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.

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Sorption of pymetrozine and dimethomorph to substrate materials
Boesten, J.J.T.I. ; Matser, A.M. - \ 2017
Wageningen : Wageningen Environmental Research (Wageningen Environmental Research report 2841) - 47
plant protection - pesticides - soilless culture - rockwool - sorption - gewasbescherming - pesticiden - cultuur zonder grond - steenwol - sorptie
Evaluation of the Dutch leaching decision tree with the substances bentazone, MCPA and mecoprop
Linden, A.M.A. van der; Beltman, W.H.J. ; Boesten, J.J.T.I. ; Pol, J.W. - \ 2015
Bilthoven : RIVM (RIVM report 2015-0095) - 78
pesticides - groundwater - drinking water - water quality - water pollution - degradation - risk assessment - sorption - decision models - leaching - bentazone - mcpa - pesticiden - grondwater - drinkwater - waterkwaliteit - waterverontreiniging - degradatie - risicoschatting - sorptie - beslissingsmodellen - uitspoelen - bentazon
Sinds 2004 wordt een beslismodel (beslisboom) gebruikt om te beoordelen in welke mate een gewasbeschermingsmiddel uitspoelt naar het grondwater. Uit een evaluatie van het RIVM, het College voor de toelating van gewasbeschermingsmiddelen en biociden (Ctgb) en onderzoekinstituut Alterra blijkt dat de beslisboom goed werkt en state of the art is. Wel laten de stofgegevens waarmee wordt gerekend te wensen over. Om de kwaliteit van het grondwater te waarborgen moeten deze gegevens zorgvuldiger worden afgeleid.
Sustainable gold mining wastewater treatment by sorption using low-cost materials
Acheampong, M.A. - \ 2013
Wageningen University. Promotor(en): Piet Lens. - [S.l.] : S.n. - ISBN 9789461737403 - 163
afvalwaterbehandeling - mijnbouw - goud - sorptie - technologie - methodologie - waste water treatment - mining - gold - sorption - technology - methodology
Uncertainty and sensitivity analysis of GeoPEARL
Berg, F. van den; Brus, D.J. ; Burgers, S.L.G.E. ; Heuvelink, G.B.M. ; Kroes, J.G. ; Stolte, J. ; Tiktak, A. ; Vries, F. de - \ 2008
Wageningen : Alterra (Alterra-report 1330) - 114
grondwater - grondwaterverontreiniging - pesticiden - uitspoelen - bodemeigenschappen - sorptie - halfwaardetijd - bodemdeeltjes - modellen - groundwater - groundwater pollution - pesticides - leaching - soil properties - sorption - half life - aggregates - models
Modelling multicomponent solute transport in structured soils
Beinum, G.W. van - \ 2007
Wageningen University. Promotor(en): Willem van Riemsdijk, co-promotor(en): J.C.L. Meeussen. - [S.l.] : S.n. - ISBN 9789085045700 - 176
bodem - bodemfysica - transportprocessen - modellen - sorptie - soil - soil physics - transport processes - models - sorption
Nickel and cobalt sorption on anaerobic granular sludges: kinetic and equilibrium studies
Hullebusch, E.D. van; Zandvoort, M.H. ; Lens, P.N.L. - \ 2004
Journal of Chemical Technology and Biotechnology 79 (2004)11. - ISSN 0268-2575 - p. 1219 - 1227.
slib - nikkel - kobalt - sorptie - kinetica - anaërobe behandeling - afvalwaterbehandeling - sludges - nickel - cobalt - sorption - kinetics - anaerobic treatment - waste water treatment - pseudomonas-fluorescens 4f39 - sphagnum moss peat - heavy-metals - aqueous-solution - waste-water - extracellular polymers - methanol degradation - removal - biosorption - ions
The kinetics and equilibria of sorption of the divalent metal ions cobalt and nickel onto anaerobic granular sludge are described. Single component and binary equimolar systems were studied at different pH values (pH 6, 7 and 8). The kinetic modelling of metal sorption by anaerobic granular sludge has been carried out using Lagergren equations
The kinetics and equilibria of sorption of the divalent metal ions cobalt and nickel onto anaerobic granular sludge are described. Single component and binary equimolar systems were studied at different pH values (pH 6, 7 and 8). The kinetic modelling of metal sorption by anaerobic granular sludge has been carried out using Lagergren equations. On fitting the experimental kinetic data both in first and pseudo-second-order equations, the regression analysis of a pseudo-second-order equation gave a higher r(2) value, indicating that both external mass transfer and intra-particle diffusion are involved in the sorption process. The experimental isotherm data were analysed using the Langmuir, Freundlich and Redlich-Peterson equations. The Redlich isotherm, a combination of the Langmuir and Freundlich equations, was found to have the highest regression correlation coefficients at pH 7. At pH 8, the Langmuir mechanism dominated for cobalt and nickel adsorption. In contrast, at pH 6, the Freundlich equation gave a better correlation coefficient which suggests a more heterogeneous adsorption at that pH. The maximal adsorption capacity of the granular sludge, as determined by the Langmuir equation, for cobalt or nickel in single systems (8.92 mg g(-1) Co TSS; 9.41 mg g(-1) Ni TSS, pH 7) compared with binary systems (8.06 mg g(-1) Co TSS; 8.43 mg g(-1) Ni TSS, pH 7) showed no great difference in the accumulation of these metals onto granular sludge. (C) 2004 Society of Chemical Industry.
Black magic in the aquatic environment
Jonker, M.T.O. - \ 2004
Wageningen University. Promotor(en): L. Lijklema; Marten Scheffer, co-promotor(en): Bart Koelmans. - [S.I.] : S.n. - ISBN 9085040280 - 254
aquatisch milieu - sediment - grensvlak tussen sediment en water - besmetters - verontreinigende stoffen - sorptie - evenwicht - aquatic environment - sediment water interface - contaminants - pollutants - sorption - equilibrium
An indicator for phosphate saturation of soils
Zee, S.E.A.T.M. van der; Chardon, W.J. ; Koopmans, G.F. ; Schoumans, O.F. - \ 2004
In: Critical evaluation of options for reducing phosphorus loss from agriculture. - Wageningen : Alterra - ISBN 9032703382 - p. 44 - 44.
bodemchemie - fosfaten - sorptie - desorptie - soil chemistry - phosphates - sorption - desorption
Feasibility of the BIOFIX-process for treatment of municipal wastewater
Temmink, B.G. ; Klapwijk, A. ; Korte, K.F. de - \ 2001
Water Science and Technology 43 (2001)1. - ISSN 0273-1223 - p. 241 - 249.
afvalwaterbehandeling - rioolwater - sorptie - nitrificatie - denitrificatie - biofilms - waste water treatment - sewage - sorption - nitrification - denitrification
The feasibility of the so-called BIOFIX-process to treat municipal wastewater was investigated in a pilot-plant. The innovative element of this process is that carrier material to which a biofilm is attached is recirculated between a sorption reactor to take up COD from the wastewater and a reactor where this COD is used for (post-) denitrification. In between the sorption and denitrification reactor the wastewater passes two subsequent reactors for removal of the remaining COD and for nitrification, respectively. The results showed that COD uptake by the biofilm in the sorption reactor with a maximum of 34% of the influent load was far below expectations and did not meet the COD requirement for denitrification (50-60% uptake). Also, 9-21% of the influent load of ammonia was taken up by the biofilm. In the denitrification reactor this ammonia was released to the bulk and in this manner discharged with the effluent without having passed the nitrification reactor. Nitrification was inhibited by the presence of high concentrations of suspended solids (50-60 mgl-1) discharged from a COD removal reactor. Together these bottle-necks caused effluent concentrations which were well above the effluent demands and it therefore can be concluded that the BIOFIX-process is not a feasible process to treat domestic wastewater.
Organic contaminants in soil : desorption kinetics and microbial degradation
Schlebaum, W. - \ 1999
Agricultural University. Promotor(en): Willem van Riemsdijk; G. Schraa. - S.l. : Schlebaum - ISBN 9789058080219 - 89
sorptie - biodegradatie - kinetica - organische verbindingen - besmetters - sorption - biodegradation - kinetics - organic compounds - contaminants
<p>The availability of organic contaminants in soils or sediments for microbial degradation or removal by physical means (e.g.) soil washing or soil venting) depends on the desorption kinetics of these contaminants from the soil matrix. When the organic contaminants desorb very slow from the soil matrix, remediation of polluted sites may be hampered. In this thesis, sorption kinetics of organic contaminants in soil was investigated with emphasis on the influence of natural organic matter on slow desorption rates, and the influence of sorption kinetics on the microbial degradation rates of these contaminants. Two organic matter fractions, humic acids and humin, were shown not only to differ in their affinity for an organic contaminant, but also in the kinetics of their interactions with the contaminant.</p><p>The results showed that humin can play an important role in the sorption behaviour of a soil. Interaction between humic acids and an organic contaminant were shown to be non-linear. Non-linear sorption behaviour was also observed for soil. The effects of non-linear sorption behaviour were demonstrated in simulations. Non-linear sorption may be caused by a limited number of sites with a high sorption energy that are present in humin. Microbial degradation of an organic contaminant in soil was predicted using measured desorption kinetics and measured intrinsic microbial degradation kinetics. It was shown that only for the slow-desorbing contaminant fraction, the desorption rate was the rate-determining step for the degradation in soil.</p><p>When microorganisms are able to degrade contaminants that become available by a slow desorption process, the natural degradation of organic contaminants in soil can be an efficient remediation method in the last stages of the <em>in situ</em> remediation of a contaminated soil. Continuous monitoring of such a contaminated soil however will be needed.</p>
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): Willem van Riemsdijk. - S.l. : Venema - ISBN 9789054856580 - 209
bodemchemie - anorganische verbindingen - mineralen - adsorptie - sorptie - ijzer - soil chemistry - inorganic compounds - minerals - adsorption - sorption - iron
<p>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.<p>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.<p>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.<p>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.<p>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
<p>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 <sub>4</sub><sup>2-</SUP>to the pore water. This HP0 <sub>4</sub><sup>2-</SUP>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.<p>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 <sub>2</sub> and 0 <sub>2</sub> , because P may limit oceanic primary production and thus determine the amount of organic material in the oceans on geological time scales.<p>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 <em>(Phaeocystis</em> 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 <sub>4</sub><sup>2-</SUP>. The enhanced HP0 <sub>4</sub><sup>2-</SUP>release was due to microbially mediated mineralization of the organic material and due to direct release of HP0 <sub>4</sub><sup>2-</SUP>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 <sub>4</sub><sup>2-</SUP>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 <sub>4</sub><sup>2-</SUP>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.<p>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 (&lt; 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.<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 <sub>2</sub> and NO <sub>3</sub><sup>-</SUP>to allow for (1) Fe and Mn reduction and (2) escape of dissolved Fe <sup>2+</SUP>and Mn <sup>2+</SUP>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 <sup>2+</SUP>and Mn <sup>2+</SUP>in the form of reduced authigenic minerals can explain the reversal of the pore water Fe <sup>2+</SUP>and Mn <sup>2+</SUP>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 &lt; 4 %); only in the depositional environment of the Skagerrak, metal oxide reduction may contribute substantially to organic C oxidation (-20%).<p>Reversible sorptive reactions can both constrain and enhance the flux of HP0 <sub>4</sub><sup>2-</SUP>from the sediment to the overlying water. The role of sorption in sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>in North Sea sediments was investigated for 15 locations in 2 seasons (Chapter 5). P sorption data, pore water HP0 <sub>4</sub><sup>2-</SUP>profiles, solid phase results and measured and calculated rates of sediment- water exchange of HP0 <sub>4</sub><sup>2-</SUP>were combined. Sorption was found to play an important role in controlling sediment-water exchange of HP0 <sub>4</sub><sup>2-</SUP>during at least part of the year in 3 of the 4 North Sea environments. At most stations, adsorption limits the flux of HP0 <sub>4</sub><sup>2-</SUP>to the overlying water. At one station in the Skagerrak, however, desorption is responsible for the maintenance of a flux of HP0 <sub>4</sub><sup>2-</SUP>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>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 <sub>4</sub><sup>2-</SUP>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 <sub>4</sub><sup>2+</SUP>diffusion are compensated for by sorption of HP0 <sub>4</sub><sup>2-</SUP>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.
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 - ponds - adsorption - sorption - phosphorus - bibliographies - netherlands - water bottoms
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 - organochlorine compounds - chlorinated hydrocarbons - water bottoms - ecotoxicology
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): Frans Leermakers. - S.l. : Israels - ISBN 9789054852315 - 100
adsorptie - sorptie - kunststoffen - industrie - colloïden - dispersie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - colloids - dispersion - macromolecular materials
<p>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.<p>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.<p>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).<p>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 <em>N</em> , the average anchoring density σ, the average segmental charge αon the chains, and the salt concentration φ <sub><font size="-2">S</font></sub> .<p>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 <em>Osmotic Brush)</em> or by non-electrostatic excluded volume interactions if the charges are low (the <em>quasi-Neutral Brush</em> regime). Upon adding salt a third regime can be found: the <em>Salted Brush.</em> 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 <em>v</em><sub><font size="-1">el</font></sub> ≈ φ <sub><font size="-2">S</font></sub><sup>-<font size="-2">1α2</font></SUP>. 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.<p>In between, at intermediate anchoring densities, the analytical theory predicts a very peculiar regime, where the thickness <em>H</em> scales as <em>H ≈N</em><sup><font size="-2">3σ</font>-<font size="-2">1α2</font></SUP>. This so-called " <em>Pincus Brush</em> ", 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 <em>Pincus regime</em> is too small to be detected. This is probably true for any reasonable set of parameters.<p>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 <em>H</em> on the salt concentration φ <sub><font size="-2">S</font></sub> for such a brush is very different from that for a conventional charged brush with constant charge density.<p>In Chapter 4 we proceed to the adsorption of ionic diblock copolymers. One block, the "anchor", consists of <em>N <sub><font size="-1">A</font></sub></em> uncharged adsorbing A segments, whereas the "buoy" block has <em>N <sub><font size="-1">B</font></sub></em> 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 <em>N <sub><font size="-1">A</font></sub></em> and <em>N <sub><font size="-1">B</font></sub> ,</em> the charge αe on the <em>B</em> segments, and the salt concentration φ <sub><font size="-2">S</font></sub> in each of the four regimes. The scaling relations axe checked using SF calculations.<p>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 <em>v</em><sub><font size="-2">e</font></sub> = α <sup><font size="-2">2</font></SUP>/φ <sub><font size="-2">S</font></sub> .<p>In the LC regime the chain density σscales as σ α( <em>N <sub><font size="-1">A</font></sub> /N <sub><font size="-1">B</font></sub></em> ) <sup><font size="-2">3/2</font></SUP><em>v</em><sub><font size="-2">e</font></sub><sup>-<font size="-2">1</font></SUP>and the layer thickness <em>H</em> as <em>H α</em> ( <em>N <sub><font size="-1">A</font></sub> /N <sub><font size="-1">B</font></sub></em> ) <sup><font size="-2">1/2</font></SUP><em>.</em> The latter scaling is independent of <em>v</em><sub><font size="-2">e</font></sub> . Using the SF model, these relations axe found to be valid for an adsorbed amount of A segments below 10% of monolayer coverage.<p>In the HC regime the adsorption is dominated by the anchoring block and the scaling relation σ α1/ <em>N <sub><font size="-1">A</font></sub></em> 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.<p>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 <sub><font size="-2">S</font></sub> * 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:<br/>φ <sub><font size="-2">S</font></sub> * ≈ <em>N <sub><font size="-1">Bα</font></sub></em><sup><font size="-2">2</font></SUP>/ <em>RN <sub><font size="-1">A</font></sub><sup><font size="-2">3</font></SUP></em><p>where <em>R</em> 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; Mieke Kleijn. - S.l. : Bos - ISBN 9789054852551 - 151
adsorptie - sorptie - eiwitten - effecten - vaste stoffen - elektrostatica - adsorption - sorption - proteins - effects - solids - electrostatics
<p>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.<p>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.<p>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.<p>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.<p>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.<p>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.<p>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.<p>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.<p>Firstly, some experiments with a simple porphyrin (tetramethyl-pyridiniurn porphyrin, H <sub>2</sub> 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 <sub>3</sub> 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 <sub>2</sub> TMPyP molecules on silica depends on the concentration of porphyrin in the solution from which adsorption takes place. At low concentration, the H <sub>2</sub> 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).<p>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.<p>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.<p>In chapter 6 the adsorption behaviour of native and porphyrin cytochrome <em>c</em> 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.<p>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.<p>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.<p><strong>Perspectives</strong><p>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.<p>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.<p>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 <sub>2</sub> TMPyP and presented in chapter 4 were promising enough to use the method for studying the orientation of several derivatives of H <sub><BLINK>2</BLINK></sub> TMPyP.<p>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.<p>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.<br/>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; Frans 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
<p>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 <em>polymer brush is</em> 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.<p>In <strong>chapter 1</strong> 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.<p>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.<p>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.<p>In all chapters except the second, the polymer chains are treated as freely jointed chains in a potential gradient. In <strong>chapter 2</strong> 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.<p><strong>Chapter 3</strong> 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 <sup><font size="-2">0.6</font></SUP>(spherical surface) and H - N <sup><font size="-2">0.75</font></SUP>(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.<p>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 <sup>-<font size="-1">4/3</font></SUP>for 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.<p>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.<p>In <strong>chapter 4</strong> 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.<p><strong>Chapter 5</strong> 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.<p>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.<p><strong>Chapter 6</strong> 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 <sup><font size="-1">1/3</font></SUP>N <sup><font size="-1">-1</font></SUP>. 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 <sup><font size="-1">1/3</font></SUP>. 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.<p>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.<p>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 (&lt;10 layers), and repulsion at very small surface separations (around 2 layers).<p>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.<p>The subject of <strong>chapter 7</strong> 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.<p>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.
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