorganic materials also play essential roles.

    In this study two optical techniques have been used: Total Internal Reflection Fluorescence (TIRF) and reflectometry. With both techniques it is possible to monitor quantitatively and qualitatively the adsorption process of proteins in situ.

    TIRF has been used over the past 20 years for the measurement of protein adsorption kinetics and adsorbed amounts, and to study the exchange of proteins between sorbent surface and solution. A relatively new research topic for which TIRF is used is to obtain information on the orientation of adsorbed (protein) molecules. The principle of TIRF is as follows: A light beam is totally reflected at an interface between two media I and 2 with the refractive index of medium I higher than that of medium 2, and the angle of incidence exceeding its critical angle value. Due to interference between the incident and the reflected light beam an evanescent wave penetrates into medium 2. The amplitude of this electromagnetic wave decays exponentially with distance normal to the interface. The penetration depth of this wave depends on the wavelength of the light used, and the refractive indices of the media. For visible light striking a quartz/water interface it is in the order of 100 nm. If medium 2 consists of a solution with fluorescent molecules, the evanescent wave will excite the molecules that are close enough to the interface; the emitted fluorescence is detected. In the case of adsorption and not too high bulk concentrations, the fluorescence signal is almost completely stemming from molecules in the adsorption layer.

    With the optical technique reflectometry the adsorption of molecules on an (optically flat) solid surface can be monitored. A linearly polarized light beam is reflected from the adsorbing surface, and the reflected beam is split into its parallelly and perpendicularly polarized components. The intensity ratio between these two components is measured continuously. This ratio changes upon adsorption, and after calibration the adsorbed amount (mass/area) is obtained.

    In this work TIRF has been used for the determination of the orientation distribution of adsorbed molecules. Therefore, the existing theory had to be extended (chapter 2). Cytochrome c has been chosen as a model protein to test and illustrate the method, because of its welldocumented crystallographic structure and its well-characterized physicalchemical properties. Furthermore, cytochrome c has a chromophoric group which can be made fluorescent by removing the Fe-atom. The protein without the Fe-atom is called porphyrin cytochrome c. From various literature data it is inferred that the native cytochrome c molecule is rather structure-stable. Another interesting feature of the protein is its relatively large electric dipole moment (325 Debye at pH 7), which might offer a possibility to influence the orientation in the adsorbed state by variation of the surface charge.

    The method for determination of the orientation of adsorbed molecules is based on the principle that by changing the polarization of the incident light beam the direction of the electric field component of the evanescent wave is modified. As a result the interaction between the transition dipole moment of the adsorbed molecules and the evanescent wave alters, which in turn, gives rise to a change in the fluorescence intensity. To obtain order parameters from which the orientation distribution can be reconstructed, one has to measure not only the intensity but also the polarization of the fluorescence as a function of the polarization of the incident light beam. The theory has been elaborated especially for orientation measurements on porphyrins and cytochrome c. In the porphyrin ring two transition dipole moments are lying perpendicularly to one another. For this system it is possible to study the orientation distribution in one orientation angle from the restricted information: the angle 0 between the plane of the porphyrin ring and the interface. The orientation distribution in θcan be reconstructed using the Maximum Entropy Method. With regard to the mobility of the adsorbed molecules, which might interfere with the orientation measurements, it is shown that rotational mobility much faster than the fluorescence lifetime would result in the disappearance of the fluorescence polarization.

    Firstly, some experiments with a simple porphyrin (tetramethyl-pyridiniurn porphyrin, H 2 TMPyP) have been performed (chapter 4). Prior to the orientation measurements, its adsorption behaviour was studied by reflectometry. For adsorption on silica from pure water, from 0.01 M phosphate buffer pH 7 and from 0.1 M KNO 3 solution different adsorbed amounts have been found. From the TIRF orientation measurements satisfying results were obtained, although the reproducibility leaves still something to desire. The orientation distribution of adsorbed H 2 TMPyP molecules on silica depends on the concentration of porphyrin in the solution from which adsorption takes place. At low concentration, the H 2 TMPyP molecules are more or less randomly oriented, while at high concentrations a broad distribution around an angle of 46° between the porphyrin plane and surface was observed. The fact that the fluorescence is polarized and the results of measurements with different solution viscosities show that the mobility of the adsorbed porphyrin molecules is on a much larger time scale than the fluorescence life time (5 ns).

    To study the influence of the electrical potential (charge) of the sorbent surface on the adsorption behaviour of proteins, a semi-conducting indium tin oxide (ITO) surface was used. This material was deposited in a thin layer (120- 140 run) on glass or silicium plates. The ITO surfaces have been characterized by streaming potential measurements, scanning electron microscopy, atomic force microscopy and resistance measurements. The results have been described in chapter 3.

    The adsorption behaviour of various proteins (serum albumin, lysozyme, ribonuclease A, superoxide dismutase, myoglobin and α-lactalbumin) as a function of an externally imposed interfacial potential has been studied using reflectometry (chapter 5). The sorbent surface was again a semi-conducting ITO layer deposited on a silicium wafer. The results obtained at the equilibrium potential as a function of pH suggest that electrostatic interactions play a decisive role in the adsorption of structurestable proteins on hydrophilic surfaces. On the other hand, protein adsorption is found to be hardly affected by externally imposed interfacial potentials, irrespective of the structure-stability of the protein. The cause for the apparent contradiction in these results must be that in both experimental approaches, but in different ways, together with the electrostatic interactions other properties of the system are also varied. (For example, on changing the pH, the net charge of the protein molecules changes, but also their structure-stability; together with increasing surface potentials, the surface becomes more hydrophilic; and, in the case a constant potential is externally applied, the adsorbing protein molecules may largely adapt their properties.) Therefore it is difficult to assess the importance of the contribution of electrostatic interactions in the process of protein adsorption. Presumably, in the past protein adsorption as a function of the pH has been interpreted in a too simplified manner, overestimating the role of electrostatic interactions.

    In chapter 6 the adsorption behaviour of native and porphyrin cytochrome c was the subject of study. Special attention is given to the adsorbed amounts, the adsorption kinetics and the influence of externally applied potentials for both forms of cytochrome c and the orientation of adsorbed porphyrin cytochrome c molecules. It was shown that the adsorption behaviour of native cytochrome c resembles that of structurestable proteins such as lysozyme and ribonuclease. In many aspects porphyrin cytochrome c behaves the same as the native form. However the adsorbed amounts at pH 7 and 10 are much higher. The adsorbed amounts and adsorption kinetics of both forms of cytochrome c are found to be hardly affected by externally imposed potentials. With regard to the orientation measurements it was not possible to interpret the data in terms of orientation distribution functions because of the scatter in the results.

    This spread is mainly caused by the low signal to noise ratio. Improvement of this ratio is difficult because of photo-deterioration of the adsorbed protein molecules. However, the total fluorescence as a function of the polarization angle of the incident light beam points to orientation distributions which do not depend on the surface coverage and cannot be influenced by imposing an electrical potential on the sorbent surface.

    In summary, in this work it is shown that TIRF is a suitable technique to determine orientation distributions of adsorbed fluorescent molecules. Further-more, it was found that it is not (or hardly) possible to influence the adsorption behaviour of proteins, irrespective of their structural stability, by externally imposing an electrical potential to the sorbent surface.

    Perspectives

    In this work we have developed a method to obtain the orientation distributions of adsorbed chromophores by making use of the optical technique TIRF. The results obtained with a simple porphyrin show that the method works. So far, application of this method to adsorbed proteins is limited, since the protein should be structure-stable upon adsorption and carry a fluorescent group. It was not possible to reconstruct the orientation distribution of cytochrome c molecules because of the scatter in the order parameters obtained. In order to obtain the orientation distribution it is necessary to improve the signal to noise ratio of the fluorescence measurements. This could be done by measuring the fluorescence over a longer time. In the case of cytochrome c this fails because the adsorbed molecule appears to undergo structural rearrangements in the presence of light.

    For other structure-stable proteins, it might be possible to determine the orientation distribution in the adsorbed state. If the molecules do not have a fluorescent group, one might consider to label them; prerequisite is that the fluorescent label is fixed in the molecule with a known orientation to the rest of the molecule. However, a caveat is that introducing a fluorescent label might lead to structural changes within the molecule and hence influence the adsorption behaviour of the protein. Another possibility to obtain orientation distributions of adsorbed proteins with TIRF is to make use of the fluorescence of the aromatic amino acids tryptophan and tyrosine. These amino acids have a fixed place in the structure and their excitation wavelength is in the UV. This sets higher demands to the optical parts in the experimental set-up. Furthermore, the presence of more than one of these amino acids can cause energy transfer from one amino acid to the other. As a result it is not known where the fluorescence is stemming from and the information about the orientation is lost. Another disadvantage is that protein molecules might be damaged by the UV light.

    Meanwhile, the TIRF method for determining the orientation of adsorbed chromophores is already used in a study concerning the development of "organic" solar energy cells, conducted in the Department of Molecular Physics of the Wageningen Agricultural University. In this study porphyrin molecules are used as sensitizers to generate charge carriers in a semiconducting surface. The adsorption of porphyrins on this surface, especially their orientation, is a prominent factor determining the efficiency of the system. The more parallel the molecules lie on the surface, the higher the energy transfer is. The results obtained here with H 2 TMPyP and presented in chapter 4 were promising enough to use the method for studying the orientation of several derivatives of H 2 TMPyP.

    In our own department the method is now also used to investigate the order in Langmuir-Blodgett (LB) layers of phospholipids, which stand model for biological membranes. In the near future the structure and permeability of such phospholipid layers will be studied as a function of the electrical potential of the substrate. To that end, the LB layers will be deposited onto optically transparent conducting ITO films on quartz slides. With different fluorescent probes it is hoped that information on the rotational mobility and/or reorientation is obtained.

    Integration of TIRF with time-resolved fluorescence measurements can provide more detailed information on the structure and orientation of adsorbed molecules and on dynamic processes taking place on the time scale of fluorescence, e.g. rotation of the whole molecule or parts of it.
    In conclusion, the potentials of TIRF to study orientations are not exhausted.

    Copolymers at the solid - liquid interface
    Wijmans, C.M. - \ 1994
    Agricultural University. Promotor(en): G.J. Fleer; F.A.M. Leermakers. - S.l. : Wijmans - ISBN 9789054852100 - 184
    adsorptie - sorptie - oppervlakten - grensvlak - vloeistoffen (liquids) - kunststoffen - industrie - vaste stoffen - oppervlakteverschijnselen - tweefasesystemen - macromoleculaire stoffen - adsorption - sorption - surfaces - interface - liquids - plastics - industry - solids - surface phenomena - two-phase systems - macromolecular materials

    Copolymers consisting of both adsorbing and nonadsorbing segments can show an adsorption behaviour which is very different from that of homopolymers. We have mainly investigated the adsorption of AB diblock copolymers, which have one adsorbing block (anchor) and one nonadsorbing block (buoy). The anchors adsorb from solution onto a surface and the buoys protrude into the solution, Thus, a polymer brush is formed. This name is derived from the resemblance between the protruding chains of B segments and the bristles of a brush. The presence of the adsorbing segments can be neglected when studying the characteristics of such a polymer brush, which is then modelled as (B-) homopolymer molecules which are terminally attached to the surface of a solid interface.

    In chapter 1 two self-consistent field (SCF) theories are introduced which give a description of such a polymer brush. The first of these theories is a lattice model. It takes into account all possible conformations that can be generated on a lattice; the molecules are treated as freely jointed chains. The overall volume fraction profile (that is, the polymer volume fraction φas a function of the distance z to the surface) is then found by weighting each conformation with an appropriate Boltzmann factor. This theory can both be applied for systems with end-attached polymer molecules and for systems with freely adsorbing chains. The volume fraction profiles for any given system must be found using a complicated numerical procedure.

    The second theory explicitly assumes that the polymer molecules are strongly stretched. Under this assumption only a fraction of all possible molecular conformations need be taken into account to find the volume fraction profile. Although this approach is less exact than the lattice model, it has as a major advantage that an analytical expression can be derived for the shape of the volume fraction profile. A simple algebraic expressions is also available for the brush height, if only the second and third order terms of a virial expansion of the free energy of mixing polymer and solvent are taken into account. If this free energy is accounted for in a more exact manner, one must (numerically) calculate the brush height from a (simple) integral equation.

    In the first chapter we make a detailed comparison of the predictions of both theories for a polymer brush at a flat surface in a low molecular weight solvent. In general an excellent agreement is found between the results of both theories. Significant deviations only occur very close to the surface and at the periphery of the grafted layer. In the lattice model there is a small depletion zone near the grafting surface, which is caused by the entropical restrictions imposed upon many polymer conformations by this impenetrable surface. The lattice calculations further show a "foot" of the volume fraction profile, which extends further away than the brush height as calculated from the strong-stretching approximation. The relative importance of these deviations increases with decreasing chain length, decreasing grafting density, and decreasing solvent quality. In order to find good quantitative agreement between the lattice calculations and the strong-stretching theory, one must incorporate the full Flory- Huggins expression for the mixing free energy of polymer and solvent into the latter theory. The derivation of elegant, analytical expressions for the layer structure by expanding this free energy in a virial series is only valid for low grafting densities.

    In all chapters except the second, the polymer chains are treated as freely jointed chains in a potential gradient. In chapter 2 more elaborate models are introduced for the polymer chains. Chain stiffness is incorporated by reducing the flexibility of the segment bonds. Stiffer chains give larger brush heights. Over a large range of chain stiffnesses the volume fraction profiles agree well with analytical expressions based on the incorporation of chain stiffness into the Gaussian approximation for the local stretching of a polymer chain. A further modification is a first order correction to the excluded volume interactions in the generation of the chain conformations. This correction slightly reduces the brush height. The opposing effects of this correction on the one hand, and chain stiffness on the other, suggest that the freely jointed chain is a good model for "real" polymers.

    Chapter 3 considers polymer brushes on cylindrical and spherical surfaces with a radius of curvature R. On such surfaces the dependence of the brush height H on the chain length N differs from that of a flat brush. SCF lattice calculations are presented to investigate this dependency as a function of R. For large values of R the scaling law H - N is recovered for both spherical and cylindrical surfaces. For R = 1 good agreement is found with the scaling laws H - N 0.6(spherical surface) and H - N 0.75(cylindrical surface). Polymer brushes on spherical surfaces can be seen as a model for AB diblock copolymers adsorbed onto small colloidal particles. For R = 1 a star-branched polymer molecule in solution is modelled.

    The volume fraction profile of the brush is also studied as a function of R. For this purpose we focus our attention on spherical brushes immersed in athermal solvents. For large radii of curvature we make the assumption that the potential energy profile of the segments can be approximated by a parabolic function, as for flat surfaces. Applying this approximation, we derived an analytical expression for the volume fraction profile which agrees reasonably well with the lattice calculations. For very small radii of curvature the lattice calculations predict volume fraction profiles which follow the scaling prediction (φ- z -4/3for spherical brushes in athermal solvents). For intermediate curvatures we propose an analytical expression for the volume fraction profile which is a combination of the parabolic potential near the surface, and the scaling form farther away from the surface. Thus, over the whole range of radii of curvature, analytical expressions for the volume fraction profiles are available which give reasonably good correspondence with the lattice calculations.

    We also studied the "dead zone" from which the free ends are excluded near the grafting surface. The lattice calculations show such a dead zone under all solvency conditions, both for spherical and cylindrical surfaces. The extension of this zone is a non-monotonic function of the surface curvature. The relative size of this zone (with respect to the brush height) is a decreasing function of R. No easy analytical expression is available for the size of the dead zone.

    In chapter 4 the adsorption equilibrium of AB diblock copolymers is considered for adsorption from solution onto small spherical particles. For adsorption onto flat surfaces it is known that the adsorbed amount shows a maximum as a function of the size of the adsorbing block, if the total chain length is kept constant. The thickness of the adsorbed layer shows a similar behaviour. Assuming that the adsorption energy is independent of surface curvature, we showed that the maximum in the adsorbed amount increases when the surface curvature increases. The hydrodynamic layer thickness of the adsorbed layer decreases strongly with increasing surface curvature. This increase occurs for all ratios of anchor to buoy sizes. On the other hand, the root- mean-square layer thickness changes much less as a function of the surface curvature. Depending on the anchor to buoy size ratio, it may either increase or decrease when the surface becomes more strongly curved.

    Chapter 5 treats the interaction between two polymer brushes, both in the presence and absence of free polymer in the solution. In this chapter we first study the effect of free polymer chains in solution on the height and volume fraction profile of an isolated polymer brush. Using self-consistent field and scaling arguments, diagrams of state are constructed, which indicate different regimes with different scaling laws for the brush height and for the interpenetration of free and grafted polymer chains, as a function of grafting density, free and grafted chain length, and bulk volume fraction of the free polymer. These scaling laws are again corroborated by SCF lattice calculations. Predictions are also given for the volume fraction profiles of free and grafted chains based on the strong-stretching approximation. In the derivation of these expressions it is explicitly assumed that the free chain length is far smaller than the brush height. When this condition is satisfied, the volume fraction profiles from the lattice calculations agree excellently with those predicted by the strong-stretching theory. When this condition is not satisfied, both approaches still predict the same height, but the strong-stretching theory gives a far too sharp interface between the grafted layer and the free polymer.

    The repulsive interaction between two compressed brushes starts at slightly larger separations according to the lattice calculations than one would expect from the strong- stretching approximation. This is caused by the "foot" of the volume fraction profile. This phenomenon occurs both in the absence and in the presence of free polymer in the solution. When free polymer is present the free energy of interaction can have an attractive part, caused by the depletion of the free chains.

    Chapter 6 deals with the interaction between two surfaces bearing adsorbed multiblock copolymer layers. We first study ABA triblock copolymers. Grafted layers of B chains with an end A block ("brushes with stickers") are used to model an adsorbed layer of such polymers. When the A adsorption energy of such a grafted layer is small, the free energy of interaction between two surfaces is purely repulsive. When this adsorption energy increases, a minimum appears, which reaches a limiting value at a certain adsorption energy. The minimum adsorption energy needed to find an attraction increases with increasing grafting density σ, and chain length N. The absolute value of this minimum also depends on N and σ. It scales as or σ 1/3N -1. The minimum always occurs at a separation d that is larger than the separation 2h at which the brushes are just in contact if the "feet" in the profiles are neglected. The difference d-2h scales as Nσ 1/3. The attraction has an entropic origin. When the surfaces are far apart, the grafted chains form loops, with the A blocks adsorbed to the grafting surface. When the surfaces are brought together, the A block of a grafted chain can either adsorb onto the surface to which this chain is grafted, or it can adsorb onto the other surface. This freedom to choose between two surfaces leads to an entropically driven attraction.

    The interaction between adsorbed layers of ABA triblock copolymers (where the adsorbed amount is determined by the equilibrium between free and adsorbed chains) has an attractive part if the copolymer chains are symmetric. The interaction curve is the same as that of a grafted layer ("brush with stickers") with a grafting density corresponding to the adsorbed amount of the triblock copolymers. If one of the adsorbing blocks is larger than the other block, the attraction decreases. For a relatively low asymmetry (one block roughly 20% larger than the other) the attraction disappears completely.

    Multiblock copolymers consisting of more than three blocks can form bridges between two surfaces comprising several blocks. We studied the interaction between two surfaces bearing adsorbed multiblock copolymer layers. The overall composition of the polymer chains was kept constant, but the chains were divided into different numbers of A and B blocks (so that the blocks become shorter when there are more blocks per chain). Chains with smaller blocks give smaller adsorbed layer thicknesses, so that the interaction starts at smaller separations. In all cases an attractive part is found in the interaction curve. Copolymer chains consisting of alternating small blocks of A and B segments very much resemble homopolymers (with properties that are some average of the A and B segments). These copolymers show a strong attraction at small separations (<10 layers), and repulsion at very small surface separations (around 2 layers).

    So far, we have only considered situations were the solvent is a good solvent for both blocks. The A blocks adsorb preferentially with respect to the B blocks, because the former have a stronger intrinsic affinity for the surface. We also consider the adsorption of an ABA triblock copolymer were both blocks have the same intrinsic affinity for the surface, but where the solvent is poorer for the A block. Now the A blocks adsorb preferentially, because of the selectivity of the solvent. We also pay attention to the interaction between two surfaces bearing adsorbed layers of such copolymers. When the interactions between the A and B segments and the solvent differ only slightly, the interaction curve resembles that of an adsorbing homopolymer, with an attraction at small separations. When these interactions differ a great deal, the interaction resembles that of a "conventional" triblock copolymer, with an attractive part at a large separation and repulsion at smaller surface separations. In the intermediate situation a more complicated interaction curve is found.

    The subject of chapter 7 is the interaction between two small particles bearing adsorbed polymer layers. An extended version of the lattice SCF theory was introduced, which takes account of gradients in two directions. In this version a cylindrical coordinate system is used, so that the volume fractions can vary both parallel to the axis connecting the centres of both particles, and in planes perpendicular to this axis. Results are presented for terminally attached polymer layers. It is first shown that this cylindrical model gives an isotropic profile around one isolated particle. This profile agrees well with the profile calculated from the "conventional" SCF lattice model, where a concentration gradient can exist in one direction only. Various free energy of interaction curves are presented for two spherical particles with terminally attached chains.

    If two spherically curved surfaces bearing adsorbed polymer layers interact, then the Derjaguin approximation relates this interaction to that between two similar flat surfaces, as long as the radius of curvature is far larger than the adsorbed layer. In chapter 7 we deal with systems where this condition does not hold. That is why we find interactions that are far less repulsive than the interaction according to Derjaguin's approximation. For increasing radii of curvature R, the interaction does move in the direction of the interaction predicted for very large R by the Derjaguin approximation. On a molecular level the decreased repulsion can be explained by the freedom of the grafted chains to mover laterally out of the gap between the two particles. Whether or not the grafting segments themselves can also move over the surface plays only a minor role.

    Sorption of micropollutants to natural aquatic particles = Sorptie van microverontreinigingen aan natuurlijke aquatische deeltjes
    Koelmans, A.A. - \ 1994
    Agricultural University. Promotor(en): L. Lijklema. - S.l. : Koelmans - ISBN 9789054852209 - 253
    rivieren - waterlopen - kanalen - water - oppervlaktewater - waterverontreiniging - waterkwaliteit - adsorptie - sorptie - aquatisch milieu - plankton - geologische sedimentatie - rivers - streams - canals - water - surface water - water pollution - water quality - adsorption - sorption - aquatic environment - plankton - geological sedimentation

    Sorption to natural aquatic particles plays an important role in the bioavailability and fate of micropollutants. The characteristics of sorption were investigated for hydrophobic organic compounds (HOCs) and heavy metals using a wide variety of natural aquatic particles. Special attention was paid to (bio)sorption to phytoplankton and detritus. The extent of sorption can be quantified by a distribution- ( K d ) or partitioncoefficient ( K p ). It was shown in this study that such coefficients in aquatic systems are variable in space and time because:

    1. At more or less constant environmental conditions K d values for trace metals depend on the content of iron(hydr)oxides, manganese(hydr)oxides and organic matter in the natural aquatic particles. It was shown for Lake Volkerak/Zoom that this content varies considerably in time and space, and that K d values for cadmium differ up to factor 30 (field data) or a factor 300 (laboratory). For mineralizing planktonic particles in the laboratory, K d increases up to a factor 10 - 20 (lead) were observed under conditions similar to those in Lake Volkerak/Zoom.

    2. When K p values for HOCs are calculated with respect to an aqueous phase which contains organic matter, the apparent K p depends on the organic carbon fraction in the aqueous phase and the particle to water ratio. Further, K p depends on organic matter content and composition. It was shown for 1,2,3,4-tetrachlorobenzene that laboratory K p values varied a factor 30 for particles from lake Volkerak/Zoom. Organic matter content of suspended solids showed a strong seasonal variation. Normalization to organic carbon removes only a part of the variation. Comparison of carbon normalised HOC concentrations (field data) in settling solids with those in simultaneously sampled suspended solids showed a difference of a factor 2 - 6. Under controlled laboratory conditions, significant K oc differences between fresh phytoplankton cells, detritus and sediments were found.

    3. Hydrophobic sorption to suspended solids may not be at equilibrium in many aquatic systems. This is caused by retarded adsorption or desorption, or for phytoplankton by the combined action of retarded bioaccumulation and algal growth.

    Biosorptie van chloorbenzenen aan mineraliserende algen.
    Koelmans, A.A. - \ 1994
    In: Kontaminanten in bodems en sediment : sorptie en biologische beschikbaarheid : een symposium, 29 april 1993, De Reehorst, Ede / Evers, E., Opperhuizen, A., Voorend, L., - p. 65 - 72.
    aquatische gemeenschappen - plantengemeenschappen - derivaten - benzeen - chloride - biologische beschikbaarheid - adsorptie - sorptie - toxicologie - algen - ecotoxicologie - bioaccumulatie - aquatic communities - plant communities - derivatives - benzene - bioavailability - adsorption - sorption - toxicology - algae - ecotoxicology - bioaccumulation
    A management model to assess the extent of movement of chemicals through soils
    Bril, J. ; Postma, L. - \ 1993
    In: Chemical time bombs : proceedings of the European state-of-the-art conference on delayed effects of chemical in soils and sediments, Veldhoven, the Netherlands, 2 - 5 September 1992 / ter Meulen, G.R.B., Stigliani, W.M., Salomons, W., Stichting Mondiaal Alternatief - p. 181 - 194.
    infiltratie - hydraulisch geleidingsvermogen - kwel - absorptie - adsorptie - modellen - onderzoek - infiltration - hydraulic conductivity - seepage - absorption - adsorption - models - research
    A model for the movement of chemicals in soils, STRASS (Simulation of TRansport and Sorption in Soils) based on the numerical solution to the one-dimensional convection-dispersion equation is presented in this study
    Vergelijking van de uitspoeling berekend met de modellen TRANSOL en PESTLA
    Kroes, J.G. ; Boesten, J.J.T.I. - \ 1993
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 238) - 40
    bodem - uitspoelen - gewasbescherming - pesticiden - pesticidenresiduen - persistentie - absorptie - adsorptie - modellen - onderzoek - soil - leaching - plant protection - pesticides - pesticide residues - persistence - absorption - adsorption - models - research
    Bepaling van de bodemfysische eigenschappen van twee soorten vliegas
    Veerman, G.J. ; Woesten, J.H.M. - \ 1993
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 261) - 46
    infiltratie - hydraulisch geleidingsvermogen - kwel - absorptie - adsorptie - rook - sintels - verbranding - as - vliegas - infiltration - hydraulic conductivity - seepage - absorption - adsorption - smoke - slags - combustion - ash - fly ash
    Sorption of phosphate by sediments as a result of enhanced external loading.
    Portielje, R. ; Lijklema, L. - \ 1993
    Hydrobiologia 253 (1993)1-3. - ISSN 0018-8158 - p. 249 - 261.
    trofische graden - voedingsstoffen - hydrobiologie - absorptie - adsorptie - meren - reservoirs - plassen - waterverontreiniging - waterkwaliteit - eutrofiëring - waterbodems - trophic levels - nutrients - hydrobiology - absorption - adsorption - lakes - reservoirs - ponds - water pollution - water quality - eutrophication - water bottoms
    Sorption of chlorobenzenes to mineralizing phytoplankton.
    Koelmans, A.A. ; Sanchez-Jimenez, C. ; Lijklema, L. - \ 1993
    Environmental Toxicology and Chemistry 12 (1993). - ISSN 0730-7268 - p. 1425 - 1439.
    plankton - adsorptie - sorptie - benzeen - derivaten - plankton - adsorption - sorption - benzene - derivatives
    Kinetics of polymer adsorption, desorption and exchange
    Dijt, J.C. - \ 1993
    Agricultural University. Promotor(en): G.J. Fleer. - S.l. : Dijt - ISBN 9789054851356 - 168
    kunststoffen - industrie - adsorptie - sorptie - macromoleculaire stoffen - plastics - industry - adsorption - sorption - macromolecular materials

    The aim of the study in this thesis was to gain more insight in the kinetics of polymer adsorption. To this end some well-characterised polymers have been systematically investigated.

    In the process of polymer adsorption one may distinguish three kinetic contributions: transport to the surface, attachment, and reconformation of the adsorbing and adsorbed chains. In order to assess the role of each of the three contributions it is necessary to measuri the adsorption kinetics under well-defined hydrodynamic conditions. For such measurements the transport (convection and diffusion) can be calculated and therefore it becomes possible to study unambiguously the interfacial processes, i.e., attachment and reconformation.

    For this study two experimental techniques were used that both fulfil the requirement that the adsorption occurs under well-defined hydrodynamic conditions: reflectometry in a stagnation point flow (chapters 2,3 and 5-7) and a streaming potential method (chapter 4). With both techniques it is possible to follow directly and continuously the buildup of an adsorbed layer. Reflectometry is a relatively new and simple optical technique for the measurement of adsorption on (optically flat) solid surfaces. In a reflectometer a linearly polarised light beam is reflected from the (adsorbing) surface, and the reflected beam is split into its parallel and perpendicular components. The intensity ratio between the two components is continuously measured. This ratio changes upon adsorption, and after calibration the adsorbed amount (mass/area) is obtained. For reflectometry there are only few restrictions on the choice of adsorbate, adsorbent and solvent.

    The applicability in this study of the streaming potential method is limited to adsorption of uncharged polymers from aqueous solution. For that case, the streaming potential can be related to the hydrodynamic layer thickness of the adsorbed polymer layer. This thickness is mainly determined by loose ends of adsorbed chains, and it is sensitive to very small changes in the adsorbed amount of long chains near saturation. Such small changes occur for desorption of long chains into solvent, so that the streaming potential method is especially suitable for the measurement of the desorption kinetics.

    In chapter I the aim and scope of this study of this study are explained, and a general introduction to adsorption of polymers is given.
    Chapter 2 deals with the measurement of adsorption by reflectometry. Using the results of an optical model we discuss the possibilities of the method for measuring the adsorption from dilute solution on a thin film on top of a silicon substrate. For a wide variety of solvents and film materials, a sensitivity can be obtained of the order of 1-2% change in reflectivity per mg/m 2adsorbed, which is quite enough for an accurate determination of the adsorbed amount. By choosing carefully the film thickness and angle of incidence of the light beam, it can be achieved that the reflected intensity varies proportionally with the adsorbed amount, independent of the concentration profile in the adsorbed layer. Under such conditions, the reflectometric signal can be simply converted into the adsorbed amount.

    In chapter 3 reflectometry is used to investigate the kinetics of adsorption of poly(ethylene oxide) (PEO) from water onto oxidised silicon. For the stagnation point flow the maximum rate of mass transfer of polymer to the surface is calculated. This rate is compared with the observed adsorption rate, and it is concluded that mass transfer is ratelimiting up to or nearly up to saturation, depending on the chain length. Only for long chains ( M >100 kg/mole) near saturation the adsorption rate is lowered by surface processes.

    In chapter 4 a model is discussed for the desorption rate of polymers into a flow of pure solvent. This model is based on the assumption that near the surface there is a rapid equilibration between free and adsorbed polymer, and that transport of free polymer away from the surface is ratelimiting for the desorption. Due to the shape of the (high affinity) isotherm, the equilibrium concentration of free chains even after a minute desorption is extremely low, so that the transport -and thus the desorptionproceeds slowly. Thus, in spite of the rapid local equilibration, the desorption is slow because of the slow mass transfer. For a logarithmic adsorption isotherm of the polymer (for which the adsorbed amount Γincreases linearly with the log of the concentration c in solution) an explicit expression for the adsorbed amount as a function of time is derived: the desorbed amount increases proportionally with log t. The model predicts that the absolute value of the slope of the (kinetic) desorption curve Γ(log t ) and the (static) adsorption isotherm Γ(log c ) are the same.

    Using the streaming potential method it is shown in chapter 4 that the above model gives an adequate description of the desorption kinetics in aqueous solutions of PEO on glass, even for high molar mass polymer (M = 847 kg/mole). Again, this shows that the equilibration of adsorbed layers of PEO is rapid as compared to the rate of mass transfer through solution.

    Chapter 5 describes the adsorption kinetics of polystyrene (PS) from decalin on oxidised silicon. On a bare surface the adsorption rate of PS is limited by mass transfer from solution, like for PEO. For PS, the adsorption rate decreases gradually with increasing coverage. This is due to a decreasing probability of attachment during a collision of a free chain with the (covered) surface. From experiments in which the chain length, the solvent quality and the adsorption energy were varied, the picture arises that the adsorption probability during a collision is the result of a balance between a gain in adsorption energy on the one hand, and repulsive interaction with the adsorbed layer on the other.

    Exchange between polymers that differ in chain length only is the subject of chapter 6. Displacement of adsorbed short chains of PEO by longer ones in solution is limited only by transport of long chains to the surface. The adsorbed layer is continuously in equilibrium with the solution near the surface. The same conclusion was drawn from the desorption kinetics of this polymer in a flow of pure solvent (chapter 4). For PS also surface processes play a role. During exchange of short by long chains of PS there is a temporary overshoot of short chains in the adsorbed layer. This overshoot may desorb either during adsorption of long chains, or by relaxation of the adsorbed layer. By interrupting the transport of long chains to the surface, this relaxation could also be directly observed. The higher chain stiffness of PS as compared to PEO possibly explains the slower equilibration of adsorbed PS.

    Finally, we present in chapter 7 some results on the exchange kinetics between three chemically different polymers: polystyrene (PS), poly(butyl methacrylate) (PBMA) and polytetrahydrofuran (PTHF). Displacement of adsorbed layers of the rather stiff polymers PS and PBMA by the very flexible PTHF is limited only by transport of the displacing polymer from the bulk solution. For mutual exchange between the two stiff polymers, surface processes play an important role: the displacer PBMA adsorbs quickly, whereas PS desorbs slowly. Possibly, the slow exchange kinetics is caused by the low mobility of the adsorbed polymers. The displacement rate of PS by PBMA increases considerably after addition of a displacer of low molar mass. The faster exchange kinetics is probably due to the lower binding strength and, consequently higher mobility of the adsorbed polymers.

    Affinity purification of polysaccharide degrading enzymes with crosslinked substrates
    Rozie, H.J. - \ 1992
    Agricultural University. Promotor(en): F.M. Rombouts. - S.l. : Rozie - ISBN 9789054850533 - 122
    fermentatie - voedselbiotechnologie - glycosidasen - adsorptie - adsorberende middelen - fermentation - food biotechnology - glycosidases - adsorption - adsorbents

    The aim of this work was to find economically favourable, affinity based, purification methods for several polysaccharide splitting bulk enzymes. The framework in which this study is done is described in Chapter 1.

    Chapter 2 describes the adsorption of endo-polygalacturonase (endoPG) from a commercial enzyme preparation (Rapidase) to calcium alginate beads. Approximately 75% of the various polygalacturonase activities from Rapidase can be adsorbed at pH 4.4 by calcium alginate beads as well as by crosslinked sodium alginate powder. Equilibrium experiments were conducted to determine a parameter (k) that represents the degree of interaction between endoPG and the adsorbent. This parameter can be influenced by a change in pH and ionic strength of the adsorbate. At pH 3.8 the degree of interaction is 20 times larger than at pH 4.2. There is increased adsorption when the ionic strength is lowered, but a small amount of CaCl 2 is required to keep the calcium alginate beads stable.

    Despite the resemblance in structure between L-guluronate blocks and polygalacturonate, a lower k value was found when the alginate, used for the preparation of the beads, contained a larger proportion of guluronic acid residues. There is no evidence that L-guluronic blocks in the alginate chain are responsible for the large affinity of endo-PG to this adsorbent. The influence of the pH and the ionic strength and the lack of endoPG inhibition by sodium alginate are indicative for ionic interactions between endoPG and the alginate chains.

    Ionic interactions were of no importance in the interaction between ct-amylase and crosslinked starch as is described in the chapters 3 and 4. Crosslinked potato starch was prepared as an affinity adsorbent for bacterial α-amylase. To this end, reaction parameters for crosslinking in an ethanol/water solvent were investigated (Chapter 3). The degree of crosslinking, and consequently the suitability of crosslinked starch as an adsorbent for α-amylase, changed by altering these parameters. An increase in the degree of crosslinking of the adsorbent caused lower affinity for bacterial α-amylase which resulted in an unfavourable decrease in adsorption capacity and a favourable decrease of degradation of the adsorbent by the enzyme.

    The adsorption and desorption characteristics of two bacterial α-amylases (B.subtilis, B.licheniformis) on crosslinked potato starch are described in Chapter 4. A capacity of about 185 mg (B.subtilis) and 71 mg (B.licheniformis) protein per g adsorbent can be realized. However, at 4 °C a smaller adsorption constant (K a ) was measured for the enzyme from B.subtilis (0.53 * 10 5L/mole) than for the B.licheniformis enzyme (3.8 * 10 5L/mole). The K a decreases with increasing temperature suggesting that association is caused by van der Waals forces. Comparison of the adsorption of the α-amylases to crosslinked starch with the activity of the enzymes on their natural substrate reveals that the velocity constant of the backward reaction of the enzyme-adsorbent complex increases strongly with increasing temperatures (B.subtilis α -amylase, k 2 (20 °C)/ k 2 (4 °C) ≈30). Desorption can be accomplished by a raise in temperature. Glycerol (20%) is added to the desorption buffer to stabilize the enzymes and protect the adsorbent against enzymic attack. The optimal desorption temperature for the B.subtilis enzyme is 60 °C. For the B.licheniformis enzyme this value is 70 °C or even higher. The adsorption velocity of α-amylases to freshly crosslinked starch is low due to the low accessibility of the adsorbent. This can easily be improved by enzymatic modification. Thus, bacterial α-amylases can be adsorbed and desorbed within short time spans (10 min) in sufficiently high amounts to make such an affinity purification process economically feasible.

    In order to facilitate the purification of xylanases from Aspergillus niger, an affinity adsorbent has been developed from oat spelts xylan (Chapter 5). A suitable adsorbent was only obtained by crosslinking oat spelts xylan with epichlorohydrin in water but not in ethanol or ethanol water mixtures. After some initial degradation of the adsorbent (approx. 4%), no further biodegradation was measured with a reused adsorbent. Up to 60% of the xylanase activity from an Aspergillus niger enzyme mixture (50 mU/ml) was adsorbed at pH 4 The degree of adsorption to crosslinked xylan of four fractions of this preparation, previously separated by DEAE-Biogel A chromatography, varied between 40 and 90%.

    Adsorption was strongly dependent on pH and ionic strength and desorption was easily accomplished by an increase in ionic strength. In addition to xylanases, polygalacturonases also adsorbed to the matrix probably due to the D-glucuronic acid moieties in xylan. No significant adsorption of β-D-xylosidase, α-L-arabinofuranosidase, β-D-galactosidase, β-(1,4)- galactanase, β-(1-3/6)-D-galactanase or cellulase activities was found.

    The binding behaviour of four commercial fungal enzyme preparations on crosslinked xylan is presented in Chapter 6. The xylanase activity in Pectinol Al (Röhm GmbH, Darmstadt, Germany) was efficiently purified with crosslinked xylan. The specific endo-xylanase activity increased from 5.5 U/mg up to 160 U/mg. Two proteins were found with SDS-PAGE in purified Pectinol (29 and 51 kD) whereas a K m of 1.1 mg/ml was measured. Equilibrium adsorption studies revealed a rather low capacity for the Pectinol endo- xylanase (1.5 mg xylanase/g adsorbent). The calculated K a was 4 * 10 6L/mole.

    Some endo-xylanases were also adsorbed by cation exchange material. However, from crosslinked xylan chromatography and additional FPLC studies it appeared that the adsorption properties of crosslinked xylan were not only due to the cation-binding properties of this adsorbent.

    Chapter 7 is an evaluation of the foregoing chapters. The adsorption properties of three kinds of economically important polysaccharide splitting enzymes are studied in this work. A cheap substrate analog and crosslinked substrates were used as adsorbents. The magnitude of the capacities of calcium alginate and crosslinked starch towards endo-polygalacturonases and α-amylases, respectively, is such that commercial applications can be considered. Only laboratory applications are foreseen for crosslinked xylan as affinity adsorbent for specific endo-xylanases since the capacity of this adsorbent is rather low.

    Proton and metal ion binding to humic substances
    Wit, J.C.M. de - \ 1992
    Agricultural University. Promotor(en): W.H. van Riemsdijk; L.K. Koopal. - S.l. : De Wit - ISBN 9789054850571 - 255
    humuszuren - organische verbindingen - bodemchemie - metaalionen - zware metalen - absorptie - adsorptie - binding (scheikundig) - neutronen - humic acids - organic compounds - soil chemistry - metal ions - heavy metals - absorption - adsorption - bonding - neutrons

    Humic substances are polydisperse mixtures of organic molecules which at least to some extent determine the mobility and bioavailability of heavy metals in soils, sediments and aquatic systems. In order to make a sound risk assessment of the fate of trace metals a good conception and preferably a sound description is essential. In this thesis mechanistic models are presented that explicitely take into account the dominant factors that determine metal ion binding. These factors are the chemical heterogeneity of the humic substances, the variable charge character, and competitive binding of ions.

    The description of the proton binding behaviour, in absence of metal ions, forms the basis of the metal ion binding model. The proton binding is described with analytical expressions for continuous heterogeneous ligands in combination with a double layer model to account for the electrostatic effects. The parameters for the proton description are obtained from the analysis of proton titration with the so called mastercurve procedure.

    In order to describe metal ion binding an approximate binding stoichiometry is assumed, in which upon the binding of one metal ion, x protons are released in solution. With respect to site competition two different limiting cases have been considered. In the fully coupled case it is assumed that the different ions bind to the same sites and that the shape of the affinity distribution is the same for all ions. In the uncoupled case each ion has its own binding sites and the affinity distribution may differ for different ions. Both models are capable of describing competitive ion binding. The uncoupled model has the advantage of a lower number of parameters that have to be specified.

    Analysis of binding heterogeneity
    Nederlof, M. - \ 1992
    Agricultural University. Promotor(en): W.H. van Riemsdijk; L.K. Koopal. - S.l. : Nederlof - 243
    bodemverontreiniging - gezondheid - organische verbindingen - bodem - bodemchemie - milieu - zware metalen - absorptie - adsorptie - soil pollution - health - organic compounds - soil - soil chemistry - environment - heavy metals - absorption - adsorption - cum laude

    Binding heterogeneity, due to different functional groups on a reactive surface, plays an important role in the binding of small molecules or ions to many adsorbents, both in industrial processes and in natural environments. The binding heterogeneity is described by a distribution of affinity constants since the different functional groups have different affinities for the adsorbing species.

    Three appraoches are discussed to obtain distribution functions on the basis of adsorption isotherms: the Local Isotherm Approximation (LIA), the Affinity Spectrum (AS) and the Differential Equilibrium Function (DEF). The methods are compared both on the basis of their derivation and on their ability to reproduce (known) distribution functions. All methods discussed need derivatives of the binding function, which are hard to obtain from experimental data. In order to apply the methods to experimental data a smoothing spline routine was adapted for the present problem. The methodology is applied to proton and copper binding to fulvic acids.

    Analogous to the heterogeneity analysis for binding under equilibrium conditions, a procedure was derived to determine first order rate constant distributions. The newly developed method is called LOcal Decay function Approximation (LODA). Also here an adapted smoothing spline routine is used to apply the method to experimental data. The method is illustrated by copper dissociation data from estuarine humic material.

    Finally it is shown how on the basis of the obtained distribution function a suitable model can be chosen for the description and prediction of binding or dissociation data.

    Field scale behaviour of cadmium in soil
    Boekhold, A.E. - \ 1992
    Agricultural University. Promotor(en): F.A.M. de Haan; S.E.A.T.M. van der Zee. - S.l. : Boekhold - ISBN 9789054850380 - 181
    bodem - cadmium - bodembiologie - absorptie - adsorptie - modellen - onderzoek - bodemverontreiniging - milieueffect - schade - nederland - geostatistiek - noord-brabant - soil - cadmium - soil biology - absorption - adsorption - models - research - soil pollution - environmental impact - damage - netherlands - geostatistics - noord-brabant

    Although total heavy metal contents of soil are often used to express the degree of contamination, they are of little value to judge environmental effects. The main objective of this thesis was to develop and test methodologies with which environmental risks of cadmium pollution of field soils can be evaluated. A method to determine the mobile and bioavailable fraction of the total Cd content may be soil extraction with 0.01 M CaCl 2 . Chemical processes that control the cadmium activity in such an extract were quantified. This is relevant information when Cd contents in CaCl 2 -extracts need interpretation in terms of bioavailability and leachability.

    Cadmium behaviour at the field scale was studied using soil samples taken from an arable field in the Dutch Kempen region. Spatial variability of total Cd contents, CaCl 2 -extractable Cd contents, soil-pH, and organic matter content was distinct and different for all parameters. Spatial variability of soil-pH and organic matter content explained a major proportion of the variability of Cd contents.

    Due to spatial variability, a high sampling density was needed for reliable estimation of pollution boundaries. When polluted soil needs remedial action, reduction of the research effort by minimizing the sampling density may well lead to an overall increase in sanitation costs because the area where concentrations exceed a critical threshold level becomes larger.

    The long-term effect of soil heterogeneity on cadmium behaviour in soil was evaluated using a stochastic simulation model. Predicted uptake of Cd by barley and leaching of Cd to groundwater was much higher in a heterogeneous field as compared to an equivalent homogeneous field. Variability of soil hydraulic properties as well as soil chemical parameters caused these differences. This demonstrated the importance of soil heterogeneity for environmental impact assessment of soil contamination.

    Comparison of different models for phosphate sorption as a function of the iron and aluminium oxides of soils.
    Freese, D. ; Zee, S.E.A.T.M. van der; Riemsdijk, W.H. van - \ 1992
    Journal of soil science 43 (1992). - ISSN 0022-4588 - p. 729 - 738.
    bodem - fosfor - absorptie - adsorptie - ijzer - soil - phosphorus - absorption - adsorption - iron
    Enzyme adsorption at solid-liquid interfaces
    Duinhoven, S. - \ 1992
    Agricultural University. Promotor(en): J. Lyklema; W. Norde. - S.l. : Duinhoven - 163
    adsorptie - sorptie - vloeistoffen (liquids) - enzymen - vaste stoffen - tweefasesystemen - adsorption - sorption - liquids - enzymes - solids - two-phase systems

    Enzymes are proteins with the capacity of catalysing various reactions. Nowadays two types of enzymes, proteases and lipases, are available for use in detergent formulations for household and industrial laundry washing. Proteases are capable of catalysing the hydrolysis of proteins while lipases enable the hydrolysis of glycerol esters, the main component in fats and non-mineral oils. In this study, two enzymes each representing one of these categories were examined: Savinase TM, an extracellular serine protease of the subtilisin family from Bacillus lentus and Lipolase TM, an extracellular lipase from the thermophilic fungus Humicola Lanuginosa S-38. To improve the soil- and stain removal in laundry washing the enzyme has to adsorb onto the soiled fabric and hydrolyse one or more of the stain components. This thesis describes a study on the mechanisms by which these enzymes adsorb at various solid-liquid interfaces from buffered solutions in the absence of other components that are found in common detergents.

    Chapter 1 gives a general introduction. It contains an overview of the physico-chemical characteristics of the two enzymes and a summary of the present insight into the protein adsorption. In Chapter 2 the investigation of the adsorption measurement techniques for Savinase is reported together with methods used to inhibit the enzyme with the small organic inhibitor PMSF (phenylmethanesulfonyl fluoride) to suppress autolysis.

    Several techniques are known to measure the protein concentration in solution and, hence, to determine the adsorption. However, the measurement of a proteolytic enzyme, such as Savinase, at such low concentrations as used in laundry washing systems proved to be a serious complicating factor. Proteolytic enzymes hydrolyses proteins and as the enzyme itself is a protein it is capable of self-destruction or cannibalism, so-called autolysis. Of the four well known methods for measurement of the protein concentration only two proved to give reliable results. The radio-active tracer technique could not be used for the measurement of Savinase adsorption because the unlabelled enzyme adsorbed preferentially over the labelled one. The latter was shown to be degraded by autolysis during the labelling reaction and subsequent storage. The competition between the numerous protein fragments resulted in a surface area- and volume- dependent adsorption comparable to the adsorption from polydisperse polymer solutions. The detection limit of the UV 280 nm extinction method was too low to determine the rising part of the adsorption isotherm. The Lowry colouring method gave reliable results but it can not be used to discriminate Savinase in a protein mixture. For the measurement of Savinase adsorption onto protein-soiled surfaces the so-called "AAPF" method proved to be applicable. This method is based on the detection of the proteolytic activity of Savinase on AAPF (N-succinyl-L-Alanyl-L-Alanyl-L-Prolyl-L-Phenylalwiine-p-Nitroanilide). As autolysis of active protease becomes more rapid at higher enzyme concentrations this method is restricted to the concentration regime of 0.1 μg ml -1to 30/μg ml -1.

    In laundry washing detergent systems the enzymes have to adsorb onto soiled cloth in order to remove stains. These practical "solid" surfaces proved to be very complex as they are mostly porous, irreproducible and difficult to characterise. Therefore, the examination of the adsorption mechanism of both enzymes, the main part of this study, is performed using well characterised, non-porous and non-hydrolysable solid surfaces. The enzyme adsorption at these solid-liquid interfaces has been approached in a similar way as protein adsorption is generally studied. In Chapter 3 the adsorption of Savinase on glass and on several polystyrene latices is reported at various pH-values, ionic strengths and temperatures. The adsorption of Savinase at solid-water interfaces was found to be driven by electrostatic interactions between the surface and the enzyme, dehydration of hydrophobic interfaces and lateral interactions between the adsorbed Savinase molecules. It was also concluded that the enzyme adsorbed as a sphere and did not unfold upon adsorption.

    As the actual adsorption depends strongly on the electrostatic interactions between the surface and the enzyme the contributions of electrostatic lateral repulsion and dehydration of hydrophobic parts of the surface to the free energy must be of the same order of magnitude. The range of the lateral repulsion extended far beyond the Debye length. The relative importance of the electrostatic interaction resulted in a decrease of adsorption on a negatively charged surface with an increase in pH. Under attractive electrostatic conditions the adsorbed amount decreased with increasing ionic strength. The uptake of an extra Ca2+ ion in the weak calcium binding site of Savinase increased the adsorption at a negatively charged interface, probably because of the increased positive charge on the enzyme. The weak binding site proved to be selective for calcium ions as magnesium ions were not sequestered.

    The adsorption of Savinase is dynamic, i.e. protein molecules replaced already adsorbed ones, although it was not reversible against dilution. On a hydrophobic surface was it proved to be reversible towards changes in pH and ionic strength. The adsorption of inhibited Savinase was temperature, independent in contrast to that of active Savinase at 30°C which was strongly determined by enzymatic autolysis.

    In Chapter 4 the influence of the properties of Savinase on its adsorption on PS-latices and glass and on the interaction with chromatographic column materials is examined. Therefore, a set of six closely related protein-engineered variants of Savinase were studied. The variants differed from the naturally occurring Savinase in their electrostatic properties such as the isoelectric point and the number of charged amino-acids. On negatively charged PS-latex differences in electrostatic interaction dominated the differences between the adsorption of the various Savinase variants. The electrostatic properties of the variants could not be described completely on the basis of either the net charge of the protein or the mean surface potential. Altering the primary structure of the protein brought about changes that could not be described properly without specifically considering the distribution of the charged residues over the protein surface. Hydration of the enzyme was a less important factor for the adsorption on hydrophobic PS-latex especially in the case of electrostatic attraction.

    The adsorption on PS-latex was compared with the retention on chromatographic columns and this proved to be useful in the investigation of the relative importance of electrostatic and hydrophobic interactions for protein adsorption. The retention of the Savinase variants on the hydrophobic interaction column (Alkyl-Superose r) showed that the retention time was inversely related to the polar - non-polar area ratio of the protein. The retention of the variants on a cation exchange column could not be described satisfactorily with the mean surface potential of the protein. A first examination of the relation between charge distribution over the protein and retention on a negatively charged column showed that strongly localised alterations of the potential could not be expected to lead to differences in retention.

    In Chapter 5 the adsorption of Lipolase on glass and various polystyrene latices was examined. The approach was similar to that for Savinase, reported in Chapter 3. The adsorption mechanism of Lipolase also proved to be comparable to that of Savinase. Electrostatic interaction and dehydration of hydrophobic parts of the surfaces are the main driving forces. Under attractive electrostatic interaction between the surface and the enzyme the plateau value of the isotherm corresponds to a monolayer coverage (2.3 mg m -2). Under experimental conditions in which dehydration of the hydrophobic surface is almost compensated by electrostatic repulsion the lateral repulsion between the adsorbed enzymes becomes important and determines the surface coverage of Lipolase. Just as in the case of Savinase it was concluded that Lipolase did not unfold significantly upon adsorption.

    In the final chapter the examined experimental techniques and the insight into the driving forces for Savinase adsorption obtained, are applied to the adsorption of Savinase at practical solid-liquid interfaces such as polyester, cotton and cotton artificially soiled with the protein BSA (Bovine Serum Albumin). These practical solid surfaces proved to be much more complex than the non-porous, non-hydrolysable, surfaces that were used in the previous chapters. A preliminary characterisation of these interfaces, based upon the adsorption of well known proteins, a determination of the time dependence of Savinase adsorption and the measurement of the isotherms under various conditions have been carried out. The specific surface area of polyester was determined to be 0.27 m 2g -1by adsorption of the protein Lysozyme. Lysozyme adsorption on polyester reached equilibrium within 1 hour. Under electrostatically attractive conditions the maximum adsorption of Savinase on polyester corresponds to monolayer coverage. The affinity of Savinase for the polyester surface was determined by electrostatic interaction together with dehydration of the hydrophobic interface.

    By Lysozyme adsorption measurement, the surface areas of clean and BSA-soiled cotton were determined to be 15 m 2g -1and 9 m 2g -1, respectively. However, the actual values may differ by as much as 50% from the ones determined in this way as Lysozyme adsorption proved to be surface-specific. Adsorption equilibrium was reached after 8 hours contact time. During this period protein diffuses into the hollow cotton fibres. The adsorption of Savinase on clean cotton resembled that on glass and is mainly determined by electrostatic interaction between the enzyme and the surface. The uncertainty in the surface areas of clean and BSA-soiled cotton prevented the determination of possible preferential adsorption of Savinase onto protein soiled cotton.

    The adsorption of active and PMSF inhibited Savinase on cotton artificially soiled with BSA differed very strongly even after short contact times. Although further experimental evidence is necessary we conclude that the lower adsorption on BSA-soiled cotton of active Savinase compared to that of PMSF inhibited Savinase was (partially) caused by the decreased affinity of the enzyme for the altered interface. Alteration of the solution by an increased concentration of hydrolysed BSA fragments did not significantly reduce the adsorption.

    The adsorption mechanism of Savinase and Lipolase indicated above can be compared with that reported for other proteins. In the literature protein adsorption in general is described as governed by a delicate balance between four categories of interactions namely protein-surface interactions (electrostatic and Van der Waals interactions), the dehydration of the protein and solid surfaces, the structural alteration of the protein and the lateral interaction between the adsorbed proteins. As a first step, proteins can be divided into two categories, "soft" and "hard", according to the resilience against structural alterations (as discussed in Chapter 1). The results reported here show that Savinase and Lipolase are even "harder" (more rigid) than the so-called "hard" proteins examined in the literature. The adsorption of "hard" proteins, such as RNase and Myoglobin, onto like charged hydrophobic surfaces is caused by their flattening upon adsorption. This flattening is the cause of an increase in the dehydrated surface area on the interface. This interaction is strong enough to compensate for the electrostatic and lateral repulsion. The rigidity of both enzymes examined here is thought to obstruct this flattening therewith limiting the dehydrated surface area. The absence of structural alterations and the relatively small contribution of dehydration make Savinase and Lipolase adsorption very sensitive to the other two important interactions: the electrostatic protein-surface interactions and the lateral interaction between the adsorbed proteins.

    Thermodynamics of the adsorption of organic cations on kaolinite : temperature dependence and calorimetry
    Mehrian Isfahany, T. - \ 1992
    Agricultural University. Promotor(en): J. Lyklema; A. de Keizer. - S.l. : Mehrian Isfahany - 199
    natuurlijke hulpbronnen - kaoliniet - kleimineralen - kaolien - adsorptie - sorptie - ionen - anionen - kationen - thermodynamica - thermische energie - natural resources - kaolinite - clay minerals - kaolin - adsorption - sorption - ions - anions - cations - thermodynamics - thermal energy

    The present work is aimed at understanding the interactions involved in the adsorption of cationic surfactants on heterogeneous surfaces. The relevance of the study derives from the environmental aspects of the adsorption of small organic molecules onto soil constituents. This thesis emphasizes the experimental aspects.

    In order to achieve a better understanding of the driving forces involved in the adsorption process, classical equilibrium thermodynamics is used to estimate the energetic and entropic parameters of the system.

    The main experimental systems were a homo-ionic kaolinite in an aqueous electrolyte solution which contained a cationic surfactant with a dodecyl tail and either a pyridinium chloride (DPC), or a trimethylammonium bromide head group (DTAB).

    In Chapter 2 a comprehensive study of the physico-chemical properties of the adsorbent has been carried out. Several techniques, such as X-ray diffractometry, electron microscopy, Ar/N2 adsorption calorimetry, and BET gas adsorption have been used to characterize the kaolinite surface. The collected evidence shows that our kaolinite is free of 2:1 clay contaminants. Combination of electrophoresis measurements and potentiometric titration results gives information on the properties of our representative of the group of 1:1 clay minerals. Contrary to the idea of some soil scientists, about 50% of our kaolinite surface charge stems from isomorphic substitution which is mainly exposed to the basal surfaces. These surfaces are homogeneous with respect to Ar and surfactant adsorption. The particles posses a variable charge (pH-dependent) on the edges. The edges have a more heterogeneous character. Constant hysteresis observed between back- and forward potentiometric titrations confirm the binary nature of the surface charge and the possibility of edge-plate interactions resulting in a so-called card-house packing. The ratio found for the edge/plate surface area depends on the method used, viz. 0.35 from DPC adsorption, 0.55 from potentiometric titration, 0.20 from Ar adsorption calorimetry and 0.41 from electron microscopy. The CEC measured by the silver-thiourea method amounts to 57 μmole/g whereas that determined by using the ammonium acetate method is 30 μmole/g. Potentiometric titrations show that the clay surface is not covered by any oxide coating. Over the experimental pH range of 4 to 10, neither an isoelectric point (iep), nor a zero point of charge (pzc) has been found for the entire particles. Extrapolation of the electrophoretic data suggests a pzc of about 2. The zero point of charge of the edges (epzc) is estimated from the inflection point of charge-potential curves which is located in the region of minimum electrolyte effect. Of the three cations Li +, Na +and Cs +, Cs +adsorbs most strongly, resulting in a lower electrophoretic mobility and a slightly lower epzc for Cs-kaolinite. The epzc is 6.7 for Li- and Na- kaolinite and 6.0 for Cs-kaolinite.

    Protons are specifically adsorbed, not only on the functional edge groups but also on the plates. Adsorption of protons on Na-kaolinite is exothermic, with the enthalpies increasing when the surface charge becomes more negative. The electrolyte effect on the proton adsorption enthalpy is very small. This suggests that the pH has a more pronounced effect on the surface potential than the indifferent electrolyte. From the fact that electrophoretic mobilities change more strongly with pH than with the electrolyte concentration we come to the same conclusion. The proton adsorption enthalpies at each pH show a qualitative similarity to that of oxides with similar pzc's.

    Chapter 3 deals with the properties of the adsorbate. Here the thermodynamic properties of a homologous series of surfactants are studied. The micellization enthalpies of three surfactants with C10, C12 and C14 tails and pyridinium head groups are directly measured as a function of electrolyte concentration and temperature. At a certain temperature and electrolyte concentration each surfactant has a characteristic critical micelle concentration (cmc) value, which is some 60-80, 10-20 and 5-10 mmoles per litre for C 10 - C 12 - and C 14 -pyridinium chloride, respectively. Enthalpies of micellization of the surfactants (Δ mic H m ) are temperature dependent. They change sign at a certain temperature, T trans . T trans is dependent on the chain length and to a lesser extent on the electrolyte concentration. The (extrapolated) Δ mic H m (T) curves of the three surfactants cross each other at - -12°C. The enthalpy at this temperature is attributed to the head group contribution; here the enthalpic contribution of the tail is zero.

    Increasing the temperature decreases the structuring of water around the tails and, consequently, the entropy rises upon association of tails, but this effect is more than compensated by the decrease in enthalpy.

    An attempt has been made to break down the Gibbs energy, enthalpy and entropy of micellization into their electrical, hydrophobic and chemical constituents. Using the mass action model, Gibbs energies of micellization are estimated. In the presence of 0.1 M NaCl, the thermodynamic parameters of micellization of the surfactants expressed per CH 2 group are very close to the corresponding parameters of transporting a methyl group from water to a hydrophobic phase. According to our calculations, hydrophobic bonding contributes most to the micellization Gibbs energy. The electrical contribution is rather small and unfavourable. The constant chemical contribution is also unfavourable and can perhaps be attributed to a decrease in hydration forces of the solvent around the head group.

    The enthalpy of adsorption may be derived from adsorption isotherms determined at different temperatures or may be directly measured by using a microcalorimetric technique. The former method is discussed in Chapters 4 and 5, the latter in Chapter 6.

    More specifically, in Chapter 4 the adsorption of DPC and DTAB on Nakaolinite is studied as a function of electrolyte concentration and pH at two temperatures. Adsorption isotherms have steep initial slopes reflecting the high affinity of the adsorptives for the surface. For both surfactants and at both temperatures, all isotherms, if measured at different salt concentrations, exhibit a common intersection point (cip) roughly around the iep. Below the cip adsorption is reduced by electrolyte addition; beyond it electrolyte promotes adsorption. Below the cip adsorption is to a large extent electrostatically driven; addition of salt reduces the attraction. However, beyond the cip association of adsorbed surfactant molecules takes place, which occurs despite the repulsion between the head groups; now electrolyte reduces this repulsion and hence promotes adsorption. The pyridinium head group shows a slightly higher affinity for the kaolinite surface than the trimethylammonium head group.

    On kaolinite, adsorption of both surfactants takes place mainly on the plates. With increasing pH the (total) adsorption increases, but not so much that all adsorption sites on the edges become covered. On the plates adsorbed protons are, at least partly, exchanged against the surfactant molecules, as the pH is increased. A bilayer adsorption model has been developed on the basis of the Frumkin-Fowler-Guggenheim isotherm equation fits the adsorption data well.

    Chapter 5 compares the temperature dependence of the adsorption of organic cations on Na-kaolinite with that on AgI. The former adsorbent is hydrophilic, the latter hydrophobic. On AgI the adsorption of tetrabutylammonium nitrate (TBAN) proceeds up to a monolayer, whereas on the hydrophilic kaolinite the adsorption of DPC and DTAB continues up to a bilayer. On AgI the adsorption of TBAN exhibits a maximum as a function of temperature. For amphiphilic molecules on kaolinite, the enthalpies of the formation of the first layer show hardly any temperature dependence, whereas those of the formation of the second layer again pass through a maximum as a function of temperature. The adsorption of surfactant molecules increases the hydrophobicity of kaolinite, hence, around the region of completion of the first layer, the surface becomes hydrophobic. Adsorption enthalpies on AgI and on the hydrophobic kaolinite show the same trend as those for the micellization of the surfactants used. This indicates that all these processes are determined by the same mechanism, viz. hydrophobic bonding.

    Chapter 6 discusses the micro calorimetric measurements of the enthalpies of adsorption of DPC on Na-kaolinite at different salt levels and temperatures. Adsorption enthalpies are temperature dependent; they are positive at low temperatures, reduce to zero at about 24°C and turn to negative at T>24°C. A break in the plots of the cumulative adsorption enthalpy as a function of adsorbed amount is detected under most experimental conditions. This break, the iep and the cip of the adsorption isotherms at different electrolyte concentrations are all identical within experimental error. Below and above the break, the adsorption enthalpies are fairly constant, suggesting that the surface is homogeneous. This is in accordance with the earlier conclusion that
    adsorption takes place mainly on the basal surfaces. The change of the slope of the ΔH(Γ) curves upon completion of the first layer shows again the different adsorption mechanism for the first and second layer. This part of the study confirms that among the different contributions to the enthalpy the hydrophobic part is the most important for the second layer formation. However, for the first layer the head group affinity for the surface dominates.

    Comparison of the isosteric adsorption enthalpy and the directly measured heat of adsorption leads to the next two conclusions: (a) For the formation of the first layer the isosteric heat of adsorption deviates somewhat from that measured by calorimetry. (b) For the formation of the second layer, good agreement has been found between the two. Both values are rather close to the enthalpy of micelle formation. Probably an important reason for the discrepancy found for the first layer is the variation of the concentrations of the other adsorbed cations at the interface as a consequence of a temperature change. Since the formation of the second layer starts when the charge of the kaolinite is compensated, the concentration of these ions at the interface is of minor importance. Therefore, beyond the iep a complete agreement between the two techniques is found.

    With regard to the adsorption mechanism our results can be summarized as follows:
    - Adsorption of cationic surfactants on charged clay surfaces takes place with a Gibbs energy of few tens of kJ/mole, implying a physical adsorption.
    - Surfactants adsorb primarily on the basal surfaces of kaolinite which possesses a negative charge. Whether the adsorption also takes place on the edges is uncertain. On the plates first a monolayer is formed, then a bilayer. For the former, electrolytes act as competitors, for the latter, where hydrophobic bonding is the driving force, as promoters.
    - Formation of the second layer is similar to micellization. The enthalpies of both processes have the same trend as a function of temperature. They are close to each other and their transition temperature agrees within a few degrees.
    - The nature of the head group does not play a dominating role in micelle formation or in adsorption.

    The adsorption of amphiphiles is sensitive to parameters such as the nature of the surface charge, electrolyte concentration, pH, and temperature. Therefore, in different climates one must expect different adsorption capacities of soils and hence, different rates of water pollution. For example, in natural systems consisting mainly of particles with a permanent charge, the adsorption capacity is probably much higher than in those systems which contain relatively large amounts of particles with variable charge. Hence, from this point of view, tropical soils may be more prone to pollution caused by the transport of organic compounds to ground water.
    The adsorption of amphiphiles is not limited to charge compensation. Therefore the value of the CEC may underestimate the adsorption capacity of soils.
    As the hydrophobic character of the charged particles changes with the amount adsorbed, there may be a large effect of soil pollution on the permeability of soils.
    The overall conclusion is that the present study not only contributes to our knowledge about adsorption forces but may also be helpful in understanding and predicting processes which occur in natural systems.

    Cationic starches on cellulose surfaces : a study of polyelectrolyte adsorption
    Steeg, H.G.M. van de - \ 1992
    Agricultural University. Promotor(en): B.H. Bijsterbosch; A. de Keizer. - S.l. : S.n. - 142
    adsorptie - sorptie - elektrolyten - zetmeel - koolhydraten - cellulose - celmembranen - celwanden - adsorption - sorption - electrolytes - starch - carbohydrates - cellulose - cell membranes - cell walls

    Cationic starches are used on a large scale in paper industry as wet-end additives. They improve dry strength. retention of fines and fillers, and drainage. Closure of the white water systems in the paper mills hase increased the concentration of detrimental substances. This might be the reason for the poor retention of cationic starches observed in the last few years.

    The purpose of the research described in this thesis was to obtain a better understanding of the adsorption of cationic starch on cellulose and how this interaction can be disturbed. In contrast to most research in papermaking we have used a colloid-chemical approach. This means that we kept our experimental system as simple as possible and therefore far from the reality of papermaking.

    In chapter 2 we tried to generalize the specific problem of cationic starch adsorption on cellulose to polyelectrolyte adsorption on an oppositely charged surface. We used a recent extension of the polymer adsorption theory of Scheutjens and Fleer for polyelectrolyte adsorption to perform model calculations. It emerged that, for the adsorption of a strong polyelectrolyte on an oppositely charged surface, two regimes can be distinguished based on the effect of salt concentration on the adsorption. We call these the screening-enhanced adsorption regime and the screening-reduced adsorption regime. In the former regime the adsorption increases with increasing salt concentration because the repulsion between the segments is screened and a nonelectrostatic interaction between polyelectrolyte and surface is present. The adsorption decreases with increasing salt concentration in the latter regime, because the mainly electrostatic attraction between polyelectrolyte and surface is screened. A transition between these two regimes can take place if the balance between the electrostatic and nonelectrostatic interactions is changed. The electrostatic interactions are determined by the segment charge and the surface charge density. The strength of the nonelectrostatic interaction is described with a χs parameter, the net adsorption energy in units kT. From the model calculations it appears that the screening-reduced adsorption regime always occurs if the interaction between polyelectrolyte and surface is only electrostatic (χs=0). The polyelectrolyte can then be completely displaced from the surface with salt ions. If there is a nonelectrostatic attraction between polyelectrolyte and surface (χs>0) the screening-enhanced adsorption regime shows up in most cases. Only for very low segment charges and not too low surface charge densities, which is often the case for polyelectrolytes used in papermaking, we are dealing with the screening-reduced adsorption regime. The theory also predicts that the adsorbed amount shows a maximum as a function of the segment charge, irrespective of the value of χs. For a very low salt concentration this occurs at a segment charge of about 0.01 unit charges, or even lower.
    If the counter ions have a specific interaction with the surface, the adsorption of a polyelectrolyte can pass through a maximum as a function of the salt concentration provided χs is not too small.
    The predictions of the model calculations agree very well with experimental results reported in literature.

    The careful characterization of the materials and the experimental methods we used are described in chapter 3. We showed that the microcrystalline cellulose, which we use as a model for cellulose fibers, is level-off DP cellulose. This means that we are dealing with fibers chemically cut into pieces. The microcrystalline cellulose had to be cleaned before use. because hemicellulose came off in aqueous solutions disturbing the determination of the equilibrium concentration of starch with a carbohydrate determination. This was accomplished by washing the microcrystalline cellulose with concentrated NaOH solutions. The surface charge of the microcrystalline cellulose, originating from carboxylate groups, was determined by potentiometric titrations to be about -1 C/g at pH=7, which is a little lower than reported for cellulose fibers. The specific surface area is a somewhat problematic quantity for a porous substrate as microcrystalline cellulose. Based on the adsorption of cationic polyelectrolytes with different molecular weights on microcrystalline cellulose, we estimated the accessible surface area for cationic starch to be about 6 m 2/g, which is only 10% of the surface area accessible to small ions.

    We used two different types of cationic starch, namely cationic potato starch and cationic waxy maize starch. Potato starch consists of two components, an essentially linear polymer of α-1,4 glucose, called amylose, and a much larger branched polymer, called amylopectin. The fraction of amylose is about 21%. Waxy maize starch consists of amylopectin. only. From their sedimentation coefficients we estimated the molecular weight of cationic amylose to be about 3.5.10 5and of cationic amylopectin from potato starch between 5.10 7and 5.10 8. The molecular weight of cationic amylopectin from waxy maize was estimated to be between 1.10 7and 6.5.10 7. Both cationic starches showed a marked decrease in viscosity and hydrodynamic radius, as measured by dynamic light scattering, with increasing electrolyte concentration. This indicates that cationic amylopectin has enough flexibility to shrink, even though it has a branched structure.
    Special attention is paid to the methods with which the starch concentration can be determined, especially to the well-known iodine determination. It is shown that it is very important to specify the iodine and iodide concentrations in the final solution with the blue starch-iodine complex and the wavelength at which the absorbance is measured.
    Finally we describe how the adsorbed amounts are measured by depletion.

    In chapter 4 we investigated the adsorption of cationic amylopectin (from waxy maize, DS(Degree of Substitution) =0.035) on microcrystalline cellulose in the presence of simple electrolytes and at different pH values. The adsorption isotherms of cationic amylopectin were all of the high affinity type, as is expected for polyelectrolyte adsorption. The plateau value of the adsorbed amount showed a maximum as a function of the salt concentration. It was also found that the adsorbed amount, in the region
    where it decreases with increasing salt concentration, was very sensitive to the type of cation used. We obtained a Iyotropic series for the alkali cations, where the adsorbed amount in the presence of the cations decreased as Li +>Na +=K +>Cs +. The trend of these experimental results could be explained very well with the theory on polyelectrolyte adsorption described in chapter 2.

    The plateau value of the adsorbed amount increased with increasing pH in the same way as the surface charge. The adsorbed amount of charge was estimated to be 10% of the titratable surface charge.

    Based on the dependence of the adsorption on the pH and the salt concentration, we concluded that for cationic amylopectin charge interactions are the main driving forces for adsorption on cellulose.

    The adsorption of cationic potato starch (usually DS=0.035) on microcrystalline cellulose is investigated in chapter 5. Special attention is paid to the fact that cationic potato starch is a mixture of 21% amylose and 79% amylopectin. It was found that amylose adsorbs preferentially. This was attributed to a larger accessible surface area for amylose due to its ability to enter the pores of microcrystalline cellulose during the equilibration time (15 hours).

    The adsorption isotherms of cationic potato starch are also of the highaffinity type. There is a strong dependence on the cellulose concentration, caused by heterodispersity of the amylose and the amylopectin fractions.

    The adsorption of cationic potato starch strongly and monotonously decreased with increasing salt concentration. It was completely displaced by salt ions at concentrations larger than 0.05 M. The divalent cations Ca 2+and Mg 2+appeared to be ten times as effective as Na+ in suppressing the adsorption of cationic potato starch, which is due both to their higher charge and a specific interaction with the cellulose surface. From the small difference in effect of Ca 2+and Mg 2+we concluded that the phosphate groups in cationic Potato starch play no relevant role in the adsorption.

    Increasing the pH led to increasing adsorption. The adsorbed amount of charge was estimated to be 10% of the titratable charge.
    Finally, we investigated the effect of DS on the adsorption of cationic potato starch. At 2 mM NaCl the adsorbed amount of the starch with the lowest DS (0.017) was largest, but at 10 mM NaCl the difference between starches with DS=0.017, 0.035 and 0.047 was very small. The adsorbed amounts decreased slightly with decreasing DS. Theory on polyelectrolyte adsorption predicts that at a salt concentration of about 0.01 M the adsorbed amounts of polyelectrolytes with various segment charges can be the same indeed. The effect of segment charge is larger at lower and higher salt concentration. At a low salt concentration the starch with the lowest DS is expected to adsorb most, whereas at high salt concentration the starch with the highest DS will adsorb best.
    We concluded from the strong dependence on salt concentration and pH that the adsorption of cationic potato starch on cellulose is mainly driven by electrostatic attraction.

    In chapter 6 we conclude that the adsorption of cationic starch on cellulose is mainly determined by the presence of charges and not by certain special properties of starch and cellulose. We point out that this thesis has general relevance because of the new light it sheds on polyelectrolyte adsorption. For papermaking this thesis is particularly relevant, because it explains the adsorption behaviour of cationic starch as that of polyelectrolytes and it therefore also improves the understanding of the adsorption of other polyelectrolytes used in papermaking. The adsorption behaviour of cationic potato starch is compared with that of cationic amylopectin from waxy maize. We suggest that this may be caused by differences in size and shape. Finally we indicate which results can be of direct relevance for papermaking in practice.

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