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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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    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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    Zacht, groen, nat & mooi: kolloidkunde in Wagenings perspectief.
    Cohen Stuart, M.A. - \ 1997
    Wageningen : Landbouwuniversiteit Wageningen - 19
    chemie - colloïden - adsorptie - oppervlakten - fysica - colleges (hoorcolleges) - oppervlaktechemie - macromoleculen - chemistry - colloids - adsorption - surfaces - physics - lectures - surface chemistry - macromolecules
    Electrochemical characterization of the bacterial cell surface
    Wal, A. van der - \ 1996
    Agricultural University. Promotor(en): J. Lyklema; A.J.B. Zehnder; W. Norde. - S.l. : Van der Wal - ISBN 9789054854920 - 101
    colloïden - bacteriën - celwanden - elektrokinetische potentiaal - elektrochemie - colloids - bacteria - cell walls - electrokinetic potential - electrochemistry

    Bacterial cells are ubiquitous in natural environments and also play important roles in domestic and industrial processes. They are found either suspended in the aqueous phase or attached to solid particles. The adhesion behaviour of bacteria is influenced by the physico-chemical properties of their cell surfaces, such as hydrophobicity and cell wall charge. The charge in the bacterial wall originates from carboxyl, phosphate and amino groups. The degree of dissociation of these anionic and cationic groups is determined by the pH and the activity of the surrounding electrolyte solution. Almost all bacterial cells are negatively charged at neutral pH, because the number of carboxyl and phosphate groups is generally higher than that of the amino groups. The presence of the charged cell wall groups leads to the spontaneous formation of an electrical double layer. The purpose of the present investigation is to elucidate the structure of the electrical double layer of bacterial cell surface. Such a study serves at least two goals. It allows the quantification of electrostatic interactions in the adhesion process and it contributes to gain better insight into the availability of (in)organic compounds for bacterial cells.

    The characteristics of the electrical double layer of bacterial cell surfaces have been revealed by applying a combination of experimental techniques, which include: chemical cell wall analysis, potentiometric proton titration and electrokinetic studies such as micro-electrophoresis, static conductivity and dielectric dispersion measurements.

    For the present study five Gram-positive bacterial strains, including four coryneforms and a Bacillus brevis, have been selected. Cell walls of these bacterial strains have been isolated and were subsequently subjected to chemical analyses and proton titration studies. Both methods provide information on the number of carboxyl, phosphate and amino groups.

    The chemical analysis of isolated cell walls involves the quantitative determination of both peptidoglycan and protein content. These analyses indicate that the chemical composition of the walls of the coryneforms are very similar, but considerably different from that of Bacillus brevis. Peptidoglycan is an important cell wall constituent of the coryneform bacteria and determines about 23 to 31 % of the cell wall dry weight. The protein fractions are somewhat lower, between 7 to 14%. The cell wall structure of the Bacillus brevis strain is more complex and multi-layered. It contains a thin peptidoglycan layer, which only determines 5 % of the cell wall dry weight. On the other hand, the protein content of these walls is higher than 56%. These proteins most likely can be attributed to a so-called S(urface)-layer, which is the outermost cell wall layer.

    The surface charge density of the bacterial cells is assessed by proton titrations of isolated cell walls at different electrolyte concentrations. Rather high values, i.e. between 0.5 and 1.0 C/m 2are found at neutral pH. The absence of hysteresis in the titration curves leads to the conclusion that the charging process can be considered as reversible. It also implies that the cell wall charge is continuously in equilibrium with the surrounding electrolyte solution, at any pH and salt concentration. This observation considerably facilitates the interpretation of the titration curves, because it allows a rigorous (thermodynamic) analysis. The anionic and cationic groups in the bacterial wall could be identified and their numbers determined by representing the differential titration curves as functions of pH and cell wall charge. The carboxyl and phosphate groups are almost entirely titrated in the pH range accessible by proton titration, allowing precise estimation of their numbers. These numbers compare very well with those based on a chemical analysis of the isolated cell walls. Estimates for the number of amino groups were less accurate, because these groups are only partly titrated in the pH range were precise titration measurements are feasible. Nevertheless, it could be concluded that the number of amino groups in the bacterial wall are lower than those of the carboxyl groups.

    Information about the ionic composition of the countercharge has been obtained from Esin-Markov analysis of the titration curves and from estimates of the cell wall potential based on a Donnan-type model. The Esin-Markov analysis is purely thermodynamic and based on first principles, whereas the Donnan model requires several assumptions about the structure of the bacterial wall. Both approaches lead to the same conclusion that at salt concentrations below 0.01 M the cell wall charge is predominantly compensated by counterions, with the excluded co-ions hardly contributing to the countercharge. This observation has considerably facilitated the interpretation of the electrokinetic properties of bacterial cell suspensions.

    Electrophoresis, static conductivity and dielectric response are related (electrokinetic) techniques and therefore share common physical bases. This also implies that the physical and mathematical problems that have to be solved in order to interpret the experimental data are very similar. Analytical solutions only exist for colloidal particles for which the electrical double layer is very thin compared to the particle dimensions. Most bacterial cells are relatively large colloidal particles and therefore the largeKa theory may be of help in the evaluation of their electrokinetic properties. However, the original theories do not include surface conductance in the hydrodynamically stagnant layer. Therefore, they had to be extended to account for the finite conductivity of ions in the bacterial wall.

    Static conductivity and dielectric dispersion both show that the counterions in the bacterial wall give rise to a considerable surface conductance. From a comparison of the mobile charge with the total cell wall charge it is inferred that the mobilities of the counterions in the bacterial wall are of the same order but somewhat lower than those in the electrolyte solution.

    Due to surface conductance the electrophoretic mobility may be strongly retarded compared to the classical Helmholtz-Smoluchowski theory, especially at low electrolyte concentrations. In 1 mM and 10 mM electrolyte solution, the Helmholtz-Smoluchowski equation underestimates the ζ-potential by approximately a factor of 2 and 1.3, respectively.

    Resolving the fundamentals of the electrochemical characteristics of bacterial cell surfaces is a key step towards a quantitative understanding of the electrostatic interactions of bacterial cells with their surroundings. The success of such an investigation depends on the state of the art of the disciplines involved. Both microbiology and colloid chemistry have the microscopically small particle as object of study. Until recently there has hardly been any exchange of scientific knowledge between these two disciplines, despite their common interest. Colloid chemists prefered to study relatively simple particles to test their basic theories and bacterial cells were considered far too complex to serve as model colloids. However, the progress that has been made during the last decades in both colloid chemistry and microbiology provide the right tools for a successful cooporation. The present study is born from such a symbiosis and shows that many physicochemical characteristics of bacterial cell surfaces are accessible with (classical) colloid chemical techniques. In fact, for testing more advanced colloid chemical theories bacteria may even be better model particles than the generally used ionorganic colloids, because of their ability to produce a homogeneous population of identical cells.

    For the time being only Gram-positive strains have been considered, because of their relatively less complex cell wall structures. Nevertheless, the techniques used may mutatis mutandis also be applied to Gram-negative cells. In fact, such a study would be highly interesting, because it would contribute to a more complete description of the composition of the electrical double layer of bacterial cell surfaces.

    Formation and colloidal behaviour of elemental sulphur produced from the biological oxidation of hydrogensulphide
    Janssen, A.J.H. - \ 1996
    Agricultural University. Promotor(en): G. Lettinga; A. de Keizer. - S.l. : Janssen - 121
    colloïden - dispersie - zwavel - colloids - dispersion - sulfur

    The formation and aggregation of elemental sulphur from the microbiological oxidation of hydrogensulphide (H 2 S) by a mixed population of aerobic Thiobacillus -like bacteria has been investigated. Sulphide is formed during the anaerobic treatment of wastewaters which contain oxidized sulphur compounds such as thiosulphate, sulphite and sulphate. This sulphide has to be removed from the effluent solution of anaerobic reactors because of its detrimental characteristics e.g. toxicity, corrosiveness, oxygen demand and bad odour. Also the biogas produced in the anaerobic treatment plants generally will contain substantial amounts (up to 3% v/v) of hydrogensulphide. For removing the sulphide, conventional physico-chemical sulphide-removing processes can be applied. The processes are based on the oxidation of sulphide with peroxide, hypochlorite or permanganate or the precipitation of sulphide with iron(III)chloride. Major drawbacks of these methods are the high costs for chemicals and the production of excessive amounts of chemical sludge. An alternative method for the chemical sulphide removal comprises the oxidation of sulphide with bacteria. At the Department of Environmental Technology (WAU) a process was developed in the mid eighties in which sulphide is oxidized into elemental sulphur. Since sulphur is an insoluble compound it can be removed from the water-phase which leads to a reduction of the total S-content. The formed sulphur can be re-used in, for instance, bioleaching processes or it can be used as a raw material for sulphuric acid production after undergoing a purification step. The objective of this PhD-research was to optimize the biological sulphide removing process which concerned the development of 1) an oxygen control strategy for maximizing the sulphur production and 2) a sulphur removal method. In order to achieve the objectives, it was necessary to understand the colloidal properties of the biologically produced sulphur particles.

    Chapter 1 presents a general introduction on physico-chemical and biological methods for sulphide removal and a general overview on the sulphur chemistry. In Chapters 2 and 3 experimental results concerning the oxidation of sulphide into sulphur and sulphate are described. The oxidation of sulphide to sulphate yields more energy than the formation of elementary sulphur and consequently the micro-organisms tend to form sulphate rather than sulphur. In environmental technology however, the formation of the non-soluble sulphur is preferred. It was shown that at sulphide loading rates of up to 175 mg S 2-.L -1.h -1complete conversion of sulphide into sulphur only proceeds if a stoichiometrical amount of oxygen is supplied, that is 0. 5 mol of oxygen per mol of sulphide. At higher oxygen to sulphide consumption ratios increasing amounts of sulphate are formed, even when the oxygen concentration remains below 0. 1 mg .L -1. This value is in the range of the lower detection-limit of the currently available oxygen sensors which means that these probes are not suited to the accurate control of the oxygen dosage. An appropriate alternative for the oxygen measurements is the application of the redox-state of the solution. Although the redox-potential is a so called 'mixed-parameter', which means that its value is determined by several dissolved compounds, e.g. sulphide, thiosulphate, oxygen and maybe also certain 'unknown' compounds, it has been shown that a linear relationship exists between the sulphide concentration and the redoxpotential. According to Eckert sulphide-ions have a much higher current exchange density than oxygen-ions which means that the electron exchange with the platinum electrode surface is much higher for sulphide than for oxygen. The measured redox-potential is therefore kinetically determined rather than thermodynamically. The optimal redox range for sulphur formation is -147 ± 5 mV/H 2 at a temperature of 30°C and pH 8.

    Dynamic experiments conducted in a fed-batch reactor revealed that the organisms are capable of switching between sulphur and sulphate formation within 0.5 h. This is far below the maximum doubling time of e.g. T hiobacillus O and T hiobacillus denitrificans, indicating that one metabolic type of organism can perform both reactions. Sulphide auto-oxidation primarily leads to the formation of thiosulphate. Its presence was recognized immediately after an increase of the sulphide loading rate during experiments conducted in a continuous flow reactor. In such a situation the biological oxidation capacity obviously becomes a limiting factor.

    In order to develop an appropriate sulphur removal step, the physico-chemical properties of biologically produced sulphur particles had to be known. Steudel et al. encountered the presence of long-chain polythionates ( -S0 3 -S n -S03-) in a sulphur dispersion formed by acidophilic Thiobacillus ferrooxidans species. They formulated a 'vesicle-model' to describe the appearance of these sulphur particles. In such a vesicle the orthorhombic sulphur crystals are included within a network of long- chain polythionates. Synthetically formed 'LaMer' sulphur, which is formed by the acidification of a sodium thiosulphate solution, belongs also to this vesicle model. In more recent papers, Steudel applies the vesicle-model to all types of biologically produced sulphur, formed by e.g. neutrophilic thiobacilli and phototrophic chromatiaceae species. However, electrophoretic mobility measurements and flocculation experiments, as described in Chapter 4, show a clear difference between sulphur originating from our reactor system and 'LaMer' sulphur. Since the sulphur particles in our system are formed under slightly alkaline conditions, i.e. pH 8, they don't belong to the 'vesicle' model. Polythionates were reported to be stable only under acidic conditions. Steudel attributes the hydrophilic character of the biologically produced sulphur to the presence of negatively charged sulphonic-groups, whereas we have evidence that (bio)polymers are attached to the sulphur core. From dynamic light-scattering measurements it can be seen that the particle diameter reduces at increasing salt concentrations which is indicative of an inward migration of an adsorbed layer. These (bio)polymers very likely contain charged groups, such as carboxylic, phosphate and ammonium groups which give the sulphur its overall hydrophilic carboxylic, phosphate and ammonium groups which give the sulphur its overall hydrophilic character. X-ray measurements of freshly formed sulphur particles indicate the presence of orthorhombic sulphur crystals (S 8 ) which are known to be hydrophobic. These crystals are therefore present in the inner-part of the sulphur particles. The negative surface charge of the particles can be measured by potentiometric titrations. Results of such titration experiments are described in Chapter 5. The point of zero charge ( pzc ) was determined at pH 5.8. At higher pHvalues the surface becomes more negatively charged whilst at pH-values below 5.8 a positive charge was measured. The pzc does not however correspond with the iso-electrical point ( iep ), i.e. the pH at which the electrophoretic mobility is zero. The iep is located at a pH below 3. A possible explanation is that within the adsorbed polymer layer charge distribution occurs. Although at pH 5.8 the overall surface charge is zero, the charge of the outside of the polymer layer may be slightly negative, as follows from the electrophoretic mobility measurements, while the charge of the inner polymer side is more positive. This positive charge is attracted to the S 8 nucleus because of its high electron density.

    In this study it was shown that biologically produced sulphur particles have the ability to aggregate into larger clusters, particularly at high sulphide loading rates. However, increasing salt concentrations lead to a deterioration of the aggregation process, indicating that not only DLVO-interactions are involved but also factors such as entrapment of sulphur particles within the biomass/sulphur film and crystallisation of the elemental sulphur particles attribute to the sulphur aggregation. The effect of certain well defined polymers was investigated in order to improve the understanding of the effect of certain complex dissolved polymers on the sulphur-aggregation such as tannins, humic acids or additives used in the paper industry (Chapter 5). It was found that longchain polymers especially affect the sulphur-aggregation detrimentally. These compounds are dissipated from the water-phase and adsorb onto the sulphur particles. In the case of approaching sulphur particles covered with these long-chain polymers approach, an increased entropy results, leading to lateral repulsion. This then hampers the aggregation of the sulphur particles. Similar observations have been made for cationic polymers but not for anionic polymers. Besides chemical factors, physical factors also play an important role in the formation of a well-settleable sulphur sludge. Fluid shear forces disintegrate the sludge. For this reason we developed a new reactor-type for sulphide oxidation, i.e. an expanded sludge bed reactor. In this reactor, shear-forces due to aeration of the reactor suspension are avoided (Chapter 6). In this new reactor type, sludge particles are formed which have an average size of about 3 mm and a mean sedimentation velocity exceeding 25 m .h -1. The sulphur content of the sludge amounted to 92% whilst the rest fraction presumably consists of active biomass, as follows from aerobic activity measurements. Because biomass is immobilized within the sludge high loading rates are achieved, viz. 14 g HS .L -1.d -1whilst only 6 g HS .L -1.d -1could be obtained for a free cell suspension. The maximal applicable sulphide loading could indeed be higher but in the experimental assembly the recirculating flow, necessary for oxygen suppletion, reached an excessive level resulting in extreme liquid upward velocities. As a consequence, wash-out of biomass occured. Under the condition that fatty acids are present in the influent, such as acetate and propionate, anaerobic conditions within the sludge prevail, leading to the reduction of sulphur into sulphide.

    Representative sampling of sulphuric acid droplets; theoretical evaluation of possibilities.
    Hofschreuder, P. - \ 1996
    Wageningen : Agricultural University Wageningen - 36
    colloïden - aërosolen - dispersie - meting - luchtverontreiniging - meteorologische instrumenten - bemonsteren - monsters - analyse - chemie - zwavelzuur - colloids - aerosols - dispersion - measurement - air pollution - meteorological instruments - sampling - samples - analysis - chemistry - sulfuric acid
    Advective - dispersive contaminant transport towards a pumping well
    Kooten, J.J.A. van - \ 1996
    Agricultural University. Promotor(en): J. Grasman; M. de Gee. - S.l. : Van Kooten - ISBN 9789054854722 - 120
    milieu - grondwater - watervoerende lagen - computersimulatie - simulatie - simulatiemodellen - waterkwaliteit - verontreinigingsbeheersing - grondwaterverontreiniging - bescherming - grondwaterstroming - modellen - permeabiliteit - gesteenten - colloïden - dispersie - pompproeven - environment - groundwater - aquifers - computer simulation - simulation - simulation models - water quality - pollution control - groundwater pollution - protection - groundwater flow - models - permeability - rocks - colloids - dispersion - pumping tests

    In this thesis we describe an analytical approximation method for predicting the advective- dispersive transport of a contaminant towards a pumping well. The groundwater flow is assumed to be stationary and essentially horizontal. Due to dispersion contaminant transport is a stochastic process. We derive approximations for the arrival probability (or fraction) of particles at a well, for the mean and variance of the arrival time and for the arrival time distribution at a well. The advective flow yields first order approximations. The effect of longitudinal dispersion is included by expanding the first and second moment of the arrival time in power series of the longitudinal dispersion coefficient. Transversal dispersion only plays a crucial role near the separating streamlines bounding the catchment area of a well. Its effect is analyzed locally with boundary layer techniques. The incorporation of linear equilibrium adsorption and first order decay is rather straightforward. The asymptotic approximations are compared with the results of random walk simulations.

    A self-contained part of this thesis is devoted to the transport of a kinetically adsorbing contaminant. We show that once the transport of a non-adsorbing contaminant has been computed, the effect of first order kinetics can be incorporated naturally by utilizing a stochastic description of the residence time of particles in the free phase.

    The results of our research have been implemented in the software package ECOWELL. The input of ECOWELL consists of a head field generated with a numerical flow model. The technical documentation of ECOWELL is part of this thesis. The use of ECOWELL is demonstrated in a case study.

    Adsorption of water-soluble polymers onto barium titanate and its effects on colloidal stability
    Laat, A.W.M. de - \ 1995
    Agricultural University. Promotor(en): G.J. Fleer. - S.l. : De Laat - ISBN 9789054854265 - 150
    colloïden - dispersie - polymeren - colloids - dispersion - polymers
    Ceramic products are usually made from powders which are processed into a green body, with a shape dictated by the final product. Organic binders are used to give the green product sufficient mechanical strength. A sintering process at high temperature converts the green body into the final ceramic product. In electronic ceramics, a high density and a homogeneous microstructure are required to obtain high quality products. For that purpose solid state sintering, in which no liquid phase is present, is applied. The result of the sintering process is highly dependent on the structure of the green body. Small pores will disappear during sintering but large ones will remain, resulting in a lower quality.

    The ideal ceramic powder has small (submicron) particles with a narrow size distribution and no hard agglomerates. Unfortunately, hard agglomerates often occur in ceramic powders. Milling of the ceramic powder in a liquid is often used to break down hard agglomerates. Voids or holes in the ceramic product can be due to these hard agglomerates or due to inadequate processing of the powder, which leads to a green body with many large pores.

    The study of the properties of powders dispersed in liquids is a branch of colloid chemistry. By using colloid chemical methods, control over the particle interactions can be achieved, which then allows the production of dense and homogeneous green bodies.

    Van der Waals attraction between particles of the same composition may cause flocculation. A dispersion with such a flocculated material will give a highly porous, low density green body. Roughly speaking, two methods are available to make the particles repulsive, allowing the production of highdensity homogeneous green bodies. The first is the adsorption of ionic species onto the surface of the particles. The particles will then repel each other electrostatically. The second method is to adsorb polymers onto the surface of the particles. If two particles with adsorbed polymer layers approach each other, the polymer concentration in the contact region between the particles is increased. The higher osmotic pressure in that region leads to repulsion. This so-called steric stabilisation can be quite effective if the adsorbed polymer layer has a sufficient thickness. The repulsion should start at a particle separation where the Van der Waals attraction still is weak.

    Adsorbed layers with sufficient thickness can be made by adsorption of homopolymers, provided the molecular weight and the coverage are high enough. Long tails then develop due to crowding on the surface. However, block copolymers with only one adsorbing block can be much more effective. Very thick adsorbed layers occur if the non-adsorbing block is highly stretched, a situation that occurs at a suitable ratio in length between the adsorbing and non-adsorbing blocks.

    Clearly, the adsorption of polymers and their conformation in the adsorbed layer are crucial in steric stabilisation. In this thesis the adsorption of polyvinyl alcohol (PVA) and polyacrylic acids salt (PAAS) onto BaTiO 3 are studied, with the objective to make sterically stabilised. dispersions in an aqueous environment. BaTiO 3 is an important material for electronic ceramics; it is used in the manufacture of capacitors and in resistors with a positive temperature coefficient.

    All adsorption experiments are analysed by Size Exclusion Chromatography (SEC). Using SEC, not only can the adsorbed amount of polymer be determined but also the fractionation in chain length upon adsorption. Even the separate adsorbed amounts from mixtures of polymers can be analysed. The principles of SEC, its possibilities, and the methods and equipment used, are presented in Chapter 2.

    In Chapter 3, the adsorption of PVA and PAAS on BaTi03 is studied. Adsorption of PVA occurs over the whole molecular weight range including short chains. The rounded shape of the adsorption isotherm indicates competition between chains of different length, pointing to preferential adsorption of the longer chains. This contradiction is probably due to the nature of PVA, which can be considered as a copolymer with short polyvinyl acetate blocks and branches distributed over the chain. Moreover, the fraction of acetate groups depends on the chain length. Accordingly, PVA must be considered as a mixture of several polymers with slightly different chemical compositions. Pure homopolymer adsorption behaviour cannot be expected with such a polymer. The adsorption of PAAS results in a rather peculiar fractionation behaviour: an intermediate molecular weight fraction adsorbs preferentially. This phenomenon is analysed in more detail in Chapters 4 and 5. In the adsorption onto BaTiO 3 from mixtures of PAAS and PVA, no adsorption of PVA occurs if enough PAAS is present to cover the surface of the particles completely. On the other hand, pre-adsorbed PVA cannot be displaced from the surface by addition of PAAS afterwards. Mixed adsorbed layers are found in this situation.

    The typical molecular weight fractionation with PAAS is studied in more detail in Chapter 4. Several PAAS samples with different molecular weights are used. It is shown that the molecular weight distribution (MWD) of the polymer has a significant influence on the result. The electrostatic repulsion between adsorbed chains on the surface and chains in solution prevents polymer chains above a certain length from reaching the surface. If the whole MWD is below this critical value, the longest chains adsorb preferentially, which is the expected behaviour for homopolymer adsorption. If the critical value lies within the MWD of the PAAS, the longer chains cannot participate in the exchange process, leaving an intermediate fraction adsorbed on the surface. PAAS samples with a MWD above the critical value and with only a few short chains show adsorption over a wide molecular weight range. Increasing the salt concentration of the solution decreases the electrostatic barrier, allowing longer chains to reach the surface. The preference in adsorption is then shifted to higher chain lengths.

    In Chapter 5 we report on the kinetics of the adsorption of two PAAS samples. The adsorbed amount and molecular weight fractionation change relatively quickly over the first two days and more gradually over a longer period. At low salt concentrations changes occur for up to 24 days. Theoretical calculations predict a depletion layer with a minimum in polymer
    concentration that strongly depends on the chain length of the PAAS. This theoretical result is in line with the chain-length dependence of the adsorption kinetics and qualitatively explains the fractionation effects. The experiments show that part of the adsorbed short chains is not exchanged by larger ones, but remains on the surface. At low salt concentration, the amount of adsorbed short chains even increases with time. An adsorption model is postulated in which the packing and the rearrangement of the adsorbed chains on the surface depend on the chain length and the salt concentration. The short chains are expected to fill the gaps between the larger ones.

    In Chapter 6 several PVA-based copolymers are tested for their ability to sterically stabilise BATiO 3 . Regular PVA is shown to be ineffective. The results obtained in Chapter 3 make it possible to define potentially suitable steric stabilisers for BaTiO 3 based on PAAS and PVA. The first choice is a block copolymer with PAAS as the anchor block and a PVA block as the stabilising moiety. Three types, with different ratios in block length, were evaluated. Each of these proved to be suitable for stabilisation. Random copolymers of PVA containing a small fraction of carboxylic acid are a possible alternative. Seven of these types were tested; only two were successful. An evaluation of the stabilising mechanism showed pure steric stabilisation with a block copolymer and combined steric and electrostatic stabilisation with one of the random copolymers. Green bodies with an improved homogeneity could be made with both the block and random copolymers, provided steric or electrosteric stabilisation is realised.

    Finally, in Chapter 7, two of the ineffective random copolymers are compared with one of the successful types. In addition, one of the block copolymers was included for comparison. The chemical compositions were studied with IR spectroscopy, and SEC was used for comparing the MWDs and for studying the adsorption behaviour. The IR analysis showed deviating chemical compositions for the ineffective random copolymers. One of these has lactone groups in the chain while the other one has a very high acetate content. SEC analysis showed a significantly lower chain length for both ineffective PVA random copolymers in comparison with the one suitable for steric stabilisation. This is the most probable reason for the ineffectiveness of these random copolymers. With short chains the steric layer thickness is too low for steric stabilisation. The block copolymer has an average molecular weight comparable to the stabilising random copolymer. Moreover, the longest chains of the block copolymer adsorb preferentially, probably resulting in a thick adsorbed layer. In the fractionation upon adsorption of the block copolymer there is no further increase in the relative amount of adsorbed long chains above a certain surface coverage. Further increase in adsorption above this coverage is possible if an increased stretching of the adsorbed chains occurs at equal relative amounts of the various chain lengths.

    In this thesis it is shown that the design of suitable block copolymers for steric stabilisation of particles can be based on adsorption experiments with the separate homopolymers. The chemical composition of stabilising random copolymers can be derived from the same experimental results. Moreover, it is shown that the adsorption kinetics of polyelectrolytes is highly influenced by the electrostatic barrier between the chains in solution and the surface of the particles.

    Reversibility and mechanism of bacterial adhesion.
    Rijnaarts, H.H.M. ; Norde, W. ; Bouwer, E.J. ; Lyklema, J. ; Zehnder, A.J.B. - \ 1995
    Colloids and Surfaces. B: Biointerfaces 4 (1995). - ISSN 0927-7765 - p. 5 - 22.
    corynebacteriaceae - pseudomonas - microbiële afbraak - chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - corynebacteriaceae - pseudomonas - microbial degradation - chemistry - colloids - adsorption - surfaces - surface chemistry
    The isoelectric point of bacteria as an indicator for the presence of cell surface polymers that inhibit adhesion.
    Rijnaarts, H.H.M. ; Norde, W. ; Lyklema, J. ; Zehnder, A.J.B. - \ 1995
    Colloids and Surfaces. B: Biointerfaces 4 (1995). - ISSN 0927-7765 - p. 191 - 197.
    micro-organismen - biochemie - fysiologie - microbiële fysiologie - microbiële afbraak - chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - microorganisms - biochemistry - physiology - microbial physiology - microbial degradation - chemistry - colloids - adsorption - surfaces - surface chemistry
    Electrochemical metal speciation in colloidal dispersions
    Wonders, J.H.A.M. - \ 1995
    Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Wonders - ISBN 9789054854708 - 88
    chemische speciatie - zware metalen - colloïden - elektrochemie - chemical speciation - heavy metals - colloids - electrochemistry

    The term "heavy metals" is connected with toxicity. They form strong complexes with enzymes, other proteins and DNA in living organisms, which causes dysfunctioning and hence poisoning. In combination with the uptake mechanism of the organism, speciation of heavy metal determines the bio-availability of heavy metals. In the environment, heavy metals are complexed by soil particles or molecules of organic and inorganic origin. This thesis deals with the speciation and the binding characteristics of heavy metals. Since complexation of heavy metals with soil particles is far too complex because of the wide range of different particles, this investigation is restricted to binding to a model system. The model system consists of polystyrene latices with and without a hydrophilic polymer shell. The surfaces of these latices contain negatively charged surface (shell) groups which can act as metaI-complexing agents. The binding can be investigated using various types of voltammetric techniques (Chapter I). To study metal binding, we first determined the amounts and types of surface groups present on the latices using potentiometry and conductometry (Chapter II). The polystyrene latex without shell showed a very high density of mainly weak, carboxylic groups on the surface. The surfaces (and shell) of the core-shell latices consist of a fraction strong acids (sulphonics) and a fraction of weak acids (carboxylics). Their shells and surfaces contain a lower total amount of groups than the polystyrene latex without shell. All conductometric results are qualitatively in agreement with those obtained by potentiometry, although the conductometric data appear to be more accurate. Potentiometry using potassium hydroxide, followed by a titration using nitric acid, was performed on one core-shell latex, indicating reversibility. During the titration with KOH, surface groups in the shell migrate to the surface. This effect is reversible. For one core-shell latex, potentiometric studies were carried out at different concentrations of supporting electrolyte (potassium nitrate). As expected, the pH increases more the lower the ionic strength during a titration. The total amount of titratable surface groups increases with higher concentration of supporting salt.

    As a following step, the metal complexes formed were characterized (Chapter III). voltammetric experiments, such as Cottrell type experiments, with all core-shell latices studied, show the formation of labile zinc(II) and cadmium(II) ion complexes at very low metal-to-site ratios in the time scale of pulse voltammetry. This means that the residence time of the metal ion in the complex form is very small compared to the pulse time. The application of the voltammetric model of de Jong et al. for dissolved complexes is succesfully used for the analysis of the binding of metal ions by colloidal particles. At a decreased metal to ligand ratio, the complexes formed were still labile, but their stability constants were slightly higher. Perhaps there is a minority of strong complexing surface groups, due to clustering or impurities in the shell, resulting in different affinities for metal ions. The metal/carboxylate surface complexes of the highly charged latex lose lability at high degree of dissociation. Also, stability constants obtained from the normalized current diverged from those obtained from the potential shift, with higher stability constants for the latter one. Some aspects of this discrepancy are discussed. The calculation of the kinetics of the lead-carboxylate complexes using the lability criterion of de Jong et al . shows that these complexes are marginally labile.

    Chapter IV deals with the characterization of surface groups by voltammetric titration, which is more complex than often assumed. This chapter tackles some of the methodological pitfalls which can be easily overlooked. Further, we estimated the amount of cadmium-complexing surface groups of some latices. The (complete) titration curves for all latices are regularly shaped. At the very onset of the titration curves complexes with larger binding constants were formed. This is probably due to the heterogeneity in surface groups described above. A procedure in which a regression line is computed using the diffusion coefficient of the latex metal complexes, can be used in the analysis. This procedure also provides one of the checks whether or not a metal complex is labile. The cadmium(II)-complexing capacity of the latices increases parallel to the fraction of carboxylic groups. Assuming a 1:2 binding ratio, roughly 30% of the sulphonate groups and 80% of the carboxylate groups bind cadmium(II) It seems that charge compensation plays a major role. Since the complexes formed by the polystyrene latex with a very high density of carboxylic groups only are not labile, the data for this latex were treated as if its surface sites would form inert complexes. An impression about the error of this treatment can be given; it seems rather small, just a few percent, due to the low diffusion coefficient of the latex particles.

    On the basis of potentiometric titrations at varied supporting electrolyte concentrations, we applied Donnan and Donnan-derived models by Ohshima and Kondo to describe the proton binding using the potential in the shell of a latex in Chapter V. In addition, the cadmium-binding properties of a core-shell type of latex were determined using differential pulse polarography. The assumptions in the shell potential model used are: the shell has a constant thickness independent of the ionic strength, the relative dielectric permittivity coefficient is 80, the degree of dissociation is constant over the shell and the site distribution is homogeneous. These assumptions did not affect the description of proton binding to a core-shell latex. Donnan's approach describes reasonably well the proton binding on the surface groups of the core-shell latex coded AOY5. Ohshima's model refines this description, by taking a Poisson-Boltzmann distribution of ions near and in the shell into account. This is an improvement. It seems that the potential correction based on the (indifferent) salt concentration is a major parameter for the binding of protons. The logarithm of the intrinsic cadmium binding constant (extrapolated to a shell charge of zero) is 1.0-1.2 for the carboxylic groups, comparable to corresponding bulk values for various organic cadmium-carboxyl complexes.

    Adsorption of charged diblock copolymers : effect on colloidal stability
    Israels, R. - \ 1994
    Agricultural University. Promotor(en): G.J. Fleer, co-promotor(en): F.A.M. Leermakers. - S.l. : Israels - ISBN 9789054852315 - 100
    adsorptie - sorptie - kunststoffen - industrie - colloïden - dispersie - macromoleculaire stoffen - adsorption - sorption - plastics - industry - colloids - dispersion - macromolecular materials

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

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

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

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

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

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

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

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

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

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

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

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

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

    Studies on the incorporation of lipase in synthetic polymerisable vesicles
    Mosmuller, E.W.J. - \ 1993
    Agricultural University. Promotor(en): H.C. van der Plas; J.F.J. Engbersen. - S.l. : Mosmuller - ISBN 9789054851172 - 121
    colloïden - dispersie - carboxyl ester hydrolasen - tannase - choline esterase - triacylglycerol lipase - synthese - organische verbindingen - colloids - dispersion - carboxylic ester hydrolases - tannase - cholinesterase - triacylglycerol lipase - synthesis - organic compounds

    This thesis describes studies on the suitability of synthetic polymerisable vesicles for the incorporation and stabilisation of lipase for the bioconversion of organic chemical compounds.

    In chapter 1 , some characteristics are reviewed of hydrolytic enzymes, and more specific those of lipases. In chapter 2 an overview is presented of the features and properties of surfactants and vesicles.

    In chapter 3 , the incorporation is described of lipase from Candida cylindracea (CCL) into polymerisable positively charged dialkylammonium bromide surfactant vesicles.

    Before incorporation the lipase has been purified and characterised. The enzyme has a molecular weight of 58.5 kD and an isoelectric point of 4.1; the pH optimum is broad, ranging from pH 4 to 6 and the optimal temperature is 45°C

    The synthesis of several polymerisable surfactants and the preparation of nonpolymerised and polymerised vesicles from these surfactants are described. The vesicle systems were characterised in terms of morphology (electron microscopy) and stability. It appeared that polymerised vesicles are considerably more stable than their nonpolymerised analogues.

    The enzyme was incorporated in the vesicle by the use of the dehydration- rehydration method or by incubation. In the latter case, trapping efficiencies are obtained of up to 100%. Activities of free and vesicle incorporated CCL are tested for three triglycerides: triacetin, tributyrin and tricaprylin and for 2,4-dinitrophenyl butyrate. Enzyme activity is lowest in homogeneous mixtures (triacetin and relatively low concentrations of tributyrin) and highest in heterogeneous mixtures (tricaprylin and relatively high concentrations of tributyrin and 2,4-dinitrophenyl butyrate). Incorporation of the enzyme in vesicular systems is advantageous for the activity, especially in homogeneous reaction mixtures, due to the presence of hydrophobic sites of the vesicles. Moreover, in the case of the production of insoluble fatty acid (caproate), inhibition by the acid is suppressed.

    The influence of several surface active additives is tested on the activity of lipase. Vesicles have a positive influence on the activity, whereas positively charged surfactant addenda act as inhibitors. In the case of tricaprylin assays, the positively charged surfactant addenda increase enzymatic activity.

    In addition, the sensitivity for tryptic digestion of free and incorporated CCL is compared. Free CCL is readily inactivated, whereas incorporated enzyme is protected from proteolytic degradation.

    In chapter 4 the stability of vesicle incorporated Candida cylindracea lipase is described.
    For this purpose, the enzyme was incorporated into vesicles of the polymerisable zwitterionic surfactant bis[2-(pentacosa-10,12-diynoyloxy)ethyll-2-aminoethanesulfonic acid WAS). Vesicle systems of BPAS were characterised
    in terms of morphology (electron microscopy) and stability. Polymerisation of vesiculated BPAS surfactants does not alter the vesicle morphology. Polymeric vesicles are considerably more stable than the monomeric analogues. CCL incorporated into the vesicle membrane by the incubation method remains fully active; especially in homogeneous assay mixtures the vesicle incorporated enzyme shows an increased activity when compared to the free lipase. The stability of free and incorporated lipase was determined by measuring the residual activity of the various systems when mixed with ethanol (50% v/v) or 2-(n-butoxy)ethanol (37.5% v/v), at 50°C and 60°C and in the presence of the proteolytic enzyme trypsin. In all cases the vesicle incorporated enzyme displays an increased stability against denaturating conditions.

    The interaction of lipase from Candida cylindracea with positively charged polymerisable surfactant vesicles was studied by the use of steady state fluorescence techniques. The results of these studies are described in chapter 5 .
    The phase transition of vesicles composed of nonpolymerised and polymerised N- allylbis[2-(hexadecanoyloxy)ethyllmethylammonium bromide was determined by measuring the change in fluorescence anisotropy of the membrane probe diphenylhexatriene. The phase transition temperature for nonpolymerised vesicles is 49°C and for the polymerised analogues 45°C. Fluorescence anisotropy and energy transfer measurements were used to demonstrate that Candida cylindracea lipase is readily incorporated into the hydrophobic bilayer of the vesicle. By using an interfacial membrane probe (trimethylammonium diphenylhexatriene) and an internal membrane probe (diphenylhexatriene), it could be determined that the lipase is incorporated more efficiently into the nonpolymerised vesicles, and that the penetration of the enzyme into the bilayer is less deeply in the case of polymerised vesicles.

    In chapter 6 , a rapid and sensitive assay for the detection of lipase activity is described. The method is based upon the increase in absorbance at 360 nm due to the formation of the 2,4-dinitrophenolate anion during the enzymatic hydrolysis of 2,4- dinitrophenyl esters. Several esters with different acyl chain length have been tested. 2,4-Dinitrophenyl butyrate proved to be a suitable standard substrate. This substrate can be used in homogeneous reaction systems and in emulsified form. In the latter case, a correction can be made for absorbance changes due to clearance of the emulsion during hydrolysis by using a diode array spectrophotometer with internal referencing. The small reaction volume and the high extinction coefficient of the product makes this method suitable for assay mixtures of low substrate and low enzyme concentration.

    In chapter 7 the results from the preceding chapters are reviewed in a general discussion.

    Partial coalescence in oil-in-water emulsions
    Boode, K. - \ 1992
    Agricultural University. Promotor(en): P. Walstra. - S.l. : Boode - 159
    emulsies - oliën - water - colloïden - coagulatie - uitvlokking - emulsions - oils - water - colloids - coagulation - flocculation
    The influence of crystals on the stability against partial coalescence at rest and during Couette flow was examined in emulsions of saturated triglycerides in SDS- or caseinate solutions and in natural cream. Partial coalescence was characterized by determining changes in globule size distribution and fat content. In the absence of crystals emulsions were stable at rest and in Couette flow. At rest partially crystallized emulsions remained stable unless numerous large fat crystals were present or a temperature cycle was applied (= rebodying process). A theory was developed to explain this temperature controled phenomenon. In Couette flow considerable partial coalescence was observed if the fat network inside the globules was continuous. Due to a lack of liquid oil crystals were sticking out of the globule further, thereby increasing aggregation. Aggregation could be nullified within a few hours after clumping by changing the wetting properties, so that the fat crystals became preferentially wetted by the aqueous phase. Deaggregation could occur also in a flow field if the solid fat fraction had exceeded the optimum, which depended mainly on the properties of the fat and on the velocity gradient applied. A theoretical model was developed that accurately describes the course of the partial coalescence process up to the point where most of the fat creamed out of the emulsion, when warming it. The model is based on Smoluchowski's frequency equation and distinguishes between singlets and clumps with and without crystals. From the model it was deduced that the kind of fat, the solid fat content and the number of globules that contains crystals are the main factors that determine the instability of the emulsion globules.
    Double layer relaxation in colloids
    Kijlstra, J. - \ 1992
    Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Kijlstra - ISBN 9789054850458 - 138
    colloïden - dispersie - elektrostatica - colloids - dispersion - electrostatics

    The purpose of the present study is to improve our insight into the relaxation of the electrical double layer around particles in hydrophobic sols. A detailed knowledge of the relaxation mechanisms is required to explain the behaviour of sols under conditions where the double layer is perturbed. Such conditions are frequently encountered in colloid science; for instance when colloidal particles coagulate or when they are subjected to an external field as in electrokinetics.

    One of the appropriate electrokinetic methods to experimentally study the dynamic properties of double layers is low-frequency dielectric spectroscopy. Previous studies have shown that latices exhibit a large dielectric response. However, these results could not be quantitatively reconciled with either electrophoresis data or existing theory. To discriminate whether the disagreements were due to theoretical or experimental imperfections, dielectric data on inorganic sols were highly desirable. The major aim of this study is to provide such data and, where necessary, to improve the existing theory. The results are described and discussed in chapters 2-5.

    The stability of sols against coagulation is of crucial importance for their applications. In principle, hydrophobic sols are thermodynamically unstable; they tend to form aggregates due to the attractive Van der Waals forces. However, in many cases the rate of coagulation is slowed down by the presence repulsive electrostatic forces. These occur if double layers overlap.

    Coagulation is a dynamic process. Particles interact on a certain time scale during which the extent of double layer overlap varies. Consequently, the equilibrium double layer structure will be perturbed, inducing relaxation processes. In principle, the colloid stability depends on the relaxation time of the double layer, which may be well of the same order as the typical time scale of a particle collision. However, the knowledge about the influence of the relaxation processes on the coagulation rate was limited. Therefore, the second aim of this study is to improve that situation. We focussed our attention on those cases where the relaxation rate of the double layer is determined by the adjustment of the surface charge density, see chapter 7. Chapter 6 discusses a related topic.

    A short summary of the results and main conclusions of each chapter is given below.

    Chapter 2 gives a description of a newly constructed fourelectrode dielectric spectrometer, designed to measure the dielectric response (or complex admittance) of sols in the frequency range of approximately 500 Hz; to 500 kHz. A four-electrode design is developed to avoid problems related to electrode polarization and, at the same time, to-enable the use of an automatic frequency response analyzer. The device is suitable for fast and accurate data acquisition, the measurement of one complete spectrum taking a few minutes only. Furthermore, it is especially designed to measure frequency-difference spectra.

    In chapter 3 it is shown that the thin double layer theory for the electrokinetic properties of dilute colloids can be extended to include surface conduction, i.e. a conduction contribution by ions behind the plane of shear. The calculations show that the occurrence of surface conduction leads to a reduction of the electrophoretic mobility and to an increase of the static sol conductivity and the dielectric response. Moreover, it also follows from the theory that an unambiguous interpretation of only one type of experimental data, for example the electrophoretic mobility, is impossible if surface conduction occurs. To assess whether this is the case, one is bound to also measure either the static conductivity or the dielectric response of the sol. The comparison between theory and experiment has been made for literature data on latices. For polystyrene latices, the mobility and static conductivity can be well explained if surface conduction is taken into account. However, the extended theory is not able to provide a quantitative explanation of the extreme dielectric increment of latices.

    Chapter 4 provides experimental data on the low-frequency dielectric response of dilute aqueous hematite and silica sols of spherical particles as a function of pH, ionic strength and particle size. The pH-sensitivity of the dielectric responses of the two sols shows that this response is a function of the surface charge density. The particle size dependence of the characteristic relaxation frequency is in fair agreement with theoretical predictions. In contrast to the case of latices, the dielectric behaviour of both hematite and silica can be well explained by classical electrokinetic theory yielding reasonable values for the ξ potentials. However, these values are systematically higher than those obtained electrophoretically. This inconsistency indicates the occurrence of surface conduction within the plane of shear, a type of conduction not included into the classical theory. By using the theory as developed in chapter 3, a distinction can be made between the (mobile) counter charge within and that beyond the plane of shear. Application to the hematite and silica data shows that a large fraction of the (mobile) counter charge is located inside this plane. This fraction increases with increasing surface charge density.

    The experimental and theoretical framework developed in the previous chapters has been applied to a spherical coryneform bacterium suspension in Chapter 5 . According to the preliminary results, approximately 95% of the total (mobile) counter charge in the double layer of the bacterium is located behind the plane of shear, i.e. probably within the cell wall itself. Such a large surface conduction contribution inhibits the possibility to determine the ξpotential of bacteria by electrophoretic measurements only. In this respect. additional information is necessary. The present investigation shows that dielectric spectroscopy is a useful technique to obtain that information.

    Chapter 6 presents a model to calculate the electrostatic interaction between two colloidal spheres, accounting for their polarizabilities. Under conditions where the potential along the surface varies during interaction, for example under those as discussed in chapter 7, the polarizability of a particle affects the electrostatic repulsion. Results are presented for spheres interacting at constant surface charge density. The calculations clearly show how the electrostatic force decreases with the polarizability of the particle. The decrease becomes larger with stronger double layer overlap, whereas it is relatively insensitive to κa. This insensitivity is a consequence of tangential screening effects inside the particles. It is pointed out that for slowly coagulating sols of particles with a fixed surface charge density, the stability ratio W is sensitive to the polarizability of the particle.

    Transient deviations from the equilibrium surface charge density during the interaction of colloidal particles and their influence on colloid stability are discussed in chapter 7 . Such deviations cause the process of particle encounter to become a non- first-order Markov process, which complicates the analysis of colloid stability. Two methods are presented to calculate a modified colloid stability ratio, taking such deviations into account in an approximate way. These methods differ by their estimates for the time scale of the Brownian encounter and its dependence on the height of the energy barrier. Despite these differences, both methods show that double-layer dynamics can have major consequences for the stability ratio. However, the predicted dependences of the rate of slow coagulation on the particle radius of the two methods are different. This indicates that double-layer dynamics could explain the experimentally found insensitivity of the stability ratio to the particle size, provided the time scale of the encounter strongly increases with growing height of the energy barrier. However, this proviso is unlikely to be satisfied. A simple statistical analysis indicates that the time scale of any individual encounter should decrease with growing barrier height!

    This thesis presents experimental and theoretical work related to double layer relaxation of colloids. It is not only of academic interest but also of significant practical importance. The results provide an encouraging basis for further research in the field of electrokinetics and stability of hydrophobic colloids.

    Fractal aggregation in relation to formation and properties of particle gels
    Bremer, L.G.B. - \ 1992
    Agricultural University. Promotor(en): P. Walstra; B.H. Bijsterbosch; T. van Vliet. - S.l. : Bremer - 202
    gels - colloïden - coagulatie - uitvlokking - fractals - fractal meetkunde - gels - colloids - coagulation - flocculation - fractals - fractal geometry - cum laude

    The purpose of this study is to gain insight into the conditions determining whether small particles in a liquid are able to jointly occupy the total volume thus forming a gel network. In order to build a network the colloidal particles have to be 'sticky', unstable. In the unassociated state the particles move at random through the liquid due to collisions with solvent molecules. This movement, called thermal or Brownian motion or diffusion, depends on the temperature and on the size of the particles. By thermal motion particles may meet and subsequently stick, thus forming clusters or floes. This process is called perikinetic aggregation. Other transport mechanisms that lead to aggregation involve velocity gradients in the dispersion (orthokinetic aggregation) or sedimentation of the flocs.

    A model describing the formation of a gel out of aggregating floes is derived in chapter II. Floes that are formed by aggregation have in general a fractal geometry. This implies that repetitive levels of detail exist on all length scales between the size of the primary particles and the size of the floe. A floe is built up of smaller floes that, on their turn, are built of still smaller floes, etc. Each separate fractal floe has its own geometry, different from that of any other floe. However, all floes share a similar average structure characterized by a stochastic fractal nature, and are in this respect scale invariant. The efficiency with which floes fill the available space is expressed by the fractal dimensionality, D, which is the exponent in the power-law relation between the number of particles in a floe and the size of the floe. A low value of D implies a small number of particles needed to build up a floe of certain size, and thus a high spacefilling efficiency. In a three- dimensional system, D may attain values between 1 and 3. Computer simulations of the aggregation process yield D = 1.8 if all collisions lead to attachment (diffusion - limited cluster aggregation) and D = 2.1 if the sticking probability is very low (reactionlimited cluster aggregation). Rearrangement of the floes during aggregation results in higher fractal dimensionalities.

    Since D is generally smaller than 3 the volume fraction of the particles in a fractal floc decreases as the size of the floc increases. When the flocs jointly become space-filling a gel is formed; now the overall volume fraction of particles in the system equals the average volume fraction of particles in the flocs. This implies that fractal flocs have the ability to become space-filling at any volume fraction of primary particles if the flocs have enough space to grow large enough, i.e., if the container is large enough. In section 2.3 a model is derived that describes the relation between the average size of clusters in a gel and the overall volume fraction of the particles.

    Gels formed at high and low volume fractions will have similar geometric stuctures, be it on different length scales, if the relative size distribution of the clusters remains constant during the aggregation process. This is shown experimentally in section 4.3 where micro-graphs of gels at various volume fractions and magnifications are compared. Scale invariance enables the derivation of scaling relations between gel properties and the volume fraction of the particles or the length scale on which the gel is studied. Relations between the permeability and the correlation length versus the volume fraction, and the correlation function and the turbidity versus length scale (wavelength), are derived in section 2.4. These relations can be used to obtain the fractal dimensionality from experimental results.

    In chapter III materials and methods are described that have been used to make and study the gels. For the preparation of the gels it is essential that aggregation occurs under quiescent conditions, because velocity gradients may lead to rearrangement of the flocs into more compact clusters with a lower space-filling efficiency. Different methods of destabilisation that have been used to obtain quiescent aggregation, are described: 1) slowly warming up a dispersion that is stable at low, but unstable at higher temperatures, 2) acidification of a dispersion that is unstable at low pH by a slowly hydrolyzing acid precursor, and 3) addition of an enzyme that removes stabilizing compounds from the surface of the particles. The permeability of these gels was studied by measuring the flow rate, caused by a certain pressure gradient, through tubes in which a gel was constrained. The geometry of the network was studied by confocal scanning laser microscopy. A small spot inside the gel, that is provided with a suitable fluorescent label, is illuminated by a focused laser beam and the fluoresced light that stems from the spot is detected via a microscope in a photon multiplier. Many positions in the gel are scanned in this way and optical sections and three- dimensional images are obtained.

    In chapter IV results of the gelation and coagulation studies are described. At quiescent conditions dispersions of small particles may gel at extremely low volume fractions. Spherical, palmitate covered polystyrene particles (α= 35 nm) formed space-filling networks even at volume fractions below 0.1 %! Aggregation caused by electrolyte addition to polystyrene or haematite sols (coagulation) may also result in continuous networks. The mixing of the electrolyte and the sol leads presumably to rearrangement of the floes into more compact clusters. The volume fraction of particles that was necessary for gel formation turned out to be roughly 5 % at high NaCl concentrations. A relative measure for the coagulation rate is the initial rate of change of the turbidity after electrolyte addition. It is found that at some critical salt concentration, the ccc, this rate attains a maximum; here, the aggregation rate is diffusion limited. In the case of coagulation by NaCl, floes of polystyrene or haematite particles were relatively compact at the ccc. This resulted in high values of the turbidity plateau that an aggregating system approaches, and in small sediment volumes. At higher NaCl concentration the floes become more ramified. Presumably, high salt concentrations cause stronger inter-particle junctions and less rearrangement, leading to more ramified floes.

    The fractal dimensionalities obtained from permeability studies were 2.34 and 2.21 for acid casein and palmitate covered polystyrene gels, respectively (section 4.2). From the absolute value of the permeability the ratio between the radius of the clusters in the gel and their effective hydrodynamic radius was estimated to be 1.13. This value is substantially lower than expected from calculations on fractal floes obtained by computer simulation. The permeability of most gels did not change during ageing, indicating that no large scale rearrangements occur. However, the permeability of rennet-induced casein gels increased during ageing, because microsyneresis causes a coarsening of the gel.

    Microsyneresis occurs if a gel tends to synerese whereas shrinkage is impossible. The process leads to local condensation of the network and the formation of large pores elsewhere.

    Changes in rennet-induced casein gels during ageing are shown on micrographs in section 4.3. The mesh-sizes of fresh rennet induced casein gels and acid casein gels with the same casein concentration are similar, but after ageing the mesh size of rennet-induced gels increases dramatically whereas that of acid gels is constant. Close to the glass surface the density of the gel is relatively high because particles stick to the surface during aggregation. Sections were taken at a depth larger than the diameter of the clusters to ensure the observation of a bulk gel. The fractal dimensionality obtained from the relation between the correlation length in micrographs of acid casein gels and the volume fraction of the particles was 2.35. A similar value was obtained from the relation between the correlation function and the length scale. Results of turbidity measurements as a function of wavelength yielded a value of D for acid casein gels of roughly 2.3.

    Various models may be derived to relate rheological properties of particle gels to the volume fraction of the particles. In addition to the geometric structure of the network, the interactions between and rheological properties of the particles are important. In chapter V two models are derived for gels built up of fractal clusters. One model applies to type 1 gels in which the stress carrying strands in a gel gradually get stretched, e.g. due to microsyneresis. The other model applies to type 2 gels in which no large scale rearrangements occur after the gel point. Rennet induced casein gels and acid casein gels made by slowly warming up a cold (4 °C) casein dispersion of pH 4.6 to 30 °C) turned out to be type I gels, whereas uninduced casein and palmitate covered polystyrene gels were type 2 gels. At the same volume fraction, type I casein gels were much stiffer than type 2 gels but the strength, i.e. the stress at which fracture occurs, was roughly similar. The strain at which the gels fracture was larger for type 2 gels. The values of D obtained by applying the models to results on the shear modulus versus the volume fraction, were 2.24 and 2.36 for type I and 2 casein gels, respectively, and 2.26 for palmitate-covered polystyrene gels. The stiffness of all gels studied increased during ageing. Since permeability studies show that no large scale rearrangements occurred during ageing (except in rennet gels), the increase in stiffness must be due to an increase of the strength of the inter-particle junctions, e.g. due to some kind of sintering.

    The aggregation time of an unstable colloidal dispersion, defined as the time after which aggregation becomes visible, depends both on the bond formation rate and on the way the structure of the aggregates develops. The latter is not taken into account in the traditional theory of aggregation kinetics where only the bond formation rate is considered. In chapter VI it is shown that the structure of colloid aggregates has a large effect on the aggregation rate and an even larger one (up to several orders of magnitude) on the aggregation time. Approximate expressions for the aggregation time at different conditions are derived. Due to the intricacy of the subject it is mostly impossible to derive exact expressions. For many situations it is possible, though, to roughly predict the aggregation time. Complications, due to deviations from the ideal case of spherical, smooth, monodisperse particles at low volume fractions, are compiled and their effects on the aggregation rate and time are estimated. It is shown that small velocity gradients often cause a huge decrease of the aggregation time. Small velocity gradients occur also in 'quiescent' systems, e.g. due to convection.

    Fractal aggregates will always fill the entire volume if they are allowed to grow without being disturbed. Factors that can disturb the gelation. or change the gel structure, are described in chapter VII. It is argued that small velocity gradients are essential for the formation of networks at very low volume fractions because the floes forming the network are so large that the time needed for diffusion over a distance equal to the radius of the floe is much longer than the time needed to settle over that distance. Larger velocity gradients may cause compaction or breakup of the floes, thus hindering gelation. All systems studied in this thesis that gel at low volume fraction show some sintering or fusion, which may be ascribed to an increase in the number of bonds per junction. This may be a prerequisite for aggregation of colloidal particles leading to a gel at low volume fraction. n.

    Fundamentals of interface and colloid science. Volume I: Fundamentals.
    Lyklema, J. - \ 1991
    London : Academic Press - ISBN 9780124605251
    chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - chemistry - colloids - adsorption - surfaces - surface chemistry
    Electrostratic stabilization of suspensions in non-aqueous media
    Hoeven, P.C. van der - \ 1991
    Agricultural University. Promotor(en): J. Lyklema. - S.l. : Van der Hoeven - 179
    suspensies - emulsies - colloïden - suspensions - emulsions - colloids
    Concentrated suspensions of detergent powder solids in a liquid nonionic surfactant are considered for practical application as liquid detergent products. If no precautions are taken, upon storage the viscosity of such suspensions increases and the pourability drops because the suspensions are colloidally unstable. It has been found that after the addition of a small amount of dodecylbenzene sulphonic acid (DoBS-acid or HDoBS) good pourability is maintained on storage. All the phenomena observed with such suspensions suggest that the addition of DoBS-acid reduces coagulation and improves colloidal stability. It was hypothesized that the colloidal stability obtained is of an electrostatic nature. In a liquid non-aqueous medium this is unexpected. A study of the mechanism of stabilization is described in this thesis.

    After a general introduction to the topic in Chapter 1, in Chapter 2 we discuss the character of the interactions which play a role in nonionic suspensions. The used nonionics are condensates of long chain alcohols and 3 to 9 alkylene oxide units. The dispersed solids are sodium salts as are usually present in current detergent powders, or oxides. They are aggregates or agglomerates of smaller crystalline primary particles and consist of irregular spheroids. The solids, the liquid nonionics and the anionic acid have been characterized with respect to a number of properties, including the molecular and crystalline structure, specific density, specific surface area, porosity, axial ratio and water content. The refractive indices and dielectric constants of the liquids and solids are also measured. Elemental analysis of the supernatants of our suspensions is carried out by Atomic Absorption, by Plasma Emission and by X-ray Fluorescence Spectroscopy. Since analysis of the supernatants indicated only very limited dissolution of the solids, it is concluded that the suspensions are lyophobic. It is demonstrated that, when DoBS- acid is added to a suspension of sodium salts in nonionic, it is converted quantitatively into anionic NaDoBS.

    Sedimentation rates, sediment volumes and viscosities are important physical characteristics of concentrated nonionic suspensions; they reflect the interactions between the suspended particles. The interactions follow the DLVO-theory, meaning that they are governed by the balance between attractive and repulsive or 'stabilizing' forces.

    The literature on van der Waals attraction (energy and forces) between particles in suspension is discussed in Chapter 3. It shows that for particles in the micron-size range, geometrical parameters (differences in particle size, interparticle distance), retardation and surface roughness are of more importance than in colloidal systems, having smaller particles. This means that the van der Waals bonding energy obtained on approach is larger, but, as a function of increasing interparticle distances, it decays more rapidly.

    In the van der Waals attraction, material properties are reflected in the Hamaker constant. Hamaker constants for the inorganic crystalline solids considered in this study are not available in the literature. Therefore it was necessary to evaluate them theoretically. Two approaches have been applied, a macroscopic theory and a microscopic theory. In a comparison they gave identical results within a few tens of percent. For the crystalline detergent solids the constants have been evaluated from their dielectric constants and refractive indices. The results showed the Hamaker constants for the detergent solids (except activated Zeolite 4A) to exceed those of the nonionics, but to be lower than those of the metal oxides. The differences between the constants of crystalline detergent solids and those of nonionics are relatively small, implying that suspensions of detergent solid particles in nonionics can be made to relatively high volume fractions and can be stabilized easily.

    In Chapter 4 the electrostatic theory for interactions of particle pairs in suspension is evaluated for its applicability in non-aqueous media, using models of plates and spheres. For both models the conclusion is that, for the calculation of the repulsive energies and forces, approximated equations can be used. They result in repulsive energies, pressures and forces, which are in good agreement with those of exact computations at distances>10 nm, but underestimate the repulsions at shorter distances.

    DLVO energy and force curves have been constructed and demonstrate the dependence of the repulsion on five parameters that govern the behaviour, viz. the dielectric constant, the ionic strength, the electric surface potential, the Hamaker constant and the particle size. For our suspensions with surface potentials ≥20 mV, significant repulsions already develop at distances between 2 and 40 nm. The theoretical repulsions are much higher than the van der Waals attractions and cause much larger repulsive barriers than those usually reported for non- polar, nonaqueous media. They are expected to play a role in the colloidal stabilization of nonionic suspensions and to influence the resistance against coagulation under pressures at the bottom of sediments. Secondary minima are only a few kT at most and coagulation is only expected at the protrusion points of contact and at relatively high ionic strengths.

    Ionic strengths in HDoBS-stabilized suspensions in the nonionics Plurafac LFRA30 and Imbentin C91/35 are evaluated from the conductivity in the supernatants and from their respective limiting molar conductivities. The methodology is described in Chapter 5. It was found that in both nonionics the limiting molar conductivity was lower than predicted from the values in water assuming Walden's rule applies. The results indicate that solvation interactions of Na +and DoBS -ions in nonionics are stronger than in water and stronger in Imbentin than in Plurafac.

    In Chapter 6 the results of the electric and dielectric measurements have been given. It is shown that the dielectric constant of nonionic is increased by HDoBS. Taking this increase into account, the ionic strengths found can be satisfactorily explained from theory. Only at high HDoBS concentration and relatively high dielectric constants are the ion concentrations lower than theoretically predicted, a feature that could be due to the formation of 'molecular associates'.

    From the limiting conductivities, at HDoBS concentrations between 10 and 150 mM, the ionic strengths have been found to range from 0.05 to 4 mM in Plurafac and from 0.08 to 30 mM in Imbentin. These results demonstrate a weak dissociation of the NaDoBS electrolyte. However, the ionic strengths obtained are considerably larger than those in supernatants of unstable suspensions and are higher than ever reported in the 'non-polar' hydrocarbon media, commonly considered in non-aqueous studies. Liquid nonionic media have a dielectric constant between 5 and 12 and are denoted 'low-polar'. At these ionic strengths, and considering the enhancement of the dielectric constant by HDoBS, in the HDoBS concentration regime between 0.5 and 150 mM, Debye lengths range from 33 to I nm in Plurafac and from 13 to 1 nm in Imbentin, i.e. in the same range as in aqueous media.

    Electrokinetic (ζ-)potentials of particles of detergent solids suspended in nonionics, given in Chapter 6, are found to be a function of the HDoBS concentration. The surface potentials tend to level off at HDoBS concentrations as low as 0.5 % w/w (15 mM dm -3), to a maximum value ranging from +25 to +70 mV, depending on the nature of the solid and the nonionic liquid. Addition of water or of a crown-complexant (15-Crown-5), reduces the ζ-potential. The formation of positive surface charges can be explained from the dissociation of adsorbed HDoBS.

    Mechanical properties of concentrated non-aqueous suspensions are discussed in Chapter 7, including their relation to the electrostatic repulsion. Rheology is used to monitor the properties under dynamic conditions. The consistency, which quantifies the particle interactions and shear thinning index was derived from the Sisko model.

    Addition of HDoBS was found to have little or no influence on the high shear rate viscosity of nonionic suspensions. This viscosity is governed by hydrodynamic interactions, which are, in turn, determined by the viscosity of the nonionic phase, the volume fraction and the temperature. The nature of the solid also has an influence on the 'infinite shear' viscosity, probably due to variations in protrusion size, causing their effective volumes to be larger than the actual volume. Measurements of the intrinsic viscosity of sodium tripolyphosphate (STP) indicated that the particles of this substance are almost spherical.

    Low shear rate viscosities monitor effects of interparticle interactions. The consistency was found to be inversely proportional to the particle volume. Addition of HDoBS reduces the consistency. As with the ~-potentials, the main effect is already obtained from 0.5 % w/w HDoBS. In this respect the behaviour of the viscosity is correlated with that of the ~-potential of the particles. It is further found that the drop in the 'normalized' consistency has a direct relation to the electrostatic force. These results support the conclusion that the nature of the obtained stabilization is electrostatic. The correlation of the viscosity with the Péclet number further supports this conclusion. It shows that under shear HDoBS-stabilized systems can be considered as hard-sphere suspensions.

    Creep compliance measurements of suspensions of STP in Plurafac at high volume fractions demonstrated that at low shear stresses the interactions are completely elastic. Under those conditions, relaxation of the stress leads to almost complete recovery. The shear moduli derived from creep compliance, drop less steeply as a function of the volume fraction than predicted from the electrostatic repulsive barrier. It is possible that this difference is a result of secondary minimum coagulation by the particle protrusions.

    In static sediments the volume fractions can be measured as a function of height by γ-ray absorption. Measurements of γ-ray absorption shows that the particle concentration from top to bottom in a stable sediment shows a concentration gradient. For HDoBS-stabilized suspensions this gradient is more continuous, whereas in unstable suspensions, due to coagulation, it is very irregular. From these results the relations between the static pressure or the network modulus and the volume fraction are derived. Pressures show an exponential relation with the interparticle distances. With low levels of DoBS-acid the interparticle distances are larger than for high concentrations of HDoBS. These results are in agreement with the dependency predicted by electrostatic repulsion, although the experimental pressure drop as a function of distance is much smoother than that theoretically predicted. The experimental network moduli derived from the pressure-volume fraction relation also drop much more slowly than theoretically predicted. This may again be a result of secondary minimum coagulation occurring by the protrusions.

    The overall conclusion is that the suspensions under consideration are electrostatically stabilized with DoBS-acid as the charge-determining electrolyte.

    Statistical thermodynamics of block copolymer adsorption
    Evers, O.A. - \ 1990
    Agricultural University. Promotor(en): G.J. Fleer; J.M.H.M. Scheutjens. - S.l. : Evers - 140
    kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - plastics - industry - chemistry - colloids - adsorption - surfaces - macromolecular materials - surface chemistry

    The aim of this study was to develop a statistical thermodynamic theory for the adsorption of linear flexible block copolymers from a multicomponent solution. This has been accomplished by a more general derivation of the self-consistent field theory of Scheutjens and Fleer for adsorption of homopolymer from a binary mixture, introducing local segment potentials for any type of segment.

    In chapter 1 the statistical thermodynamic analysis for a multicomponent mixture (including block copolymers) near a surface is given in detail. Near the surface, a density gradient for every type of molecule is found due to spatial restrictions and mutual interactions between segments and between segments and the surface. Every individual segment is subjected to a local (segment) potential, which depends on the distance from the surface and on its chemical nature. We use a lattice model to evaluate the contact energies and the conformation count. The segment potential is derived from the maximum term in the canonical partition function. Like in the original derivation of Scheutjens and Fleer we maximize the canonical partition function with respect to the number of molecules in each particular conformation. However, to perform the necessary partial differentiations under the appropriate boundary conditions we apply the method of Lagrange multipliers. From the segment potentials we can calculate for every particular conformation its statistical weight as a multiple product of Boltzmann factors (one for each segment) and its contribution to the overall segment density profile. In Appendix III of chapter 1 a set of equations is formulated from which the segment potentials can be found in a self-consistent manner by standard numerical techniques.

    A number of results on the segment distribution of di- and triblock copolymers is given. Diblock coplymers are found to adsorb with the adsorbing block rather flat on the surface and the less or non-adsorbing block in one dangling tail protruding far into the solution. A comparison with terminally anchored chains shows overall agreement but also typical differences.

    In chapter 2 the physical background of the theory is briefly reviewed. Results on the adsorbed amount and the hydrodynamic layer thickness of adsorbed di- and triblock copolymers are given. We find a strong dependence of these parameters on the chain composition. When the total length and bulk solution volume fraction of a diblock copolymer are kept constant, a maximum is found in the adsorbed amount as a function of the fraction of adsorbing segments. The fraction of adsorbing segments corresponding to this maximum could be named "the optimal fraction"; it is found to decrease with increasing chain length, increasing bulk solution volume fraction, increasing surface affinity of the more strongly adsorbing block, and decreasing surface affinity of the weakly adsorbing block. From these results we have been able to relate in a simple way the adsorbed amount of an AB-diblock copolymer (where A adsorbs more strongly than B) to the adsorbed amount of an A-homopolymer of equal length. A linear relation is obtained between the adsorbed amount of AB-diblock copolymer (as compared with an A-homopolymer) and the block length ratio r B /r A . where r A and r B are the lengths of the A-block and the B-block, respectively. Usually, diblock copolymers form thick adsorbed layers, with a hydrodynamic layer thickness that depends strongly on the adsorbed amount. This thickness is of the order of 10 to 30 % of the length of the B-block. For BAB-triblock copolymers with adsorbing A-segments and non- adsorbing B-segments we find lower adsorbed amounts as compared to an AB-block copolymer with the same total number of A- and B-segments

    The interaction between adsorbed layers of block copolymers is examined in chapter 3. The calculation of the free energy of interaction is straightforward. We elaborate the concept of full equilibrium and that of restricted equilibrium for a multicomponent mixture. Full equilibrium refers to the case that all molecules in the mixture are free to diffuse out or into the gap between the surfaces. Hence, in full equilibrium all molecules have a constant chemical potential when the surfaces are brought closer. If one or more of the components are unable to leave the gap when the surfaces come closer we have a restricted equilibrium and the chemical potentials of those molecules will not be constant. Usually, the interaction between adsorbed layer of adsorbed diblock copolymers at full equilibrium is found to be repulsive. in contrast to the case of homopolymers where the interaction is always attractive. At full equilibrium, when the surfaces are brought closer, homopolymers desorb and form bridges resulting in attraction between the surfaces. Since diblock copolymers hardly form any bridges when the surface affinities of both blocks differ enough, no attraction is found at full equilibrium. For the same reason we find always repulsion in a good solvent when the amount of diblock copolymer is kept constant (restricted equilibrium). The onset of the repulsion increases with increasing adsorbed amount and with increasing length of the non-adsorbing block. The interaction curves at various lengths of the adsorbing- and non-adsorbing block could be scaled onto approximately one master curve. When the solvent quality for the non-adsorbing block becomes poor (χ>0.5). there is an attraction at large separation as a result of osmotic forces (phase separation), even at restricted equilibrium. In fact, adsorbed diblock copolymers behave like infinitely long homopolymer chains in solution, which phase separate when χis above 0.5. For ABA-triblock copolymers with adsorbing A-segments and non-adsorbing B-segments, we find attraction at not too small separations in a good solvent for the B-blocks, because now bridging is again possible: adsorbing segments are found at both extremities of the chains.

    This model has provided a detailed insight in the properties of adsorbed block copolymer layers and should be a useful tool for the development and optimization of experiments and products in which copolymer adsorption plays a role.

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