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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    Letter to the editor: Elevating the conversation about GE crops
    Gould, Fred ; Amasino, Richard M. ; Brossard, Dominique ; Buell, C.R. ; DIxon, Richard A. ; Falck-Zepeda, Jose B. ; Gallo, Michael A. ; Giller, Ken E. ; Glenna, Leland L. ; Griffin, Timothy ; Hamaker, Bruce R. ; Kareiva, Peter M. ; Magraw, Daniel ; Mallory-Smith, Carol ; Pixley, Kevin V. ; Ransom, Elizabeth P. ; Rodemeyer, Michael ; Stelly, David M. ; Stewart, C.N. ; Whitaker, Robert J. - \ 2017
    Nature Biotechnology 35 (2017)4. - ISSN 1087-0156 - p. 302 - 304.
    Nanoscale cell wall deformation impacts long-range bacterial adhesion forces on surfaces
    Chen, Y. ; Harapanahalli, A.K. ; Busscher, H.J. ; Norde, W. ; Mei, H.C. van der - \ 2014
    Applied and Environmental Microbiology 80 (2014)2. - ISSN 0099-2240 - p. 637 - 643.
    staphylococcus-aureus - microscopy - biofilms - attraction - mechanisms - dlvo - van
    Adhesion of bacteria occurs on virtually all natural and synthetic surfaces and is crucial for their survival. Once they are adhering, bacteria start growing and form a biofilm, in which they are protected against environmental attacks. Bacterial adhesion to surfaces is mediated by a combination of different short- and long-range forces. Here we present a new atomic force microscopy (AFM)-based method to derive long-range bacterial adhesion forces from the dependence of bacterial adhesion forces on the loading force, as applied during the use of AFM. The long-range adhesion forces of wild-type Staphylococcus aureus parent strains (0.5 and 0.8 nN) amounted to only one-third of these forces measured for their more deformable isogenic ¿pbp4 mutants that were deficient in peptidoglycan cross-linking. The measured long-range Lifshitz-Van der Waals adhesion forces matched those calculated from published Hamaker constants, provided that a 40% ellipsoidal deformation of the bacterial cell wall was assumed for the ¿pbp4 mutants. Direct imaging of adhering staphylococci using the AFM peak force-quantitative nanomechanical property mapping imaging mode confirmed a height reduction due to deformation in the ¿pbp4 mutants of 100 to 200 nm. Across naturally occurring bacterial strains, long-range forces do not vary to the extent observed here for the ¿pbp4 mutants. Importantly, however, extrapolating from the results of this study, it can be concluded that long-range bacterial adhesion forces are determined not only by the composition and structure of the bacterial cell surface but also by a hitherto neglected, small deformation of the bacterial cell wall, facilitating an increase in contact area and, therewith, in adhesion force.
    Kunnen er planten groeien op Mars?
    Wamelink, Wieger - \ 2013
    Growing plants on Mars
    Wamelink, Wieger - \ 2013
    Plans for growing Earth plants on Mars revealed
    Wamelink, Wieger - \ 2013
    Growing plants on Mars
    Wamelink, Wieger - \ 2013
    Colloids in ultra-low dielectric media : surface forces and self-assembly
    Banerjee, S. - \ 2013
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Mieke Kleijn. - S.l. : s.n. - ISBN 9789461736857 - 257
    schoonmaken - was - kooldioxide - oppervlaktechemie - oppervlakte-interacties - oplosmiddelen - cleaning - laundry - carbon dioxide - surface chemistry - surface interactions - solvents

    This thesis aims at gaining fundamental insight on colloidal interactions in two types of apolar media, namely, liquid CO2 and (as a model for liquid CO2) n-hexane. The other components playing major roles are surfactants and water. The background of the work was to address the challenges met in the use of liquid CO2 as a dry-cleaning solvent, in particular the insufficient removal of particulate soil.

    Since the dielectric constant of liquid CO2 is extremely low (1.6 at 60 bar and 10 °C), it has a low individual Hamaker constant. This in turn leads to a many orders of magnitude higher van der Waals force between interacting surfaces through liquid CO2 as opposed to traditionally employed solvents like perchloroethylene (PERC), which are toxic and environmentally unfriendly. Both the absence of charge on surfaces and the high Van der Waals force mean that a high solvodynamic force (high Reynolds number) is required to dislodge particles from fabrics.

    The situation becomes worse in the presence of water (without a suitable surfactant), which is a minor component in any dry-cleaning formulation. Our atomic force microscopy results indicate that water-mediated capillary bridges can lead to higher adhesion forces between the interacting surfaces. The roughness and softness of the surfaces were found to affect the kinetics, magnitude and range of the interaction force.

    Further, we have shown that using suitable surfactants these forces can be reduced. Following a systematic selection approach based on the hypothesis that a hydrocarbon surfactant for liquid CO2 should have a low molecular weight and a branched t-butyl tail in its alkyl part, Igepal CA520 was chosen. This surfactant has an ethylene oxide (EO) chain as a headgroup (CiEOj type surfactant). The surfactant solubility was tested first in the model solvent, followed by measuring its cloud point in the liquid CO2 system, which showed that the surfactant is soluble at ~ 50 bar and 5 - 10 °C (CO2 dry-cleaning conditions). Furthermore, we found that Igepal CA520 was surface active at the water - liquid CO2 interface. Igepal CA520 was further tested in a pilot scale dry-cleaning apparatus, where it showed marked improvement in detergency of particulate soil.

    The interfacial behaviour of the surfactant - water - liquid CO2 system was also studied using the self-consistent field theory of Scheutjens and Fleer (SF-SCF). We showed that the interfacial tension of bare water - CO2 interface decreases with increasing pressure and becomes invariant of pressure beyond the saturated vapour pressure. The water contact angle on a hydrophilic surface in CO2 increases with increasing pressure. The first phenomenon was explained from the increasing Gibbs excess of CO2 at the water - vapour interface. The increase in contact angle was shown to result from the adsorption of CO2 on the -OH populated surfaces with increasing pressure. The model further predicted complete wetting of the water - vapour interface by a CO2 layer, in line with the fact that the system conditions were chosen not far from criticality.

    The model was further extended to describe and predict the interfacial and bulk properties of the liquid CO2/surfactant/water system. The experimental water - CO2 interfacial tension data and the SF-SCF modeling of the Igepal/water/liquid CO2 system indicated that Igepal adsorbs at the water - liquid CO2 interface. The model also predicted the formation of reverse micelles both at the three-phase (water/liquid CO2/gaseous CO2) coexistence (at P/Psat = 1 ) and for P/Psat > 1. With increasing pressure the critical reverse micellar concentration (CRMC) increases and the aggregation number at the CRMC decreases. A higher pressure leads to a stronger stopping mechanism for reverse micellization due to the better solvation (better solvency power of liquid CO2) of the surfactant tails by CO2.

    Apart from the bulk phase behaviour, the presence of the surfactant gives rise to interesting wetting phenomena at the water - vapour interface. Partial wetting by CO2 was noted, followed by a re-entrant wetting transition as the surfactant concentration in the bulk water phase was increased.

    The theoretical phase behaviour was validated by small angle X-ray scattering experiments (SAXS) on Igepal/water/liquid CO2 ternary systems. The SAXS results indicated conclusively the presence of self-assembled structures. The role of water in driving the self-assembly has been mapped and it was concluded that water acts as a mesogen (promoter of liquid crystals) in CiEOj type surfactant - liquid CO2 systems. In the absence of water, at a particular range of surfactant concentrations, the system contains isotropic reverse micellar mesophases (often termed as L2) and with the addition of water L2 undergoes a phase transition to a lamellar phase, Lα. The lamellar repeat distance increases with increasing water content. Following these findings, a partial phase diagram in liquid CO2 has been generated. Comparing the phase diagrams of Igepal CA520/water in liquid CO2 and in n-hexane it is clear that water plays similar roles in the two systems. Based on this we also conclude that n-hexane is a good model for liquid CO2.

    The model was used to arrive at design guidelines for surfactants for liquid CO2. It is interesting to note that the stability window for reverse micelles in liquid CO2 is rather narrow with respect to the two key Flory-Huggins interaction parameters, namely the χC3Dand χOW, characterizing the interaction between methyl groups in the surfactant tail and CO2, and between the head group oxygen and water, respectively. The first interaction parameter dominates the stopping mechanism for micellization, while the latter determines the driving force for this process. The fact that this window is narrow essentially points out the difficulties involved in designing amphiphiles for liquid CO2. The design criteria emerging from modeling are based on the numerical results that for smaller molecules (< C10), branching is important and for longer molecules (C10 and above), CH3 type interactions are more important over branching.

    Apart from CO2 dry-cleaning, the knowledge gained in this thesis can be beneficial to many other environmentally friendly industrial processes involving liquid CO2, such as enhanced oil recovery and extraction of polar compounds. The outcome of this thesis can also be extended to alleviate the problems associated with the geological storage of CO2 at high pressure under the ocean floor (deep saline aquifers).

    Colloidal interactions in liquid CO2 - A dry-cleaning perspective
    Banerjee, S. ; Sutanto, S. ; Kleijn, J.M. ; Roosmalen, M.J. van; Witkamp, G.J. ; Cohen Stuart, M.A. - \ 2012
    Advances in Colloid and Interface Science 175 (2012). - ISSN 0001-8686 - p. 11 - 24.
    supercritical carbon-dioxide - electrostatic stabilization - hydrocarbon surfactants - high solubility - phase-behavior - water - microemulsions - capillary - solvents - pressures
    Liquid CO2 is a viable alternative for the toxic and environmentally harmful solvents traditionally used in dry-cleaning industry. Although liquid CO2 dry-cleaning is being applied already at a commercial scale, it is still a relatively young technique which poses many challenges. The focus of this review is on the causes of the existing problems and directions to solve them. After presenting an overview of the state-of-the-art, we analyze the detergency challenges from the fundamentals of colloid and interface science. The properties of liquid CO2 such as dielectric constant, density, Hamaker constant, refractive index, viscosity and surface tension are presented and in the subsequent chapters their effects on CO2 dry-cleaning operation are delineated. We show, based on theory, that the van der Waals forces between a model soil (silica) and model fabric (cellulose) through liquid CO2 are much stronger compared to those across water or the traditional dry-cleaning solvent PERC (perchloroethylene). Prevention of soil particle redeposition in liquid CO2 by electrostatic stabilization is challenging and the possibility of using electrolytes having large anionic parts is discussed. Furthermore, the role of different additives used in dry-cleaning, such as water, alcohol and surfactants, is reviewed. Water is not only used as an aid to remove polar soils, but also enhances adhesion between fabric and soil by forming capillary bridges. Its role as a minor component in liquid CO2 is complex as it depends on many factors, such as the chemical nature of fabrics and soil, and also on the state of water itself, whether present as molecular solution in liquid CO2 or phase separated droplets. The phenomena of wicking and wetting in liquid CO2 systems are predicted from the Washburn–Lucas equation for fabrics of various surface energies and pore sizes. It is shown that nearly complete wetting is desirable for good detergency. The effect of mechanical action and fluid dynamic conditions on dry-cleaning is analyzed theoretically. From this it follows that in liquid CO2 an order of magnitude higher Reynold's number is required to exceed the binding forces between fabric and soil as opposed to PERC or water, mainly due to the strong van der Waals forces and the low viscosity of CO2 at dry-cleaning operational conditions.
    De waterhuishouding van een loofbosaanplant in de droogmakerij De Purmer; resultaten van onderzoek gedurende de periode 1990-2000
    Hamaker, P. ; Beets, C.P. ; Moors, E.J. ; Dolman, A.J. - \ 2002
    Utrecht [etc.] : STOWA [etc.] - ISBN 9789057731846 - 172
    waterbeheer - bebossing - hydrologie - bossen - oppervlakkige afvoer - bosbeleid - recreatie - bosbedrijfsvoering - sloten - nederland - noord-holland - bosbeheer - boshydrologie - waterhuishouding - Purmer - water management - afforestation - hydrology - forests - runoff - forest policy - recreation - forest management - ditches - netherlands - noord-holland
    Water in de groene ruimte van Noordwest Nederland : quick scan voor de Bouwsteen Water in het tweede Structuurschema Groene Ruimte
    Kwakernaak, C. ; Hamaker, P. ; Massop, H.Th.L. ; Stuijt, L.C.P.M. ; Westein, E. - \ 2001
    Wageningen : Alterra (Alterra-rapport 322) - 48
    ruimtelijke ordening - waterbeheer - hoogwaterbeheersing - stroomgebieden - retentie - nederland - physical planning - water management - flood control - watersheds - retention - netherlands
    In de voorbereiding van het Tweede Structuurschema Groene Ruimte speelt 'water' een belangrijke rol. Daarbij wordt ingehaakt op de stroomgebiedbenadering die in het waterbeheer (Waterbeheer 21e Eeuw) en de ruimtelijke ordening (Vijfde Nota R.O.) is uitgezet. In opdracht van de directie Noordwest van het Ministerie van Landbouw, Natuurbeheer en Visserij heeft ALTERRA een quick scan uitgevoerd van de huidige knelpunten in de waterhuishouding en van de verbeteringsmogelijkheden door aanpassingen in de inrichting en het beheer van land en water. De quick scan heeft betrekking op drie (deel)stroomgebieden, die in het advies van de Commissie Waterbeheer 21e Eeuw zijn onderscheiden: Noord-Holland, Amstelland en de IJsselmeerpolders. Voor deze gebieden is aangegeven welke maatregelen getroffen kunnen worden om meer water te kunnen vasthouden en bergen, en het water langer en in een groter gebied schoon te houden. Deze maatregelen zijn uitgewerkt tegen de achtergrond van verwachte ontwikkelingen in het ruimtege- bruik in deze gebieden.
    Surface forces studied with colloidal probe atomic force microscopy
    Giesbers, M. - \ 2001
    Wageningen University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer; J.M. Kleijn. - S.l. : S.n. - ISBN 9789058083586 - 135
    colloïden - krachten - colloids - forces

    Forces between surfaces are a determining factor for the performance of natural as well as synthetic colloidal systems, and play a crucial role in industrial production processes. Measuring these forces is a scientific and experimental challenge and over the years several techniques have been developed to measure the interaction between surfaces directly as a function of their separation distance. Colloidal probe atomic force microscopy (colloidal probe AFM) offers the possibility to study such forces between virtually all kinds of surfaces. Furthermore, the time scale of the measurements can be short enough to monitor relaxation effects and to study the interaction at Brownian-like collision rates. Combining this with the original application of the AFM, namely the imaging of surfaces at nanometer resolution, makes the AFM a versatile instrument in surface science.

    In this thesis the forces that play a role in colloidal systems, especially with respect to the role of surface groups and polymer layers are studied using colloidal probe AFM.

    A Colloidal Probe (a silica particle) glued to an AFM cantilever.

    An introduction to forces acting between (colloidal) surfaces is given in chapter one . In addition, this chapter presents a short overview of the development and various applications of the atomic force microscope, especially with respect to its application as a surface force apparatus. In colloidal probe AFM a micrometer-sized particle (the colloidal probe) is glued to the end of an AFM cantilever and is moved towards and from a flat surface with the use of a piezo element. The deflection of the cantilever is measured as a function of piezo position and reflects the forces acting between the surfaces. The chapter concludes with an overview of the various techniques to directly measure surface forces. A comparison of three of these techniques, i.e ., the surface force apparatus (SFA), colloidal probe AFM, and a relatively new technique called MASIF is made.

    In chapter two the experimental ins and outs of the colloidal probe technique are described in detail. The chapter deals with topics such as colloidal probe preparation, cantilever calibration and conversion of the raw data into force-distance curves.

    Chapter three presents colloidal probe force measurements on a silica-silica system in aqueous solutions of varying pH and electrolyte concentration. The results are compared to similar measurements by other authors and were found to be in good agreement with these earlier experiments, which confirmed the proper working of our surface force technique. The experimental data were fitted to the DLVO (Derjaguin, Landau, Verwey and Overbeek) theory. No indication whatsoever was found for Van der Waals interaction, which is in itself surprising but is in line with what is generally reported in literature. Most probably the Van der Waals interaction is obscured by non-DLVO short-range interactions, in particular hydration forces, and by surface roughness effects.

    In the same chapter the interaction between gold-coated surfaces as a function of pH is described. For comparison, streaming potential measurements were performed as well. The zeta-potentials thus obtained for the gold-coated surfaces are in good agreement with the surface potentials derived from the gold-gold force measurements through Poisson-Boltzmann fits. As for the silica-silica systems, we found no evidence for a contribution of Van der Waals forces to the interaction. Of course, also in the gold-gold system the Van der Waals interactions may be partly hidden due to surface roughness or the presence of hydration layers. However, because of the high literature value for the Hamaker constant of gold, a significant contribution of the Van der Waals interaction was expected at distances up to 10 - 20 nm (!). The only possible conclusion is that the high Hamaker constant for bulk gold is not applicable for the systems studied, but the reason is not clear at all.

    Finally, we studied the interaction between silica and gold surfaces. Overall, the results are in agreement with expectation. All experimental force curves are well in between the calculated Poisson-Boltzmann limits for two surfaces maintaining either constant charge or constant potential. In the case of dissimilar surfaces it is not possible to determine the potential of one of the surfaces from the interaction curves without knowledge of the potential of the other surface and of the charge regulation mechanisms. Depending on the latter, the interaction on approach between surfaces of opposite charge sign may change from attraction into repulsion, or repulsion between surfaces of the same charge sign may change into attraction. Indications of such phenomena was found for the gold-silica system around the i.e.p. of the gold surface, where the ratio between the surface charge densities is the most extreme.

    In chapter four interaction forces are described between polymer-covered surfaces for different polymer chain lengths. The polymer used was poly(ethylene oxide) (PEO). The interaction on approach is dominated by electrostatic interaction. On separating the surfaces, however, a strong adhesion is observed, which is attributed to bridging. The adhesion shows a strong dependence on the chain length of the polymer. A linear relationship between the adhesion force and the surface coverage ( i.e ., the adsorbed amount in mass per unit area) is found. However, adhesion occurs only for chain lengths above a certain threshold value. In order for this bridging to occur the surfaces have to be pressed together to some extent. At some pH values electrostatic repulsion inhibits this bridging and no adhesion is found. In these cases bridging can be induced by increasing the electrolyte concentration or increasing the load-force.

    The topic of chapter five is interactions between acid- and base-functionalised surfaces. Silica and gold-coated silica surfaces were modified with self-assembled monolayers with amine terminal groups and carboxylic acid terminal groups, respectively. Especially for the NH 2 modified silica surfaces, we found that variations in the pretreatment of the surface results into differences in the density of functional surface groups. The interaction upon approach between the different combinations of surface layers can be explained from electrostatics, assuming that for the NH 2 -NH 2 and COOH-COOH combinations the surface layers on the colloidal probe and the flat surface are not identical (due to differences in pretreatment of probe and flat surface). On approach the NH 2 -NH 2 system and the COOH-COOH system show the same trends: repulsive when the surface layers carry a large charge, but as the pH changes in the direction where more surface groups become uncharged the repulsion changes into an attraction. On retraction all combinations of modified surfaces show a pH dependent adhesion, the strongest between NH 2 and COOH surfaces. This is attributed to acid-base interaction (between -COO -and -NH 3+) and hydrogen bonding (between -NH 2 and -NH 3+and between -COOH and -COOH). As compared to literature data, the adhesion forces are low. Probably, the roughness of the surfaces, which reduces the real physical contact area, is the most important cause for this weak adhesion. Surface roughness may also lead to the large influence of the ionic strength on the adhesion force since a part of the adhesion force originates from electrostatic interaction especially just outside the actual contact area.

    Microflotation suppression and enhancement caused by particle/bubble electrostatic interaction
    Mishchuk, N.A. ; Koopal, L.K. ; Dukhin, S.S. - \ 2001
    Journal of Colloid and Interface Science 237 (2001). - ISSN 0021-9797 - p. 208 - 223.
    The processes of attachment and detachment of small or medium-sized particles to relatively large bubbles during microflotation are considered in terms of the heterocoagulation theory. Calculations are made for the conditions that the surface potentials are of similar sign and constant, that one of the surface potentials is small, that hydrophobic attraction is absent, and that there are no surface deformations. Under these conditions bubble–particle aggregates may form as a result of an electrostatic attraction which exceeds the repulsive van der Waals force at intermediate distances. Next to electrostatic and van der Waals forces, hydrodynamic and gravitational forces are considered. These forces may overcome the electrostatic repulsion at large distances and promote particle bubble attachment. Strong electrostatic attraction at small distances, arising at a large difference of the surface potentials of the bubble and the particle and of low electrolyte concentrations, can prevent subsequent detachment by hydrodynamic and gravitational forces. With increasing electrolyte concentration the electrostatic barrier increases and the attractive electrostatic force diminishes. As a result, a critical electrolyte concentration for microflotation exists. Above this concentration attachment may still occur but it is followed by detachment. At lower electrolyte concentrations the electrostatic attractive force prevents the detachment. The dependence of the critical electrolyte concentration on the values of the bubble and particle potentials and the Hamaker constant is calculated. The critical concentration does not depend on particle or bubble size if the absolute values of the total detachment force and the total pressing force coincide, which is the case for Stokes and potential flow. For every electrolyte concentration lower than the critical value there are two critical particle sizes that limit the flotation possibility. For small particle sizes attachment is impossible because the pressing force is smaller than the electrostatic barrier. For large particle sizes detachment cannot be prevented because the detachment force exceeds the maximum electrostatic attraction. A microflotation domain of intermediate particle sizes exists in which irreversible heterocoagulation occurs.
    Brushes and soap : grafted polymers and their interactions with nanocolloids
    Currie, E.P.K. - \ 2000
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : s.n. - ISBN 9789080347069 - 209
    borstels - polymeren - colloïden - oppervlaktespanningsverlagende stoffen - brushes - polymers - colloids - surfactants - cum laude

    Layers of polymer chains end-attached to a grafting plane at high densities, so-called brushes, are a curious state of matter. The (average) monomer density within the brush is as high as in a semi-dilute polymer solution, resulting in a high osmotic pressure in the brush. Due to the grafting, however, this isotropic osmotic pressure results in an anisotropic stretching of the chains normal to the surface. This degree of stretching can be quite extensive; in this thesis PEO-chains of 700 monomers are considered which are stretched up to 20% of their total contour length, i.e. form a brush with a thickness of 50 nm, merely by the presence of similar grafted chains.

    It is evident that such extended polymer layers may strongly modify the properties of the grafting surface. To this end brushes are applied as, for instance, adsorption inhibitors or colloidal stabilisators. In this thesis we focus on the thermodynamic and structural properties of polymer brushes, both neutral and charged, and on their interactions with nanocolloids. A mean-field model is developed that describes the effect of complexes formed by polymer (or polyelectrolyte) chains and nanocolloids on the polymer conformation, and the phase behaviour of such mixtures. These two modes of investigation converge in the theoretical and experimental investigation of the interaction between neutral brushes and nanocolloids which may form complexes with the polymer chains in a bulk solution.

    A general introduction to brushes and polymer-nanocolloid complexes is presented in Chapter 1. The concepts underlying scaling and analyticalself-consistent-field (aSCF) models of brushes are briefly discussed, as are a number of technological applications of grafted polymers. The difficulties encountered in the preparation of a brush of a controlled chain length and grafting density are also considered.

    In Chapter 2 surface pressure isotherms of neutral, end-grafted chains that can adsorb to the grafting plane are modelled with the numerical Scheutjens-Fleer self-consistent-field (nSCF) model. These numerical results are compared to experimental isotherms of PS-PEO block copolymers irreversibly adsorbed at the air/water interface. Semi-quantitative agreement between the numerical and experimental isotherms is found. It is shown that for long chains the experimental and numerical isotherms obey the power law for the brush surface pressure as a function of the grafting density predicted by aSCF models.

    The predicted power law for the brush thickness is only obeyed when the experimental surface pressure isotherms also follow the aSCF power law. The adsorption/desorption transition of grafted polymers upon increasing grafting density is investigated numerically by considering the chemical potential of the grafted chains and its derivative with respect to the grafting density. It is shown that this adsorption/desorption transition is continuous, irrespective of the chain length and the adsorption strength. The behaviour of the chemical potential at large adsorption energies is reminiscent to that of a (mean-field) magnetic system approaching its critical point.

    The monomer density profiles of monodisperse and bimodal PEO-brushes are determined with neutron reflectivity and compared to profiles predicted by the nSCF model in Chapter 3. The monomer density distribution predicted by aSCF-models, namely a parabolic profile, is only found at a relatively high grafting density. At lower densities the contribution of a `tail' region at the edge of the brush to the reflectivity spectra is considerable. In this distal region, which originates from fluctuations of the extended chains, the density smoothly drops to zero. Good agreement is found between the experimental and nSCF density profiles. When short and long PEO-chains are mixed at relatively high grafting densitites a bimodal brush is formed. This biomodal density distribution is enhanced by unequal chain length ratio's and mixing ratio's at high grafting densities of such mixed layers. As expected on the basisof theoretical predictions, the long chains in the bimodal brush are additionally stretched by the presence of the shorter ones.

    In Chapter 4 the properties of annealed polyeclectrolyte brushes, consisting of grafted polyacrylic-acid (PAA) chains in contact with aqueous solution, are examined with surface pressure measurements, optical reflectivity and ellipsometry. When the ionic strength of the subphase is high and the pH relatively low, the predicted power law for the surface pressure as a function of the grafting density in the salted brush (SB) regime is found. At low ionic strength and pH, however, the PAA-chains are found to adsorb at the air/water interface.

    Due to such adsorption the predicted osmotic brush regime is not observed at the air/water interface. A novel manner to prepare brushes on a solid substrate, namely Langmuir-Blodgett deposition of PS-PAA block copolymers from an air/water interface on a hydrophobic modified silicon wafer and subsequent thermal annealing, is developed. Using this technique the average degree of dissociation of grafted PAA chains as a function of pH is measured with reflectometry. It is shown that dense grafting of the PAA-chains shifts the titration curves significantly to higher pH, as predicted by scaling models and numerical studies.

    The thickness of the PAA brushes on hydrophobic modified silicon wafers is measured with ellipsometry as a function of pH, ionic strength and grafting density. At a pH not far from the monomeric pKa, the brush thickness is theoretically predicted to initially increase with increasing ionic strength and to decrease again at high ionic strength. This non-monotonic behaviour of the brush thickness is now observed experimentally for the first time.

    The initial increase in brush thickness with increasing ionic strength is, however, experimentally less pronounced than predicted by theory.

    An analytical mean-field theory for long polymer chains that form complexes with nanocolloids is developed in the following chapters. In Chapter 5 the complexation between single polymer chains in a good solvent and surfactants in micellar aggregates is considered, using a Flory-like approach. It is shown that the number of complexed micelles on a polymer chain continuously increases with increasing surfactant concentration, in agreement with experimental evidence. The size of the coil can monotonously increase, decrease, or have a maximum as a function of the surfactant concentration. Comparison with experimental data for PEO-gels complexed with SDS shows a reasonable agreement between the predicted dependence of the gel volume on the ionic strength and experiments.

    In Chapter 6 semi-dilute solutions of complexed chains are considered. Osmotic interactions are found to strongly influence the degree of complexation in a semi-dilute solution. The degree of loading of the chains by nanocolloids decreases with increasing monomer density when the osmotic interactions between complexed particles are strong compared to those between bare monomers. If, however, the complex-monomer osmotic interactions are strong compared to both the complex-complex and monomer-monomer, phase separation into a relatively dilute phase consisting of highly loaded chains coexisting with a relatively dense phase of bare chains may occur. Such phase separation is promoted when the solvent quality decreases. If the solution is below the Theta-temperature of the bare polymer, a first-order phase transition from a bare, collapsed globule to a swollen coil with increasing particle density is predicted.

    Such a first-order phase transition is reported experimentally for collapsed polymer globules with increasing surfactant concentration. An analytical self-consistent-field theory for polymer brushes, in the presence of particles capable of complexation is presented in Chapter 7. As a monomer density gradient is present in a brush, the density of complexed particles is also predicted to vary across the brush. Roughly speaking, the complexes are predominantly located in the distal region of the brush, where the average monomer density is low. In the proximal region of the brush, close to the grafting plane, the density of complexed particles is low. Microphase separation may occur in the brush under the same conditions for which macroscopic phase separation occurs in a bulk solution.

    The overall number of complexed particles is predicted to have a maximum as a function of the grafting density. The height of the brush is found to either increase monotonously with increasing grafting density, or have a local maximum and minimum. The adsorption of the protein BSA on hydrophobic silicon wafers covered with grafted PS-PEO-chains is experimentally examined in Chapter 8. The amount of adsorbed BSA is measured with reflectometry at several grafting densities and different PEO chain lengths.

    Conventional models for the interaction between a brush and adsorbing proteins predict the adsorbed amount to decrease with increasing grafting density and chain length as the interaction between PEO and BSA in the bulk is purely repulsive. However, it is observed that the adsorbed amount has a maximum as a function of the grafting density for long chains, whereas it decreases monotonously in the case of short chains. This maximum is qualitatively understood with our aSCF model presented in Chapter 7 and indicates that some (unknown) attraction between grafted PEO and BSA may exist.

    Finally, in Chapter 9, our theoretical model is extended to complexation of polyelectrolyte chains with oppositely charged nanocolloids. In a given system (particle size, charge densities of the chain and particle) the ionic strength is the main parameter which controls complexation. At high ionic strength the attractive electrostatic interactions are suppressed and the degree of complexation is negligible. As the ionic strength decreases the attractive electrostatic interactions induce complexation. The transition from a bare polyelectrolyte to a complexed chain is predicted to be either continuous or abrupt, depending on the ratio of the charge densities and the Hamaker constant of the particles. In the former case the complex remains soluble, in the latter a non-soluble coacervate is formed. Both kinds of loading processes have been reported in the literature.

    Gewenste grond- en oppervlaktewatersituatie voor de landbouw in Fryslân; landelijke kennis en normen en toepassing daarvan op vier peilgebieden
    Bakel, P.J.T. van; Hamaker, P. - \ 1998
    Wageningen : DLO-Staring Centrum - 95
    drainage - regulatie - opbrengsten - nederland - grondwaterspiegel - grondwater - oppervlaktewater - waterstand - friesland - drainage - regulation - yields - netherlands - water table - groundwater - surface water - water level - friesland
    Evaluatie waterhuishoudkundige normen en richtlijnen bij Schieland; normen voor het waterkwantiteitsbeheer
    Hakvoort, H.A.M. ; Schuurmans, W. ; Lemkes, J.C. ; Bakel, P.J.T. van; Hamaker, P. - \ 1998
    Het Waterschap 83 (1998)22. - ISSN 1380-4251 - p. 980 - 983.
    waterbeheer - watervoorraden - watergebruik - kostenanalyse - nederland - normen - waterschappen - raden - zuid-holland - water management - water resources - water use - cost analysis - netherlands - standards - polder boards - boards
    Effectiviteit bodembeschermende voorzieningen voor spoelbassins in de bloembollensector
    Boels, D. ; Groenendijk, P. ; Stuyt, L.C.P.M. ; Hamaker, P. - \ 1998
    Wageningen : DLO-Staring Centrum - 74
    bodemverontreiniging - carbendazim - hydrologie - isolatie - soil pollution - carbendazim - hydrology - isolation
    Op basis van literatuurgegevens is de stof carbendazim geodentificeerd als de meest milieukritische stof die naast enkele andere stoffen waarschijnlijk hoofdzakelijk verantwoordelijk is voor bodemverontreiniging onder bezinkbassins. Berekeningen latenzien dat de zone waarin bodemverontreiniging onder bassins optreedt, beperkt blijft door biologische afbraak van bestrijdingsmiddelen in de bodem. De dikte van deze zone is maximaal 6-7 m zonder bodemafdichting en minder dan 1 m met bodemafdichting. Emissies vanuit bezinkbassins naar de bodem zijn geringer dan emissies via drainage naar het oppervlaktewater, maar niet verwaarloosbaar en bovendien geconcentreerd op een relatief gering oppervlak. Emissies kunnen met 90-99% worden gereduceerd. De goedkoopste oplossingen, die echter geen absolute bescherming van de bodem bieden, lijken afdichting met plaatselijk aanwezige klei, hydrologiche isolatie al dan niet in combinatie met een afdichtingslaag, en adsorptielagen. Praktijkervaring met deze oplossingen ontbreekt en nader onderzoek naar de effectiviteit is aanbevolen.
    Gevolgen van het beperken van de waterinlaat en doorspoeling van het gebied Voorne-West : beknopte versie
    Hamaker, Ph. ; Aarnink, W.H.B. ; Peerboom, J.M.P.M. - \ 1997
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 435.2) - 52
    hydrologie - wateraanvoer - waterbeheer - kanalen - oppervlaktewater - chemische eigenschappen - modellen - zuid-holland - glastuinbouw - zuidhollandse eilanden - hydrology - water advance - water management - canals - surface water - chemical properties - models - zuid-holland - greenhouse horticulture - zuidhollandse eilanden
    Gevolgen van het beperken van de waterinlaat en doorspoeling van het gebied Voorne - West : uitgebreide versie
    Hamaker, P. ; Aarnink, W.H.B. ; Peerboom, J.M.P.M. - \ 1997
    Wageningen : DLO-Staring Centrum (Rapport / DLO-Staring Centrum 435)
    hydrologie - wateraanvoer - waterbeheer - kanalen - oppervlaktewater - chemische eigenschappen - modellen - zuid-holland - glastuinbouw - zuidhollandse eilanden - hydrology - water advance - water management - canals - surface water - chemical properties - models - zuid-holland - greenhouse horticulture - zuidhollandse eilanden
    Gevolgen van het beperken van de waterinlaat en doorspoeling van het gebied Voorne-West; beknopte versie
    Aarnink, W.H.B. ; Hamaker, P. ; Peerboom, J.M.P.M. - \ 1997
    Wageningen : SC-DLO (Rapport 435.2) - 52 p.
    Onderzocht is of verzilting door zoute kwel afdoende kan worden bestreden door de inlaat van gebiedsvreemd water te beperken en het oppervlaktewatersysteem door te spoelen. In het veld zijn de water-, chloride- en nutriëntenhuishouding onderzocht en met een model zijn de gevolgen van verminderde waterinlaat berekend. Als de waterinlaat en het doorspoelen worden afgestemd op de chlorideconcentraties volgens de functietoekenning volgens het Integraal Waterbeheersplan Zuid-Holland Zuid, kan de inlaat met 40 worden beperkt. Het oppervlaktewater wordt daarbij eutrofer en de glastuinbouw ondervindt nadelige gevolgen in de vorm van directe zoutschade en/of hogere productiekosten.
    Gevolgen van het beperken van de waterinlaat en doorspoeling van het gebied Voorne-West; uitgebreide versie
    Aarnink, W.H.B. ; Hamaker, P. ; Peerboom, J.M.P.M. - \ 1997
    Wageningen : SC-DLO (Rapport / DLO-Staring Centrum 435.1) - 149 p.
    Onderzocht is of de interne verzilting door zoute kwel afdoende bestreden kan worden met minder inlaat van gebiedsvreemd water en doorspoeling van het oppervlaktewatersysteem. Het betrof enerzijds veldonderzoek naar de water-, chloride- en nutriëntenhuishouding en anderzijds modelberekeningen naar de gevolgen van verminderen van de waterinlaat. Door het inlaten van water en het doorspoelen af te stemmen op de chlorideconcentraties volgens de functietoekenning volgens het Integraal Waterbeheersplan Zuid-Holland Zuid kan een beperking van de inlaat met 40 bereikt worden. De eutrofie van het oppervlaktewater neemt daarbij toe en de glastuinbouw ondervindt nadelige gevolgen in de vorm van directe zoutschade en/of hogere productiekosten.
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