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Staff Publications

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

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

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

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

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    Bacterial adhesion
    Loosdrecht, M.C.M. van - \ 1988
    Agricultural University. Promotor(en): A.J.B. Zehnder; J. Lyklema. - S.l. : van Loosdrecht - 113
    micro-organismen - morfologie - cytologie - microbiologie - chemie - colloïden - adsorptie - oppervlakten - oppervlaktechemie - microorganisms - morphology - cytology - microbiology - chemistry - colloids - adsorption - surfaces - surface chemistry

    As mentioned in the introduction of this thesis bacterial adhesion has been studied from a variety of (mostly practice oriented) starting points. This has resulted in a range of widely divergent approaches. In order to elucidate general principles in bacterial adhesion phenomena, we felt it was necessary to start from a fundamental level i.e. using welldefined model systems. In our study colloid chemical principles are applied to microbial systems. Although both colloid chemists and microbiologists have investigated the behaviour of small microscopic particles, there has been only limited cooperation between them in the past. Nevertheless, this study reveals that such a cooperation can be very fruitful.

    After a general (Chapter 1) and a theoretical (Chapter 2) introduction, we deal in Chapters 3 and 4 with the relation between bacterial surface characteristics and adhesion to sulphated polystyrene (a hydrophobic, charged surface). The cell surface hydrophobicity and electrokinetic potential were determined by the contact angle measurement and electrophoresis, respectively. Adhesion increases with increasing bacterial hydrophobicity or decreasing electrokinetic potential. The effect of the electrokinetic potential increases with decreasing hydrophobicity. An interesting finding is the increase with growth rate in surface hydrophobicity of bacteria.

    In Chapter 5 we show that initial adhesion to sulphated polystyrene is reversible and can at least qualitatively be described by the DLVO theory for colloidal stability, i.e., in terms of Van der Waals and electrostatic interactions. From adhesion isotherms we found an adhesion Gibbs energy of -2 to - 3 kT per cell. This corresponds to calculations using DLVO theory that predict adhesion in the so-called secondary minimum, a case where no direct intimate contact is made between bacterium and surface. Finally, the implications of our findings for natural and (bio)technical processes are discussed.

    In Chapter 6 we report on the applicability of the DLVO theory for the interpretation of bacterial adhesion to glass and to more practical surfaces (Rhine river sediment and protein-coated surfaces). In all these cases adhesion could be interpreted in terms of the hydrophobicity and electrical properties of the surfaces.

    The possible influences of adhesion on bacterial activity are discussed in Chapter 7, in the form of a critical literature review. Despite the opinion regularly heard that there might be a direct influence of adhesion on bacterial physiology we have not been able to find any experimental evidence in support of this hypothesis. Different activities of attached and free cells are often due to changes in substrate transport (e.g. diffusion, desorption, or convective transport) or differences in hydrophobicity of active and resting cells. For the conversion of adsorbed substrates the dissolved concentration determines the conversion rate. With strongly adsorbing compounds the conversion can become desorption-limited, whereas non-desorbing compounds are often not degraded.

    In this thesis it is shown that application of colloid chemistry to microbial systems can lead to interesting new viewpoints. More specifically, the DLVO theory for colloidal stability was found to give a quantitative description of the initial stage of bacterial adhesion both to model surfaces as in more applied situations (Chapters 5 and 6). Generally, in the studies dealing with interaction between bacteria themselves or between bacteria and surfaces electrostatic interactions are often neglected, despite the fact that this interaction is often desicive whether strong adhesion can occur or not.

    The insights derived from a colloid chemical approach can be used, as complementary to a more biological approach, in understanding the (auto-) immobilization of bacteria in natural and biotechnological systems, as e.g. in UASB- reactors.

    The experimental methods developed in this study may also be successfully applicable in other research areas. Due to the sensitivity of the contact angle and electrophoretic mobility measurements they can for instance be applied as a rapid screening
    method of new isolates or cell surface mutants. Especially with urface mutants the methods mentioned here are much faster than conventional biochemical or immunological methods.

    The contact angle and electrophoretic mobility measurements may also be useful for obtaining information on the structure of the outer part of the cell wall. In particular electrophoresis, at different pH and electrolyte strength, combined with chemical modifications of specific groups (e.g. -NH 2 groups) may be very powerful. Preliminary experiments with lipopolysaccharide mutants of Pseudomonads are very promising. For this and other applications it is necessary to improve the electrochemical characterization of bacteria, especially with respect to the influence of bacterial conductivity.

    Other areas in microbiology that may be successfully treated by colloid chemical theories concern firstly the biological availability of substances, in particular micro-pollutants, to bacteria. This availability is mainly determined by substrate adsorption to inert solid material and substrate transport through the cell wall and membrane. A second interesting field might be the relation between molecular composition and function or stability of membranes in different bacteria, or under different environmental conditions.

    Single particle optical sizing : aggregation of polystyrene latices by salt and polymer
    Pelssers, E.G.M. - \ 1988
    Agricultural University. Promotor(en): G.J. Fleer, co-promotor(en): M.A. Cohen Stuart. - S.l. : Pelssers - 158
    analytische methoden - coagulatie - colloïden - uitvlokking - spectraalanalyse - spectroscopie - optica - analytical methods - coagulation - colloids - flocculation - spectral analysis - spectroscopy - optics

    The subject of this thesis is the development of a Single Particle Optical Sizer (SPOS) which is capable of measuring in detail discrete particle size distributions in the colloidal size range. With this instrument we studied the aggregation of latices induced by polymer and salt, and found evidence for non-equilibrium flocculation.

    Chapter 2 is an inventory of the existing methods of measuring aggregation. A comparison is made with our SPOS instrument. The techniques are classified into three groups: classical, multi particle detection and single particle detection methods. Only very global information is obtained about the aggregation process with the classical methods. In the case of turbidity, only the initial rate of the total aggregation process can be obtained. Multiparticle detection methods are able to determine accurately a particle size in monodisperse samples (laser beat spectroscopy). For large spherical particles (d>1 μm) a particle size distribution can be measured (laser diffraction spectroscopy). With small angle light scattering an initial rate of the aggregation can be determined. Single particle detection methods are able to measure discrete particle size distributions. With electron microscopy very small particles can be individually sized. However this technique is rather tedious and unsuitable for the study of aggregation kinetics. With SPOS, also a single particle detection method, fast and reliable particle size and aggregate distribution can be measured as a function of time.

    The SPOS instrument operates on the principle of low angle light scattering. In order to determine the measurable size range of the SPOS, we present in chapter 3 numerical results of the light scattering intensity as a function of size, type, solvent and detection angle, as obtained with the Mie theory.

    In chapter 4, the design of the SPOS is described and several test experiments on the operation of the instrument are presented. In the instrument the particles are hydrodynamically focused into a very narrow stream, and they pass one-by-one through an elliptical laser focus. Upon passage, each of them emitts a flash of light which is detected by a photomultiplier and converted into an electronic pulse which is stored according to its intensity in a multichannel analyzer. The number of signals of each size can be displayed and renders a complete particle size distribution.

    Much attention is paid to the possible influence of the hydrodynamic forces in the instrument on the disruption of aggregates. We conclude that only for very weakly bond aggregates de-aggregation may occur before monitoring.

    In chapter 5 the instrument is used to study the coagulation process (aggregation induced by salt) of latex dispersions. We describe the preparation of the latices and determine the rate constants of three initial aggregation steps (singlet+singlet, singlet+doublet and singlet+triplet). This enables us to check the primary assumption of the Von Smulochowski theory which states that all these rate constants are the same. We measured a difference between the value of the determined rate constants. Furthermore we used three different mixing cells to study the effectivity of mixing and the influence on the aggregation rate.

    In chapter 6 we use the SPOS method to study the aggregation induced by polymer. The experimental results are on first sight rather surprising. In many cases the flocculation does not obey second order kinetics. Nevertheless, the data can be well understood if the dynamical aspects of the polymer adsorption are taken into account. From our experiments a clear distinction in behaviour was found between polymers in a relaxed or a non-relaxed state on the latex surface, leading to equilibrium or non-equilibrium flocculation, respectively. In the latter case the rates of polymer attachment and particle collision are faster than the rate of reconformation of adsorbed polymer. We propose a new model for polymer induced bridging flocculation which incorporates these two mechanisms and predicts the occurrence of these mechanisms as a function of particle concentration, molecular weight of the polymer, shear forces and double layer repulsion.

    Statistical thermodynamics of association colloids : the equilibrium structure of micelles, vesicles, and bilayer membranes
    Leermakers, F.A.M. - \ 1988
    Agricultural University. Promotor(en): J. Lyklema, co-promotor(en): J.M.H.M. Scheutjens. - S.l. : Leermakers - 149
    colloïden - dispersie - entropie - membranen - statistiek - thermodynamica - colloids - dispersion - entropy - membranes - statistics - thermodynamics - cum laude

    The aim of the present study was to unravel the general equilibrium physical properties of lipid bilayer membranes. We consider four major questions:
    1. What determines the morphology of the association colloids (micelles, membranes, vesicles) in general?
    2. Do the apolar tails of the lipids in the bilayer organise themselves more like matches in a box or rather like hot spaghetti in a pan?
    3. How does this membrane organisation depend on temperature?
    4. How do additives like surfactants or polymers interact with the bilayer?
    These four questions cover a wide range of topics currently subject to intensive research. Each one of them calls for a rigorous answer. We believed that it would be possible to design one single theory covering the whole field. The development of such a theory is undertaken in the present thesis.
    Recently, the statistical thermodynamics of homopolymers at interfaces has been worked out by Scheutjens and Fleer (SF). This theory is an extension of the Flory Huggins (FH) theory for polymers in solution in the sense that it allows for inhomogeneities in one dimension. In the other two dimensions a mean field, i.e., an average segment density, assumption is applied. One of the strong points of this theory is that, by using a Markov-type approximation, all possible conformations of the chains are considered with a minimum of computational effort. The SF theory can be extended to describe copolymers at interfaces.
    For well-chosen amphipolar molecules the theory is able to deal with local phase separation phenomena. Preliminary calculations on surfactant bilayers showed that the SF theory needed some modifications in order to be relevant to the four topics given above. The main reason for this is that for the very small surfactant molecules the Markov-type approximation is not very accurate. Five extensions of the theory are presented in this thesis:
    1. For the chain statistics the Markov-type approximation is extended to the so called rotational isomeric state scheme. This scheme prevents backfolding in chain sections of five consecutive segments. The improvement allowed us to adjust the stiffness of the chain as a function of temperature.
    2. The theory is generalised for arbitrary geometries. With this extension the polymorphism of association colloids could be studied.
    3. The theory is extended to account for branched chain molecules. This has been used to simulate lipid molecules with two apolar tails and one polar head group.
    4. In the SF theory the statistical weight of each conformation is found by Boltzmann statistics. The potential of each conformation depends on segment-segment interactions, hard core contact potentials, and the number of gauche bonds in the chain. A new weighting factor is introduced which accounts for the average orientation of the molecules. The statistical
    weight of a conformation is increased when its bond directions match with those of the surrounding molecules. With this molecular orientational field co-operative phenomena like crystallisation can be studied.
    5. Allowing for inhomogeneities in two dimensions enables us to study membrane-"protein" interactions.

    The properties of the theory with these new features are thoroughly examined in five chapters. A short summary of the results and main conclusions of each chapter is given below.

    Chapter 1 dealt with the morphology of association colloids. In this chapter we prove that the formation of micelles is a first order transition. However, the theoretical critical micelle concentration is not observed very sharply, because it is very low. We showed that, with Increasing concentration of bipolar molecules, the micelles first grow and eventually change their shapes. Lecithin-like molecules prefer lamellar aggregates over globular ones.

    In chapter 2 the rotational isomeric state scheme is presented and details of the statistics of branched chain molecules is given. We present an overview of the behaviour of the membranes as a function of the four energy parameters. There is no need to restrict the molecules to pre-assigned positions in the system. The membrane thickness adjusts itself. The equilibrium membrane is free of tension. Its excess free energy per surface area is very small. When fluctuations and long range Van der Waals attractions are neglected the excess free energy is essentially zero.

    Vesicle systems are studied in chapter 3. We show that the excess free energy of curvature per vesicle is constant for vesicles composed of one type of lipid, irrespective of the radius of the vesicle. This remains true for bi-lamellar and hence for multilamellar vesicles. We show that as a rule, the thicker a membrane is the more energy it costs to bend it. Adding surfactants to a system containing vesicle is disastrous for the vesicle structures. Increasing the surfactant/ lipid ratio causes the vesicles to brake up in micelles. When vesicles are formed by two compatible lipid molecules, the free energy of curvature varies linearly with their composition. If the two bipolar molecules do not mix, they partition themselves over the two membrane sides and the excess free energy of curvature shows, at constant vesicle radius, a minimum as a function of composition. For a given composition the vesicle adopts an optimal vesicle radius.

    The membrane structure predicted by the theory significantly improves when the orientational dependent molecular field is applied. We derive the partition function for this SCAF (Self-Consistent Anisotropic Field) theory in chapter 4. Among other things, the order parameter profiles now show the well known plateau along the lipid tails. In agreement with experiments, we find a first order phase transition which transforms the membrane from a high temperature liquid into a low temperature gel state. In the gel phase the lipid tails are virtually in a all trans conformation. Because of this, the density in the gel membrane is higher than in the liquid phase. For the model membrane we observed two possible gel phases. One gel phase was about twice as thick as the other. The thin, intercalated, gel membrane was found in the case that the membranes were isolated, i.e., when they did not interact with each other, while the other gel phase, obviously with non-intercalated membranes, was found in the concentrated regime.

    In the final chapter we studied two cases of the interaction of long copolymers ("proteins") with a model membrane. In the first example the molecule is in a trans membrane configuration. In the second example a group of four molecules is clustered together and forms a hydrophilic pore, through which polar molecules can pass the membrane. In this chapter we also study the boundary region between two areas of lipid molecules which do not mix (lateral phase separation). It is characteristic for membrane system, that the lipids in the membrane are very efficient in camouflaging the inhomogeneities in the boundary layers. No big differences in solvent profiles are observed along the boundary layers. This ability of the lipid molecules to compensate Irregularities explains why membranes are not easily disrupted.

    It is the first time that a statistical thermodynamical theory is presented that can deal with association phenomena without the requirement to fix the head groups to pre-assigned positions. We showed that this theory does give a very detailed insight into equilibrium membrane properties. The correspondence with experimental data is satisfactory. The theory can be easily extended to incorporate more details in the calculations and better quantitative agreement with experimental data Is well feasible.

    The stability of recombined milk fat globules
    Melsen, J.P. - \ 1987
    Agricultural University. Promotor(en): P. Walstra. - S.l. : Melsen - 146
    dierlijke producten - colloïden - zuivelindustrie - dispersie - emulsies - vet - melkproducten - oliën - malsheid - textuur - water - vetgehalte - animal products - colloids - dairy industry - dispersion - emulsions - fat - milk products - oils - tenderness - texture - water - fat content

    The stability of the fat globules in recombined milk products against creaming, flocculation, clustering, partial coalescence and real coalescence, with the emphasis on partial coalescence, was studied. (partial) Coalescence was characterized by determining changes in globule size distribution and fat content. Without crystals the emulsions were mostly stable at rest and during flow. If crystals were present, natural cream and emulsions of milk fat-in-whey were unstable in a flow, while emulsions of milk fat and skimmilk or milk fat and buttermilk remained fairly stable and only gave partial coalescence if high shear rates were applied to emulsions with a high fat content and a large average diameter.

    In some cases partial coalescence resulted in the formation of a few large clumps that coalesced into floating fat upon heating the emulsion to above the melting point of milk fat thus causing a reduction of the fat content of the underlying emulsion, in other cases partial coalescence resulted in the formation of many small clumps that coalesced into larger fat globules upon heating, thus causing an increase in average globule size. Coalescence course and rate appeared to depend on emulsion type and applied treatment. Partial coalescence of milk fat-in-whey emulsions nearly always resulted in a decrease of the fat content. With a model starting from a small fraction of reactive globules gradually growing into clumps during the treatment, the coalescence process of these emulsions was fairly good quantitatively described. The ideas whereupon this model is based were used to qualitatively explain the different partial coalescence processes observed with the other emulsions.

    Interfacial electrochemistry of colloidal ruthenium dioxide and catalysis of the photochemical generation of hydrogen from water
    Kleijn, J.M. - \ 1987
    Agricultural University. Promotor(en): J. Lyklema; H.P. van Leeuwen. - S.l. : Kleijn - 141
    ruthenium - colloïden - waterstof - elektrochemie - fotochemie - katalyse - ruthenium - colloids - hydrogen - electrochemistry - photochemistry - catalysis

    The formation of hydrogen from water using solar energy is a very attractive research topic, because of the potential use of hydrogen as an alternative, clean fuel. It has been shown by many workers in the field that photochemical hydrogen generation can be achieved in an aqueous system, containing a sensitizer (a light absorbing solute), an electron relay, and a dispersed catalyst. The electron relay transfers electrons from the light-excited sensitizer to the surface of the catalyst, where subsequent reduction of H +takes place. In an ideal photochemical system for solar energy conversion, water itself would ultimately provide the necessary electrons for hydrogen formation, under simultaneous oxygen evolution. However, complete ("cyclic") photodissociation of water involves a number of complications, like the recombination of intermediate photoproducts. To separately study the formation of hydrogen, these additional problems can be bypassed by adding an electron donor, which decomposes after having reduced the oxidized sensitizer. Such simplified systems are known as "sacrificial".

    The present thesis is concerned with the generation of hydrogen in such a sacrificial photochemical system. The main purpose has been to gain insight Into the processes that take place at the catalyst/solution interface. Because of its wide application in photochemical model systems for hydrogen production, methylviologen (MV 2+) was chosen as the electron relay. Via its reduced form MV +., electrons are transferred from the sensitizer to the catalyst. Colloidal ruthenium dioxide (RuO 2 ) was used as the catalyst compound. It has the advantage over the more commonly used Pt catalysts, that it does not catalyze the undesired, irreversible hydrogenation of MV 2+.

    The heterogeneous processes in a hydrogen photoproduction system cannot be investigated without taking into account the reactions in solution too. Therefore, ruthenium trisbipyridyl (Ru(bipy)32+) and EDTA were chosen as photosensitizer and sacrificial electron donor, respectively: most of the (light-induced) homogeneous reactions that take place in the Ru(bipy)32+/MV 2+/EDTA/colloidal catalyst system have been studied extensively by different groups of researchers. In our experiments, the standard reaction mixture (58 ml) for photogeneration of hydrogen contained 2 x 10 -4M Ru(bipy)32+, 5 x 10 -4M MV 2+, 0.02 M EDTA, and 0.05 M acetate buffer (pH 4.6).

    Colloidal RuO 2 was prepared by thermal decomposition of RuCl 3 at ca. 400 °C. The material obtained is crystalline and only slightly contaminated with residual Cl, which is mainly present at the surface of the particles. The BET surface area is 20-30 m 2/g. Dispersions of RuO 2 are colloid-chemically very unstable, even in the presence of polymers or surfactants. They manifest the same electric double layer characteristics as many other oxide dispersions. The point of zero charge (p.z.c.) in indifferent electrolyte (KNO 3 ) is positioned at pH 5.7-5.8.

    Experiments with RuO 2 film electrodes, prepared from the same colloidal material and sintered at 700 °C, revealed that the hydrogen evolution reaction is chemically reversible. Hydrogen evolution at moderate overpotentials does not modify the RuO 2 . In the presence of 0.05 M acetate buffer (pH 4.6), the mass transport limited current density for H +reduction is high since it is related to the buffer capacity and not to the actual proton activity. In the potential range studied, the hydrogen evolution reaction can be described by the Butler-Volmer equation, with a transfer coefficient αof about 0.33, and an exchange current density i o of ca. 0.09 mA/cm 2geometrical surface area. The true exchange current density is smaller by a factor depending on the surface roughness of the film electrodes.

    Adsorption, of MV 2+at the RuO 2 /solution interface is mainly a result of attractive coulombic interactions (above the p.z.c. of RuO 2 ), but it has been shown that there are also more specific interactions. However, the specific adsorption is weak and not noticeable at high concentrations of back-ground electrolyte and pH values below the p.z.c. of RuO 2 . No indications were found that MV 2+adsorbs at the catalyst surface under operational conditions of hydrogen evolution. Under these conditions, the sensitizer Ru(bipy)32+does not adsorb either. On the other hand, the electron donor EDTA strongly adsorbs on RuO 2 from a 0.05 M acetate buffer solution of pH 4.6. However, this seems not to affect the electron transfer between methylviologen and RuO 2 film electrodes, a process which takes place with a transfer coefficient αof ca. 0.35 and a standard heterogeneous rate constant k oof ca. 1.4 x 10 -5m/s (referred to the geometrical surface area).

    The colloidal RuO 2 turned out to be a good catalyst for photoproduction of hydrogen, in spite of the strong tendency of the particles to form aggregates. During the hydrogen evolution process, it does not loose its catalytic properties. It was confirmed that RuO 2 does not catalyze the hydrogenation of methylviologen. A disadvantage of RuO 2 is that it absorbs light throughout the entire visible region.

    Upon illumination of the reaction dispersion and after a certain induction time, hydrogen production takes place at a constant rate (steady state). After several hours, the production rate gradually decreases to zero. The maximum attainable amount of H 2 is determined by the initial amount of electron donor: each EDTA species can regenerate three oxidized sensitizer ions. However, in most experiments the total H 2 yield was less due to gradual destruction of methylviologen in the bulk solution.

    The steady state ratio [MV +.]/[MV 2+] appeared to be always low, even in the absence of catalyst. This must be the result of a yet unspecified reaction which reconverts MV +.into MV 2+. Probably, a photogenerated intermediate species is involved in this process.

    In all the experiments with the hydrogen photoproduction system, the incident light intensity was a rate-determining factor. The steady state rate of hydrogen production depends also, but to a lower extent, on the sensitizer concentration. It has been shown in a simple way that the first step in the hydrogen evolution process, i.e. the excitation of Ru(bipy)32+, is first order in the light intensity and less than first order in the sensitizer concentration.

    The hydrogen production rate increases with EDTA concentration up to a plateau above ca. 0.02 M. At the plateau, the oxidized sensitizer is regenerated efficiently, preventing back-reaction with MV + .As a function of methylviologen concentration, the production rate exhibits a maximum around 2 x 10 -3M.

    At low quantities of RuO 2 (< 10 mg), the available catalytic surface area is rate-limiting. At higher catalyst amounts, the production rate is fairly constant; it decreases slightly with increasing RuO 2 amount due to the absorption of light by the RuO 2 particles.

    For any amount of RuO 2 , the stirring rate affects the rate of hydrogen evolution. Mass transfer of H +to the catalyst surface is not rate-limiting, as is also confirmed by the insensitivity of the production rate to the buffer concentration. This implies that the mass transfer of MV + .to the catalyst surface is a rate-determining factor.

    Most of the abovementioned experimental results can be satisfactorily simulated using a quantitative model, in which the homogeneous reactions are described by steady state kinetic equations and the heterogeneous processes as electrode reactions. The catalytic properties of RuO 2 can be understood and predicted by considering the RuO 2 aggregates as microelectrodes. Probably, the electrical conductivity of RuO 2 -on the level of a metallic conductoris essential for its catalytic performance.

    Hydrogen evolution at the catalyst surface takes place near the equilibrium potential of the H +/H 2 couple. At these potentials, reconversion of MV 2+into MV + .at the catalyst surface is negligible. The rate of the heterogeneous processes is determined by the rate of mass transfer of MV + .to the surface and, to a lower degree, by the rate of interfacial electron transfer. The mass transfer coefficient of methylviologen, under the standard stirring conditions, appeared to be in the order of 10 -5m/s.

    Mass transfer of methylviologen would undoubtedly be favoured by a better dispersion of the catalyst, since aggregation of the RuO 2 particles makes the surface less accessible. If the same or higher hydrogen production rates could be reached with lower catalyst amounts, the disadvantage of light absorption by the RuO 2 particles would become less important. Therefore, it seems worth trying again to stabilize dispersions of RuO 2 , for example by covalently linking polymers to the oxide surface.

    The simulations further indicate that, if the total surface area of the RuO 2 particles is assumed to be catalytically active, the kinetic parameters i o and k oare only ca. 10 times lower than the corresponding values found for the RuO 2 film electrodes per unit geometrical surface area. This is surprising, because the roughness factor of these electrodes was estimated to be in the order of several hundreds. This point deserves further attention. Aspects that could be investigated, are the influence of heat treatments on the reductive catalytic properties of RuO 2 and the comparison with kinetic parameters for single crystal RuO 2 electrodes.

    The presented model for the hydrogen production system does not account for the maximum in hydrogen production rate as a function of methylviologen concentration. The differences between model predictions and experimental results point to a progressive inhibition of the heterogeneous processes with increasing MV 2+concentration. This aspect will be the subject of further study, including investigation of the dependency of the electron transfer rate constant on the bulk concentration of methylviologen.

    The overall quantum yield of the hydrogen production in our standard system is low; even with an excess of catalyst, it is less than 4 %. Since reconversion of MV 2+into MV + .at the catalyst surface does not take place (each MV + .species that reaches the surface is used for hydrogen production), the low efficiency of the system results from the homogeneous proceases. Reconversion of MV + .into MV 2+in solution is competitive with the production of hydrogen and makes the system less efficient. The quantum yield is also limited by the low efficiency of the quenching of the excited sensitizer by methylviologen. At pH 4.6, less than 25 % of the quenching acts results in charge separation (according to our numerical simulations ca. 16 %). Furthermore, the gradual destruction of methylviologen under illumination of the reaction mixture, makes this compound unsuitable for use in any practical device for photogeneration of hydrogen.

    Combination of information regarding the homogeneous and interfacial aspects of the hydrogen production system leads to a picture that is at least semiquantitatively, and in many aspects quantitatively consistent. Extentions of this approach could be useful for the rational design of catalytic systems for solar energy conversion.

    Colloidal stabilization of beer = Colloidale houdbaarheid van bier
    Anonymous, - \ 1985
    Wageningen : Pudoc (Literatuurlijst / Centrum voor Landbouwpublikaties en Landbouwdocumentatie no. 39)
    bieren - bibliografieën - bierbereiding - colloïden - dispersie - houdbaarheid (kwaliteit) - beers - bibliographies - brewing - colloids - dispersion - keeping quality
    The preparation and stability of homodisperse colloidal haematite (alpha-Fe2-O3)
    Penners, N.H.G. - \ 1985
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema, co-promotor(en): L.K. Koopal. - Wageningen : Penners - 97
    colloïden - dispersie - emulsies - hematiet - ijzerhydroxiden - ijzeroxiden - isolatie - verwerking - zuiveren - suspensies - colloids - dispersion - emulsions - haematite - iron hydroxides - iron oxides - isolation - processing - purification - suspensions

    Since the foundation of colloid chemistry as a branch of science, much attention has been paid to the subject of colloid stability, i.e. the stability of colloid systems against aggregation. Gradually, our knowledge of the mechanisms involved has improved and models were developed, comprised in the DUO theory, which form the basis of a quantitative description of the stability of a colloidal system. There is plenty of experimental evidence which substantiate the correctness of the principles of the DUO theory, and hence, this theory is regarded as one of the fundaments of colloid chemistry. However, in one respect the theory is not confirmed by experiments: calculations predict pronounced size effects, but in practice stability seems to be little affected by particle size.

    It was the purpose of this study to gain insight in this contradictory matter. Chapter 1 offers a more extended introduction to the problem as well as the outline of this study.

    Chapter 2 focusses attention on a model system which meets the outlined requirements regarding surface charge, homodispersity, sphericity and particle size range: a method is described for the synthesis of homodisperse haematite (α-Fe 2 O 3 ) sols containing particles whose sizes vary from 35 nm up to 700 nm. This method is principally based on the gradual growth of haematite seeds in supersaturated FeCl 3 solutions (heterogeneous nucleation) up to the desired paticle size. As slight deviations in the composition of the growth medium have drastic effects on the shape of the final colloid, emphasize is given to the description of optimal synthesis conditions for spherically shaped particles. Kinetic experiments, performed to unravel the principles of particle growth revealed that the precipitation process is governed by diffusion.

    The coagulation experiments described in chapter 5 are monitored by turbidity measurements. Therefore the characterisation of the optical properties of the sols is a prerequisite for further studies. The sols under investigation are homodiperse and contain spherically shaped particles. This makes them particularly suited for such an evaluation, as their scattering behaviour can be interpreted in terms of the Mie theory. Such a comparsion, leading to values for the refractive index (n) and the absorption coefficient (K) in the wavelength range from 400 to 800 nm is made in chapter 3 . The agreement between calculations and experiments is good for any wave length in the visible range, and for any particle size studied, although the particles are monocrystalline and no perfect spheres.

    Any study dealing with electrostatic stabilisation demands some knowledge of the electrochemical behaviour of the system under study. Chapter 4 pays attention to the electrochemical characterisation of the haematite surface by comparing haematite samples from different origines. In these studies, potentiometric titrations, streaming potential measurements on haematite-coated capillaries and micro electrophoresis were used as the experimental tools to get access to the surface properties. Instead of providing unambiguous data, being valid for all iron oxides occuring in the (α-Fe 2 O 3 ) modification, the reported experiments emphasize that the crystal structure of the bulk phase is not the exclusive parameter in determining the electrochemical behaviour of an oxide. The purification procedure, or a heat treatment of the sol (aqueous or dried) plays an important part as well. There is some evidence that the crystal habit of the haematite surface is pH-dependent and that in some cases precipitated amorphous oxide may share in determining the surface properties. Though not going too much into details, the study gives evidence of the fact that the charging mechanisms of haematite are more complicated than expected on the grounds of purely crystallographic considerations.

    Inevitably this finding has its impact on the question of colloid stability, which is the dominating item of chapter 5 . Considering the outcome of the electrochemical study, is it still justified to assume that the haematite particles meet the demands of the stability theory regarding the sharpness of the boundary between bulk material and surrounding liquid? How could a diffuse surface layer be accounted for in existing stability models? Within the restrictions set by such questions, chapter 5 deals with the item of colloid stability with special reference to the influence of particle size on stability: though the value of the critical coagulation concentration depends on particle size and shows a minimum (!), the slopes of the log W - log C plots are virtually size independent. Such trends can be accounted for by the concepts of the DUO theory, if coagulation reversibility and shear effects are incorporated in the analysis. Deviations from sphericity, which are definitely observed for the systems under investigation. might explain some of the observed effects as is shown by some simple double layer calculations dealing with orientational effects in the interaction of a cubic particle with a half space.Finally, chapter 6 reflects on the preceding Items and pays attention to its limitations. Furthermore, it points to subjects which deserve further elaboration and mentions the means to make them experimentally accessible.

    Electrokinetic properties and conductance relaxation of polystyrene and silver iodide plugs
    Hoven, J.J. van den - \ 1984
    Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch. - Wageningen : Van den Hoven - 160
    aromatische verbindingen - colloïden - dispersie - jodide - polymeren - polystyrenen - zilver - elektrochemie - elektrokinetische potentiaal - aromatic compounds - colloids - dispersion - iodide - polymers - polystyrenes - silver - electrochemistry - electrokinetic potential

    This thesis describes an experimental study on the electrokinetic and electrical properties of concentrated polystyrene and silver iodide dispersions. The purpose of the study is to obtain information on the structure of the electrical double layer at the solid-liquid interface. Special attention is paid to the various polarization phenomena that may interfere in electrokinetic and conductance measurements on condensed systems.

    After a general introduction in chapter 1, in chapter 2 we discuss the preparation and characterization of the polystyrene latices. The geometry of these colloids is well-defined and easy to control, their surface properties. on the contrary, are not. The number and nature of the surface groups may change upon characterization, due to contact with a solution of high ionic strength as well as during storage. In order to preclude any effect of these phenomena on the electrokinetic experiments, after preparation the latices have been treated in a special way.

    In chapter 3 the afore-mentioned polarization phenomena are studied with a four-electrode cell. The characteristics of this cell are investigated for both reversible and irreversible electrodes. In both cases electrode polarization phenomena considerably interfere with the streaming current measurements. Particularly disturbing are the polarizations that occur in the dispersed medium itself. The four- electrode method, however, enables correction for both forms of polarization. As a result. the electrokinetic data thus obtained are independent of the material of the measuring electrodes. When using reversible electrodes, another complication can be that electrolysis processes at the current-carrying electrodes disturb the system under investigation. When irreversible (platinum black) electrodes are incorporated in the cell, such processes do not occur. Another important advantage of the latter electrodes is that the four-electrode method is then most accurate and , moreover, generally applicable. Two independent experimental procedures (streaming potential and streaming current method) yield identical results, confirming the reliability of the four-electrode technique.

    Chapter 4 deals with the choice of the theoretical models that have been applied to infer double layer parameters from electrokinetic and conductance data. Zeta potentials are calculated by means of capillary models. In these the effect of double layer interaction has been taken into account, but not that of polarization of the double layer. For the inference of surface conductivities both capillary and cell models are considered. The latter take account of double layer and concentration polarization. The chapter concludes with a theoretical consideration of the relation between zeta potential and surface conductivity.

    In chapter 5 the electrokinetic techniques described in chapter 3 are applied to polystyrene plugs. The results show the slipping plane to shift away from the solid surface upon decreasing the ionic strength. Considerable conductance takes place in the corresponding fixed layer. The relaxation phenomena, observed in the streaming current measurements, are also studied by measuring frequency-spectra of the plug conductance. In the low-frequency range dispersions occur, being the more pronounced the lower the salt concentration. At high salt concentrations the dispersions probably stem from processes taking place at the measuring electrodes. Below concentrations of 10 -2M relaxation processes in the plug itself contribute increasingly to the dispersions. These intrinsic processes as well as the electrokinetic data are accounted for by assuming the surface layer of polystyrene particles to consist of protruding, partly mobile polymer chains. The thickness of this "hairy" layer and, consequently, the position of the shear plane is controlled by the salt concentration.

    In chapter 6 the techniques developed are used to investigate the influence of adsorbed charged species (tetraalkylammonium (TAA +) ions) on the structure of the polystyrene-solution interface. To this end, adsorption isotherms are also determined. When the surface is (partially) covered with TAA +ions, the position of the slipping plane is independent of ionic strength. Between this plane and the solid surface under these circumstances only very little or no conductance takes place any more. The polarization phenomena in the plug practically disappear when TAA +ions adsorb. The conclusion is that TAA +adsorption largely eliminates the hairiness of the polystyrene particles.

    Chapter 7 describes the measurements on silver iodide plugs. These have been performed as a function of ionic strength and surface potential. Intrinsic polarizations do also occur in plugs of this material, but they are much less pronounced than those observed in polystyrene plugs. No satisfactory explanation for these polarization phenomena can be offered. The experiments reveal a number of peculiar features. For high salt concentrations the electrokinetically displaceable charge density considerably exceeds the surface charge densities reported in literature. This inconsistency is ascribed to the fact that the latter data is based on an improper value of the specific surface area. Upon decreasing the ionic strength, for this system the slipping plane also shifts outwardly. This phenomenon is interpreted in terms of water structuring due to influences of the silver iodide surface. The concept of a stagnant layer can also account for the results obtained at variable surface potential. Procedures to calculate the thickness of the adhering layer are indicated, but definite conclusions cannot be drawn, because of uncertainties as to the extent of specific adsorption. The layer behind the slipping plane substantially contributes to the conductance of the system. To a small extent this stems from ionic transfer in the solid phase.

    It can be concluded that the electrokinetic and electrical techniques described in this thesis are useful tools to obtain insight into double layer structures. They have provided detailed information on the structure of the interface in aqueous polystyrene and silver iodide dispersions.

    Syneresis of curd
    Dijk, H.J.M. van - \ 1982
    Landbouwhogeschool Wageningen. Promotor(en): P. Walstra, co-promotor(en): J. Schenk. - Wageningen : Van Dijk - 87
    melkeiwitten - colloïden - coagulatie - uitvlokking - kunststoffen - industrie - wrongel - macromoleculaire stoffen - milk proteins - colloids - coagulation - flocculation - plastics - industry - curd - macromolecular materials
    This study deals with the syneresis of curd. Rennet gels are primarily considered; some comparisons with acid milk gels are given.

    After curdling the milk, the curd tends to shrink; in other words, the network of aggregated paracasein micelles (PCM) will be under stress. If the curd is cut or - as was the case in our expirements - a curd surface is wetted, syneresis starts. The rate at which the whey is expelled depends on the pressure gradient in the whey and on the permeability of the network.

    In Chapter 2 the materials and methods generally used are described. Unless mentioned otherwise, standard conditions were used in the experiments. By standard conditions is meant: reconstituted skim milk with the saw dry matter content as the original milk, to which 500 ppm rennet was added; the temperature during the whole experiment was kept at 30 °C; no CaCl 2 was added.

    The endogenous syneresis pressure ( Ps) appeared to be very low, about 1 Pa. In Chapter 3 two methods are described which give an order of magnitude of the stresses involved. Moreover, the weight of the network can cause an additional pressure. The maximum pressure caused by the weight ( Pg) at a level hc below the interface is (ρ curd - ρ curd ) ghc ≈75hc Pa (hc in m).

    The permeability measurements are described in Chapter 4. Two methods were used; in both, the flow of whey through a vertical column of curd was measured as a function of head pressure. A problem is that the curd is deformed during the experiment. In the "tube" method, deformation is a function of the pressure gradient (d Pt /dx), the diameter of the tube holding the curd (d t ), and the rigidity of the gel. In the second method the "torsionflux" method, the deformation was adjustable. 'The tube method led to the following results.
    - The permeability is of the order of 10 -13m 2.
    - Permeability increases with time, which is ascribed to "microsyneresis", i.e. syneresis at local sites in the gel. The rate of increase is approximately constant.
    - The increase in permeability (d B /d t ) is higher for a higher pressure gradient or a wider tube; both lead to larger deformation of the curd.
    - The change of the permeability with time in the absence of deformation (d Be /d t ) was obtained by applying the head pressure at different times after addition of rennet. Shortly after clotting permeability increases fastest. Between 1 and 24 h
    d Be /d t was constant.
    - The permeability of curd made from ultrafiltered skim milk ( B ( i )) and its change with time (d B ( i )/d t ) were determined. This
    yielded the permeability as a function of concentration and time ( B ( i,t )).
    - The permeability also depends on temperature, CaCl 2 concentration, acidity, fat content and type of skim milk.
    - In acid milk geld permeability was of the same order of magnitude, but it hardly changed with time.

    The rheological behaviour of curd is discussed in Chapter 5. The dynamic measurments with the "Den Otter" rheometer show that the moduli G ' and G " kept increasing for a long time (~3 h) after rennet addition. From the dependence of G' and G" on the angular frequency it was deduced that G" is due to the relaxation of bonds and that the relaxation time is a few times 10 s.

    The instantaneous shear modulus ( G0 ) was determined as a function of protein concentration. The obtained relation can be explained in term of an only partly effective contribution of the casein to the network; this contribution being relatively smaller at lower concentrations. Also from the creep measurements it was concluded that the endogenous syneresis pressure was less than 10 Pa.

    If both permeability and pressure are known for all values of concentration (or relative remaining volume ( i )) and time ( t ), the syneresis can in principle be calculated. This is in the model described in Chapter 6, in which the equation of Darcy is combined with the equation of continuity. A numerical procedure is developed, for a one dimensional case; the syneresis of a thin slab.

    The pressure in the whey is the sum of the endogenous syneresis pressure ( Ps) and the pressure caused by the weight of the network ( Pg). For Ps( i ) and Pg( i ) some trial functions were considered.

    In Chapter 7 the syneresis of slabs is studied. The results of the experiments show that initially Γ= dlogΔH/dlog t is about 0.5. For t >0.5 h Γincreases to ~0.78. Γis independent of the original thickness of the slab ( H0 ) during a certain period (penetration period). The length of this period depends on H0.

    After one day H did not change any more and H∞ / H0 was about one third. The best fit between model calculations and experimental results was obtained if it was assumed that:
    - the permeability increases with time ( t ) and decreases with i , as was found in the experiments,
    - endogenous syneresis pressure (Ps) decreases only with shrinkage, - maximum gravitational pressure ( Pbg) is constant,
    - P0s= Pbg= 1 Pa ( H0 = 10 mm).

    P0swas found to be a function of time after renneting, at first increasing, then (after 1 - 2 h) decreasing. However, the introduction of such a relation in the model did not improve the fit to the experimental results. After all, the pressure cannot relax twice, both by shrinkage and by "ageing".

    The effects of several parameters (pH, temperature, Ca concentration, etc.) on milk clotting, gel permeability, syneresis and curd rigidity are interrelated. A survey is given in Table 7.2 and a tentative explanation is summarized in Table 7.3.

    In Chapter 8 it is shown that external pressure has a dramatic effect m the syneresis rate. Extrapolation to zero external pressure yields, again, an endogenous syneresis pressure of about 1 Pa.

    Electrolytic analogue study of the effect of openings and surrounds of various permeabilities on the performance of field drainage pipes
    Dierickx, W. - \ 1980
    Landbouwhogeschool Wageningen. Promotor(en): W.H. van der Molen. - Wageningen : Dierickx - ISBN 9789070142131 - 238
    buisdrainage - drainage door leidingen - permeabiliteit - hydrodynamica - stroming - laminaire stroming - turbulente stroming - chemie - colloïden - adsorptie - oppervlakten - elektrolyten - elektrische geleiding - landbouw - intreeweerstand - oppervlaktechemie - tile drainage - pipe drainage - permeability - hydrodynamics - flow - laminar flow - turbulent flow - chemistry - colloids - adsorption - surfaces - electrolytes - electrical conductance - agriculture - entrance resistance - surface chemistry
    The effect of various openings and surrounds of various permeabilities on the performance of field drainage pipes was studied by means of an electrolytic analogue. The results obtained were compared with these of analytical solutions. Rather simple and sufficiently accurate solutions exist to determine the entrance resistance of pipes with smooth outer surface. These theoretical solutions cannot be applied to pipes with corrugated cuter surface provided with perforations in the valley of the corrugations for which the corrugations are filled with soil. The shape of the corrugations and the boundary of soil and corrugation present additional difficulties in obtaining an exact theoretical solution for such drains.

    From the investigations performed it follows that the smallest entrance resistance is obtained at the greatest subdivision of a given perforation area or perimeter per unit drain length. The most favorable perforations which confer the lowest entrance resistance are these with the smallest area or perimeter such as circular perforations and, for rectangular slits, those with the smallest length. Except for circular perforations, an increase of the actual perforation area of 20 - 25 cm 2/m to about 50 cm 2/m will considerably reduce the entrance resistance.

    Using permeable envelopes. the entrance resistance decreases considerably up to an envelope thickness of about 5 mm after which a constant value is obtained. The effective radius, however, continues to increase with increasing envelope thickness due to the decrease in radial resistance. Increasing the permeability of the envelope reduces the entrance resistance and increases the effective radius up to a permeability ratio of 20. Any further increase of the permeability ratio is of less significance. For a constant value of pipe radius plus envelope thickness much the same effective radius is obtained if the thickness of the envelope is at least 5 mm.

    A less permeable drain surround increases the entrance resistance enormously and inadmissible values are quickly reached. A constant entrance resistance is obtained for thicknesses of about 10 mm and upwards. The effective radius decreases due to the increase of radial resistance and exceptionally small values are obtained. The increase of entrance resistance and decrease of effective radius are particularly marked for permeability ratios less than 0,2.

    The entrance resistance only changes slightly when a drain is surrounded by an envelope which has a reduced permeability over a certain percentage of its original thickness. Due to the effect upon radial resistance. the effective radii will decrease with decreasing permeability and increasing thickness of the blocked zone. The effective radius never assumes such extremely small values as are obtained with a drain directly surrounded by a wholly less permeable layer.

    The entrance resistance of a drain pipe is constant and depends only on the geometrical characteristics of the pipe itself. It is, however, important to give an exact description of the flow pattern, since its omission can result in faulty conclusions being drawn about the entrance resistance.

    Due to the entrance resistance, the hydraulic gradient in the vicinity of the perforations can reach high values and massive invasion of soil particles may occur. These gradients are markedly reduced when the drain is surrounded by a permeable envelope.

    Although the approach flow conditions are mere favorable if water is standing above the drain, the entrance resistance which causes a certain water level above the drain will raise the water table midway between drains more then an ideal drain operating with the game head.

    Electrokinetic investigations on the system polystyrene/aqueous electrolyte solution : verification of model theories on dilute and concentrated dispersions
    Put, A.G. van der - \ 1980
    Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch. - Wageningen : van der Put - 223
    chemie - colloïden - adsorptie - oppervlakten - aromatische verbindingen - polymeren - polystyrenen - elektriciteit - magnetisme - oppervlaktechemie - elektromagnetisme - chemistry - colloids - adsorption - surfaces - aromatic compounds - polymers - polystyrenes - electricity - magnetism - surface chemistry - electromagnetism
    This thesis presents a systematic experimental and theoretical study on electrokinetic and electroconducting properties of disperse systems. The increasing interest in transport processes through charged porous systems has recently brought about a corresponding growth of models and theories since real systems are extremely complex. Monodisperse polystyrene latex seems to constitute a good geometrical model system to theoretically and experimentally assess the influence of the interfacial properties of the dispersed particles and the overall properties of the dispersed system.

    The preparation and characterization of the polystyrene latices are dealt with in chapter 2. The concentrated dispersions (plugs), with a volume fraction of the solid in the order of 0.6, were obtained by centrifugation of the latex samples as described in chapter 3.

    Chapter 3 recalls the basic principles of the theory of irreversible thermodynamics which are relevant in connection with the determination and mutual dependency of electrokinetic quantities. All possible forms of polarization, viz. concentration polarization at the electrodes and/or at the plug surfaces, ohmic polarization and intrinsic polarization of the plug, are studied with a 4-electrode equipment. The main features of these polarization phenomena are:

    (i) concentration polarization at the plug-solution interface (negative adsorption) may cause transient streaming potential or an apparent nonlinear electro-osmotic flow-current relationship, when this cause is not properly recognized, the results can, e.g., erroneously be interpreted in terms of special transitions in the water structure at the interface.

    (ii) the retardation behaviour of the streaming current is caused by concentration polarization at the current-processing electrodes and/or is a result of a slow intrinsic relaxation process.

    (iii) the current-voltage characteristics may be obscured by induced concentration polarization processes (transport number effect) but also by slow intrinsic polarization processes.

    The employment of this new technique has led to the correct measurement of the pertinent phenomenological coefficients. Furthermore, it enables to obtain the time-dependency of a possibly present slow intrinsic polarization process and the transport number in the plug.

    A theoretical and experimental study also shows how to evaluate the phenomenological coefficients in case various forms of polarization take place simultaneously. The 4-electrode technique also enables to determine the concentration charge at the low pressure side of a plug during per meation. A quantitative expression has been derived which interrelates the ionic strenght outside the plug, the electrolyte concentration in the porous phase and the volume flow.

    Finally, the time-dependency of the depletion process is used to calcu late effective charge densities (dynamic negative adsorption method) which appear to be completely consistent with the electrokinetic results (chapter 4).

    Chapter 4 summarizes and discusses the basic geometric models. The electrokinetic and hydrodynamic data are then presented and discussed in relation to these various models. Special attention is given to the electroviscous effect and the concept of anomalous conduction. To that purpose theoretical predictions for the cell and the capillary model of Levine are made suitable or are extended to the concept of anomalous conduction.

    The main features of the electrodynamic and hydrodynamic results are:
    - the severe restriction to low potentials causes the cell model to be less usefull than Levine's capillary model;
    - a mutual comparison of various theoretical models demonstrates the effect of polarization in concentrated dispersions; when surface conduction is not negligible, porous systems are even polarized un der streaming current conditions, which may lead to an apparent lyotropic sequence in the streaming current;
    - our latex plugs show a strong deviation from 'model behaviour'. The experimental data are interpreted in terms of an outward shift of the shear surface when decreasing the electrolyte concentration, coinciding with a change in the double layer structure. The boundary layer thicknesses evaluated from the hydrodynamic and from the electroviscous retardation results both show the same concentration dependency. The difference between the absolute values is related to the structure of the double layer.

    Chapter 5 is concerned with the conversion of experimental data on the conduction of concentrated and dilute dispersions into double layer characteristics. A very clear view on the applicability on the various theoretical formulae is obtained. The conductivity of dilute dispersions can quite reasonably be described by an equation that allows for concentration polarization according to the Dukhin - Semenikhin theory, provided that the experimental conditions are chosen such that the particles are fully polarized. For concentrated systems, the concentration polarization mechanism is suppressed by short-circuiting of the interacting double layers near the contact points of the particles. This keeps the conductivity finite as the ionic strength approaches zero; consequently all polarization models finally underestimate the conductivity of a concentrated dispersion upon decreasing ionic strength, especially when the concentration polarization according to Dukhin - Semenikhin is incorpora ted. The 'classical' Bruggeman equation proves to be most usefull for the evaluation of double layer properties. The influence of the decrease in concentration polarization upon concentrating a particle dispersion is also noticed from the shift of the isoconductivity point to higher electrolyte concentrations.

    Also in chapter 5, the combined information on the influence of the na ture of the counter ions, the temperature and the surface charge density on the surface conductance is used to analyse the structure of the electrical double layer of our model system. The main conclusions are:
    - the counter charge effective in the electric conduction remains more or less constant upon variation of the ionic strength whereas the 'hydrodynamically immobilized' part of the counter charge increases sharply upon diluting the electrolyte;
    - the hydrodynamic and electroviscous data indicate a shift of the shear plane upon decreasing ionic strength;
    - contacting polystyrene with a medium of high ionic strength causes an increase in the titration charge density (charging phenomenon);
    - a alkali-specificity is noticed for the surface conductance and the activation energy of conductance.

    The 'hairy layer' model (Δ-layer) is used to explain these peculiar findings. Arguments are given that protruding polystyrene chains with terminal charges are present and that they are distributed in patches. Increase of ionic strength causes the thickness of the hairy patches to shrink; the shear surface enveloping the end-groups of the most protruded chains shifts inwardly and the counter ions originally accommodated be tween the headgroups become electrokinetically 'visible'. Furthermore, ion-paired groups in the innermost regions of the boundary layer are ac tivated and contribute to the surface conductance. Irreversible structural rearrangements in the Δ-layer are responsible for the fact that upon subsequent dilution of the electrolyte a larger fraction of the end-groups remains active.

    The concentration dependent surface diffusion of ions, the amount of specific adsorption and the observed alkali-specificity are strongly related to the shrinkage of the Δ-layer and the evoked permittivity change within that layer.

    In chapter 6 the effect of concentration polarization on electrophoresis under conditions of anomalous conduction is verified with the theory of Dukhin and Semenikhin. Several procedures are discussed to obtain information about the boundary layer thickness and the average mobility of the ions inside the shear surface. The results are in line with the concept of the concentration dependent boundary layer thickness formulated in the preceding chapters. In passing from Li +to Cs +the average ionic mobility in that layer seems to decrease.

    Chapter 7 presents an account of the dielectric dispersion occurring on a time-scale of seconds. The slow relaxation process is closely related with the 'hairy boundary' layer. The effect of salting out the hairy layer on the relaxation time and the conductivity increment confirms the proposed hairy layer model.

    In conclusion, this study shows, that electrokinetic investigations on concentrated dispersed systems, in combination with electrophoresis, are very useful in studying electrokinetic transport processes. Admittedly, our latex system, due to its simple geometry of dispersed spheres, is an idealized model for systems that are as complex as porous plugs or membranes. However, the presented results are of considerable help in visualizing the immensely complex transport processes and the present shortcomings in their theoretical description.

    Relaxation of the silver/silver iodide electrode in aqueous solution
    Peverelli, K.J. - \ 1979
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema, co-promotor(en): H.P. van Leeuwen. - Wageningen : Peverelli - 63
    chemie - colloïden - adsorptie - oppervlakten - zilver - jodide - elektriciteit - magnetisme - oppervlaktechemie - elektromagnetisme - chemistry - colloids - adsorption - surfaces - silver - iodide - electricity - magnetism - surface chemistry - electromagnetism
    The aim of this study is to detect and characterize relaxation processes on silver/silver iodide electrodes in aqueous electrolyte solution. The information obtained is to be used for an estimation of the consequences of similar processes on colloidal AgI particles during encounter.
    In chapter 1 a general introduction is given. It is explained how electrode kinetic data can be used to obtain insight in the dynamic aspects of the stability of the lyophobic colloids.
    Some fundamental properties of the Ag/AgI film electrode which are important in the interpretation of the relaxation measurements are described in chapter 2. The preparation of the electrode, the silver layer and the AgI film is described. The specific resistance of the AgI film was obtained from measurement of the electrode resistance and the thickness of the AgI film. An estimation of the roughness factor of the AgI surface is made using electron micrographs and results of other investigators who worked with the same type of electrodes.
    In chapter 3 the relaxation behaviour of the Ag/AgI electrode in 0.1 M KNO 3 solution is described. The used pulse technique, the experimental set-up and the method of analysis are treated. It is shown that there are two relaxation processes: a fast one on the Psec scale and a slower one on the wee scale. They are probably connected with charge transfer processes in the Ag/AgI and AgI/ electrolyte solution interface respectively. The standard exchange current density and the transfer coefficient for the slow relaxation of both the Ag + and the I - reaction are calculated. The double layer capacitances are also obtained and their values agree with the results of other studies.
    Relaxation measurements in various KNO 3 concentrations ' and various types of inert electrolyte are reported in chapter 4. One of the main conclusions is that the slow relaxation is not influenced by the concentration and nature of the inert electrolyte.
    In chapter 5 an attempt is made to interpret the results obtained with the electrodes in terms of relaxation processes occurring at interacting AgI sol particles. It is indicated that some of the possible relaxation processes are fast as compared with the process of encountering of two colloid particles.
    Diffuse double layer relaxation in the aqueous solution and migration of interstitial silver ions in the solid are examples of such fast processes. On the other hand, the transfer of potential determining ions (silver or iodide ions) through the Stem layer is a rather slow process with a time constant which is lower than the particle encountering time by a factor of about ten. Thus, in the case of silver iodide sols, exchange of charge between particle and solution will not occur. However, redistribution of charge in the region of short distance between the interacting particles is possible via the fast relaxation processes. Thus, the silver iodide colloid appears to be intermediate between the constant charge case and the constant potential case.

    Dielectric relaxation of dilute polystryene lactices
    Springer, M.M. - \ 1979
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Arnhem : Werkvoorziening Midden-Gelderland - 123
    aromatische verbindingen - polymeren - polystyrenen - chemie - colloïden - adsorptie - oppervlakten - diëlektrische constante - elektriciteit - magnetisme - oppervlaktechemie - elektromagnetisme - aromatic compounds - polymers - polystyrenes - chemistry - colloids - adsorption - surfaces - dielectric constant - electricity - magnetism - surface chemistry - electromagnetism
    The main aim of the investigations, described in this thesis is to provide data which can be used to test theories of dielectic phenomena in dilute colloid systems.
    The possibility of preparing homodisperse polystyrene latices with surfaces uncontaminated by emulsifiers, means that dielectric methods can be applied to these colloid dispersions. The equipment used and the latices which where prepared are described in chapters two and three respectively.
    The experimental results for latex A 1 , with KCl as the added electrolyte, are particularly relevant. These results, which are characteristic of the dielectric behaviour of dilute polystyrene latices, are used in chapter five for calculations with different theories. Based on the dependence of the static dielectric increment Δε st on c b , the concentration of added electrolyte, it appears that until now no theory exists, by which the experimental results obtained are explained conclusively. The best agreement between theory and experiment is obtained both with the theory of Dukhin and Shilov and with that of Schurr. Using the calculated relaxation times, it is impossible to discriminate between these two theories, because in both cases a relaxation time is calculated which is of the right order of magnitude compared with the experimental value.
    Figure (5-2) demonstrates that Schurr's theory is the best in accordance with the experimental results. However, in Schurr's theory the diffusion fluxes of the ions in the diffuse part of the double layer are not taken Into account. In view of their theoretical starting points the theory of Dukhin and Shilov must be considered as the best one. The larger discrepancy between the theory of Dukhin and Shilov and the experimental results for Ac st as a function of c h may be caused by an overestimation of the influence of radial fluxes in their theory.
    In conclusion, based on the established discrepancies between theory and experiment It is thought to be pertinent to perform further dielectric measurements on other colloid model systems such as chromium hydroxide sols. Probably, a part of these discrepancies is due to irregularities in the polystyrene latices used. on the other hand, it is necessary to develop the theory further. In this case special attention must be paid to the role played by the diffusion coefficients and diffusion fluxes around the colloid particles.

    Inference of polymer adsorption from electrical double layer measurements : the silver iodide-polyvinyl alcohol system
    Koopal, L.K. - \ 1978
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 141
    adsorptie - sorptie - zilver - jodide - polymeren - zuurstof - kunststoffen - industrie - chemie - colloïden - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - adsorption - sorption - silver - iodide - polymers - oxygen - plastics - industry - chemistry - colloids - surfaces - macromolecular materials - surface chemistry

    The purpose of this study was to investigate how the double layer properties of charged particles are modified by the presence of adsorbed polymer molecules and to obtain information on the conformation of the polymer layer from the observed alterations in the double layer properties.
    In chapter 1. the use of double layer investigations to obtain insight in the adsorbed layer conformation is briefly outlined. Some theoretical and experimental aspects of the studies of polymer adsorption are shortly reviewed.
    For the experiments the AgI-PVA system is chosen. Double layer charge and potential of AgI dispersed in aqueous electrolytes can be determined and controlled. Much is known about the surface area determination and the stability of Agl. PVA is a water soluble, flexible and uncharged polymer of which the concentration in solution can be determined readily. The combined AgI-PVA system is well suited for the purpose of this study. For the interpretation of the results recourse can be made to similar information previously obtained with low molecular weight alcohols.
    The general procedures and the preparation of the Agl precipitates and sols are given in chapter 2.
    Chapter 3. deals with the characterization of the specific surface area of the AgI samples. Several independent methods are used: capacitance measurements, N2- adsorption, adsorption from solution and electron microscopy. Accepting an uncertainty margin of 10 to 20% the areas obtained by the last three methods compared well mutually. However, the capacitance areas were always 3 to 4 times greater. This disparity was observed before by VAN DEN HUL and LYKLEMA. Surface areas of AgI sols strongly reduce upon coagulation or precipitation. A subsequent heat treatment enhances this effect.
    We used the capacitance area in electrochemical studies, whereas for the adsorption of organic molecules the methylene blue adsorption area is chosen, otherwise unrealistically low adsorption values were found.
    In chapter 4. the properties and solution characteristics of PVA are described. The samples used, differing in molecular weight and acetate content, are characterized by IR and UV spectroscopy. It could be concluded that our samples are atactic, contain no or very little 1,2-glycol units and one or two conjugated carbonyl groups per molecule. The acetate groups in PVA 88 (12 mole % acetate groups) are predominantly distributed in blocks along the polymer chain, whereas in PVA 98 the distribution of these groups is probably random. Molecular weights and the molecular weight distributions were determined by viscometry and gel permeation chromatography.
    The solution properties of the polymers in water have also been studied by viscometry. Unperturbed dimensions, linear expansion factors and polymersolvent interaction parameters are calculated, taking heterodispersity into account. The relative magnitude of the steric hindrance or the characteristic ratio, combined with the fact that water is a poor solvent for PVA suggest that intramolecular interactions occur in the polymer chain.
    Chapter 5. covers the measurement of the mass of PVA adsorbed per m 2 AgI, Γ p , as a function of the PVA concentration. Special emphasis is. given to the influence of molecular weight, acetate content and of the surface charge and state of dispersion of the Agl. The adsorption isotherms show a high-affinity character, leading to a maximum amount adsorbed of 1.5 to 2.6 mg m -2 . The saturation adsorption increases with increasing molecular weight and acetate content. The surface charge of the Agl and its state of dispersion have no measurable influence on the adsorption. The reduction in adsorption by the addition of KNO 3 UP to 10 -1 M is due to a decrease in available surface area.
    No desorption of the polymer could be detected upon dilution with solvent, but the increase in adsorption with time shows that the adsorbed segments are reversibly bound.
    In chapter 6. the principles of the double layer investigations are explained, whereafter a description is given of the potentiometric titrations and electrophoresis studies. The titrations reveal three important features upon adsorption of PVA:
    - the double layer capacitance decreases,
    - the p.z.c. moves to more positive values,
    - the curves pass through a common intersection point, characteristic for the type of PVA.
    These features reflect changes in the Stern-layer, caused by adsorbing polymer trains. Adsorbed polymer trains and low molecular weight adsorbates having a composition comparable with that of the polymer segments behave very similarly in the Stern-layer. From this resemblance it could be concluded that in PVA 88 segments with an acetate group adsorbed preferentially in the first layer. This is promoted by the blocky distribution of these groups in PVA 88. The similarity has further been used to develop a theorem to obtain the degree of occupancy of train segments in the Stern-layer, θ. The obtained result is confirmed by electrophoresis studies. A measure of the effective layer thickness can be found from the slope of the electrophoretic mobility against pAg curve around the isoelectric point. It was shown that this procedure is superiour to the classical one, in which the effective layer thickness is deduced from the reduction of the mobility in the plateau region.

    The main conclusion is that the fraction of polymer adsorbed in trains and the effective layer thickness are a function of the adsorbed amount only. Molecular weight and acetate content affect the adsorbed amount and thus indirectly influence the occupancy in the first layer and the effective thickness. Except for PVA 3-98, where probably preferential adsorption of higher molecular weight species occurs, the layer thickness is proportional to the square root of the degree of polymerization, as expected theoretically. The dependence of the train segment adsorption on the surface charge, though in principle present, is
    too small to be practically important. The layer thickness is also independent of the surface charge.
    In chapter 7. some further parameters of the adsorbed layer are calculated, such as the fraction of segments adsorbed in trains, p , and the amount adsorbed in loops and tails. It can be concluded that molecules adsorbed in the isolated state (Γ p ->0) do not completely unfold, probably due to the presence of intra molecular interactions. The average segment density in the loop region is relatively high, indicating that the average loop length is much shorter than the total chain length.
    The obtained adsorption parameters are used for a quantitative check of the Hoeve model for polymer adsorption. The adsorbed amount at large polymer concentration is reasonably well predicted, including the molecular weight dependence. Also the trends in pp ) and θ(Γ p ) were confirmed. However, the adsorbed layer thickness is very poorly predicted and the physical significance of the model parpameters is obscure. Our general conclusion therefore is that the Hoeve model cannot fully describe the adsorption of AgI on PVA.

    In conclusion, this study shows that double layer investigations combined with polymer adsorption measurements provide a valuable tool to investigate the conformation of adsorbed polymers. The degree of occupancy of the first layer, the effective thickness of the adsorbed layer and the kind of segments directly adsorbed onto the surface could be determined over a wide adsorption range.

    Interactions between adsorbed macromolecules : measurements on emulsions and liquid films
    Vliet, T. van - \ 1977
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 131
    kunststoffen - industrie - chemie - colloïden - adsorptie - oppervlakten - suspensies - emulsies - macromoleculaire stoffen - oppervlaktechemie - moleculen - intermoleculaire krachten - plastics - industry - chemistry - colloids - adsorption - surfaces - suspensions - emulsions - macromolecular materials - surface chemistry - molecules - intermolecular forces

    The aim of this study was to gain more insight into the factors, determining the inter- and intramolecular interactions between adsorbed macromolecules. To that end several experimental and theoretical approaches were followed, using well-defined systems. It was shown that these interactions could conveniently be studied by measurements on emulsions and thin free liquid films. Two different macromolecules were used: a nonionic one: polyvinyl alcohol (PVA) and an ionic one: a copolymer of methacrylic acid and the methyl ester of methacrylic acid (PMA-pe) in the molar ratio 2:1.
    The characterization of the used materials has been described in chapter 2. The conformational transition, occurring in dissolved polymethacrylates was briefly discussed. At low pH, the molecules occur in a compact form, the hypercoiled form, or a-conformation. At pH above ~ 6 the molecules occur in the common. more extended b-conformation. From viscometry on PVA solutions conformational parameters, such as the root mean square end-to-end distance, the length of a statistical chain element and the linear expansion factor were determined. These conformational parameters were determined in a I M aqueous glycerol solution because in the film experiments 1 M glycerol was present in the PVA solutions in order to lower the water vapour pressure.
    In chapter 3 the experimental methods have been described. In the first part attention was paid to the preparation of the emulsions and to the determination of basic properties, such as specific area and adsorbed amount. A variety of rheological measurements were described in the second part. A more detailed description was given of the apparatus for the dynamic measurements (the rheometer) and of that for creep measurements.
    The end of chapter 3 deals with the thickness measurements of polymerstabilized free liquid films. First, a description of the apparatus and the experimental procedure was given. Subsequently, a discussion followed of the calculation of thicknesses from the intensity of the reflected light. It was shown that, for the calculation of the correction to be applied to the equivalent aqueous solution thickness, the smeared out adsorbed polymer segment layers may be formally replaced by a block distribution.
    The inter- and intramolecular interactions between the PMA-pe segments and the effect of these interactions on the conformation of the polyelectrolyte molecule and on the rheological properties of emulsions stabilized by this polyelectrolyte, have been discussed in chapter 4. As possible attractive forces responsible for the compact conformation at low pH, VAN DER WAALS attraction and hydrophobic bonding between the methyl groups in the main chain were considered. In addition, the strength of the COULOMBIC interaction between the carboxyl groups also plays a role in the conformational transition.
    The conformational transition from the a- to the b-conformation in free and adsorbed PMA-pe, was studied by potentiometric titration. Data for adsorbed PMA-pe were obtained by titrating polyelectrolyte-covered emulsion droplets. It was found that the conformational transition also occurs in adsorbed PMA-pe. This conformational transition is reflected in the rheological properties of paraffin in water emulsions, stabilized by PMA-pe. It could be concluded both from viscosity and dynamic data, that strong attraction between the emulsion droplets occurs only at a low degree of neutralization α, that is, if a substantial part of the adsorbed PMA-pe is in the a-conformation. Then both the dynamic moduli and the viscosities are very high. On the contrary at high a the emulsions were very fluid with little or no indication of attraction between the adsorbed polyelectrolyte sheets.
    The main conclusions from the potentiometric titration data and the rheological measurements are:
    a. the attraction between the polyelectrolyte segments, observed at low αin solution occurs also between loops and/or tails, adsorbed on one emulsion droplet;
    b. the high values of the dynamic moduli and of the viscosities at low αare due to attraction between extending loops and/or tails, adsorbed on different droplets;
    c. the two types of interaction are very similar.
    This conclusion was confirmed by the influence of methanol on Na-PMA-pe stabilized emulsions and the effect of temperature. Moreover, from these experiments it could also be concluded, that probably the hyper-coiled conformation at low α, is to a large extent due to hydrophobic bonding.
    The influence of Ca ++ ions on the properties of the polyelectrolyte was also investigated. Potentiometric titration showed that, in the presence of Ca ++ ions, the conformational transition is moved to higher a. Again the transition is reflected in the rheological properties of emulsions, stabilized by Ca-PMA-pe. The balance between the inter- and intramolecular interaction forces and the interactions themselves are more complicated than in the case of Na-PMA-pe. This complex character is reflected in the more complex rheological functionalities (η(α), G' (α) curves) of emulsions stabilized by Ca-PMA-pe.
    The interactions between adsorbed macromolecules were further investigated by studying the properties of polymer stabilized thin free liquid films. Measurements on films, stabilized by PVA or PMA-pe, were reported in chapter 5.
    The interaction forces which must be taken into account in a PVA film are VAN DER WAALS attraction, hydrostatic pressure and steric interaction. The VAN DER WAALS attraction over a film can be calculated. The equilibrium film thicknesses of the films were determined at varying hydrostatic pressure. Then the steric repulsion force Fs between the two adsorbed PVA layers was obtained by equalizing - Fs with the hydrostatic pressure and the VAN DER WAALS attraction. So the steric repulsion force could be calculated for different equilibrium thicknesses. Next the free energy of steric interaction was found by graphic integration of the force-distance curve. These values can be compared with theoretical predictions.
    In order to calculate the free energy of steric interaction theoretically, a model of the segment density distribution had to be developed. The proposed semiquantitative model was based on the consideration that the molecular weight distribution of the used PVA samples is wide and the presumption that a large fraction of the segments is adsorbed as tails. Indications for this presumption were found by comparing the extrapolated ( Fs - 0) film thickness with the ellipsometric thickness of an adsorbed layer. This model leads to the conclusion, that the properties of the outer part of the adsorbed layers are dominated by a few extending tails. The free energy of steric repulsion, thus calculated with the HESSELINK et al. (1971b) theory of steric repulsion, between two adsorbed PVA ( M v = 27,000 or 86,000) layers, agrees well with the experimentally determined values for reasonable lengths of the tails.
    In chapter 5 also the drainage behaviour and the equilibrium thicknesses of PMA-pe films, made at different values of the degree of neutralization a, were discussed. The measured equilibrium thicknesses correlated well with ellipsometric measurements of an adsorbed layer. The drainage pattern changes if or is varied. At low αthe films are rigid, whereas at high αthey are mobile. Also the dilatational modulus decreased from α= 0.1 to α= 1.0. Probably the interaction forces between the polyelectrolyte segments which are responsible for these phenomena, are the same as those which induce the conformational transition in the molecule or which are responsible for the drastic changes in the rheological properties of emulsions stabilized by PMA-pe if αis varied.
    A more elaborate discussion of the rheological properties of PMA-pe stabilized emulsions is given in chapter 6. Both dynamic and creep measurements were reported.
    In the dynamic experiments the storage modulus G' and the loss modulus G'' were measured as a function of the frequency ω. The degree of neutralization, the polyelectrolyte supply and the salt concentration were variables. By comparing the gel point concentration of free PMA-pe with the polyelectrolyte concentration in the layer between two emulsion droplets, it was concluded that there also a gel could be formed if attractive forces between the polyelectrolyte segments dominate. This conclusion is supported by analyses of the G' (ω) and G'' (ω) curves. In cases where such a kind of gel is formed, it is possible to relate G' to the number of polyelectrolyte cross-links between two droplets. Equations were given for the case of an ideal network model and for an aggregate model of the emulsion structure. For both models equations were also derived relating G' to the VAN DER WAALS attraction between the droplets. It was found that the ideal network model was good enough to interpret semiquantitatively the results obtained for the viscoelastic emulsions. The VAN DER WAALS attraction between the emulsion droplets proved to be much less important than the interactions between the polyelectrolyte sheets. It was calculated that at αPMA-pe = 0.1, about 400-1000 polyelectrolyte bonds were formed between two emulsion droplets at interparticle distances of 30 to 50 nm. It implies that about 10-20% of the polyelectrolyte molecules, present in the contact region between two emulsion droplets, are directly involved in the formation of these bonds.
    A short discussion was given of the unusually high values of the loss factor tg δ. The suggestion was put forward that these high values follow from the fact that liquid must move in and out of the micro gels between adjacent emulsion droplets or from the relaxation of the polyelectrolyte cross-links, during a deformation cycle.
    The creep curves were analysed by assuming the existence of both strong and weak bonds between the emulsion droplets. If measured under the proper conditions the weaker (secondary) bonds are broken, but the stronger (primary) bonds are not. Then it is possible to calculate from a non-linearity in the deformation as a function of the shear stress, if any, the contribution of the secondary bonds to the shear stress. The secondary bonds were identified as VAN DER WAALS attraction between the emulsion droplets and the primary bonds as interactions between polyelectrolyte molecules adsorbed on different droplets. Again it was found that the VAN DER WAALS attraction is relatively unimportant. From the found contribution to the shear modulus of an emulsion due to VAN DER WAALS attraction and steric repulsion between the droplets the interparticle distance was calculated to be 25-30 nm. This value was of the same order of magnitude as the results of the film thickness measurements. A semiquantitative assessment of the activation energy necessary to break a polyelectrolyte-polyelectrolyte bond showed that the interactions between the methyl groups must have a cooperative character.
    It was concluded that the results of creep and dynamic measurements support each other.

    In conclusion, this study shows that both rheological measurements of sterically stabilized dispersions and the investigation of polymer stabilized thin liquid films are excellent tools for investigating the interactions between adsorbed macromolecules. Intramolecular interactions and interactions between macromolecules adsorbed on different interfaces are very similar. The latter interactions are dominated by the outer part of the adsorbed macromolecule layers.

    Kolloidale stabiliteit en dubbellaageigenschappen van zilverjodide in water-ethyleenglycol mengsels
    Wit, J.N. de - \ 1975
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 117
    alcoholen - colloïden - derivaten - elektriciteit - ethyleenglycol - glycolen - jodide - magnetisme - polyethyleenglycol - zilver - thiolen - elektromagnetisme - alcohols - colloids - derivatives - electricity - ethylene glycol - glycols - iodide - magnetism - polyethylene glycol - silver - thiols - electromagnetism

    The purpose of this study was an analysis of the colloidal stability in mixed aqueous-alcoholic media. Addition of alcohol to water gives the opportunity of changing the dielectric constant of the medium, which is a very important parameter in sol stability.

    However, addition of alcohol not only influences the properties of the diffuse part of the double layer, it results also in changes of the STERN layer, such as an increase of the STERN layer thickness and a change of the potential ψ d of the outer HELMHOLTZ plane.

    In order to gain more insight in the relative influences of both parts of the double layer, we studied systematically the stability and the double layer properties of colloids under comparable conditions. In doing so, we explicitly incorporated the behaviour of the STERN layer in the stability research.

    Special attention was paid to a quantitative check of important parameters of the diffuse part of the double layer, resulting in an extension of the DLVO-theory.

    Silver iodide in the water-ethylene glycol system was chosen as the model for experimental research. In Chapter 1, the motivation of this choice is given as well as the outline of the present study. It was pointed out that it can be used both for stability experiments and for double layer investigations. This gives the possibility of combining information from two sources under nearly identical conditions. Ethylene glycol (EG) was introduced because it is a suitable medium for stable colloidal solutions of AgI, and it can be mixed with water in all proportions. By changing the EG content, a gradual transition of the dielectric constant to half its value in water is obtainable.

    However, a consequence of reducing the dielectric constant of the medium is an increased tendency of ionic association rendering the salt concentration and, for multiply charged ions, also the valence no longer a univocal measure for the amount of countercharge.

    In Chapter 2, the extent of ion association is checked by conductivity experiments of two representative electrolytes, KNO 3 and Ba(NO 3 ) 2 , in different EG- water media. From the results it appears that no significant association of Ba 2+-ions occurs at the concentrations encountered in the stability experiments. However, the concentrations of KNO 3 required in order to ensure coagulation are about 50 times higher than those of Ba(NO 3 ) 2 and, as a consequence, appreciable association in EG media must be taken into account.

    A further analysis of the results of the conductivity measurements revealed that there is an interaction between EG and water molecules, which is maximal at about 30 mole % EG. It appears that in this region two water molecules are bound by one EG molecule.

    In Chapter 3, the stability experiments are described and the results discussed in terms of the DLVO-theory with several modifications. The stability of the AgI-sols against some 1-1 and 2-1 electrolytes was determined using the kinetic method. The rates of coagulation were measured by a Stopped Flow Spectrophotometer. This apparatus mixes equal volumes of sol and salt solution in a very short time and allows interpretation of particle aggregation in the very early stages, up to doublet formation. Several corrections have been applied, among which the optical corrections accounting for the difference in light scattering between a dumb-bell of two spheres and two separate spheres, and the hydrodynamical correction on the coefficient of diffusion required when two spheres approach each other closely.

    From the experimental results, a collection of log W -log c plots were obtained, where c is the electrolyte concentration and W the stability ratio defined with respect to the fast coagulation rate of the sols.

    The flocculation rates revealed a defect in either the technique of observation or in the kinetics of flocculation in the sense that the flocculation seems not to be a bimolecular process.

    From the log W - log c plots, no special attempts have been made to determine critical coagulation concentrations, but instead at each salt concentration values of ψ d were calculated. This evaluation was based on the FUCHS integration procedure after introduction of some improvements. The major improvement is that we accounted for the fact that the electrostatic repulsion V R acts only over the diffuse part of the double layer, whereas the VAN DER WAALS attraction V A operates over the total distance between the particles. Hence, V R has a range that is 2Δshorter than the range over which V A operates where Δis thickness of the STERN-layer. The values for Δin the different EG-water mixtures were derived from model considerations based on a tetrahedal buildup of the STERN layer. It was assumed, that upon collision the particles do not penetrate into each others STERN layers.

    The new procedure leads to the following deductions. A consequence of the improved model of double layer interaction is the fact that sometimes the maximum interaction energy is located within the STERN layer, which needs further elucidation. Values of ψ d are obtained that are relatively low as compared with the unmodified model. The calculated values of ψ d are very sensitive to the choice of the thickness of the STERN layer. To a lesser extent, they depend also on the choices of the HAMAKER constant and the particle radius.

    In Chapter 4, the electrical double layer was studied in EG-water mixtures in the presence of 10 -1M KNO 3 or 10 -3M Ba(NO 3 ) 2 , using the well-known titration technique. The replacement of water by EG has the following consequences:

    a. The solubility of silver iodide increases by about a factor of two. This was mainly attributed to the change in standard free energy of solvation of the silver ion and, at the higher EG-water contents, to the change of the activity coefficients.

    b. The point of zero charge moves in a positive direction. This is in a minor part attributable to a change in solvation free energy of the silver and iodide ions, but to a large extent to a change in χ-potential. The maximum shift of the zero point of charge is = 89 mV in 10 -1M KNO 3 . It was deduced that the χ-potential causes about 72.5 % of this shift. This was explained by the preferential orientation of EG molecules with their negative sides to the AgI, surface; similarly water molecules are also oriented with their negative sides to this surface, but with a much bigger net moment. The shift of χis much smaller than that reported for EG at the mercury/aqueous solution interface. On mercury, it has been suggested that the component of the molecular dipole perpendicular to the surface is very small and, in any case, with the positive end toward the metal.

    c. All titration curves at different x EG pass through a common intersection point located at -3.3 μC/cm 2in 10 -1M KNO 3 and -3.8 μC/cm 2in 10-3 M Ba(NO 3 ) 2 . This point can be identified as the surface charge where the relative surface excess of EG is maximal. It appears to be dependent on the nature of the counterion. It should be remarked that the intersection point in EG is located at more negative surface charge than that reported for butanol. This can be explained by the stronger competition of the EG dipoles with water at the AgI, surface as compared with the butanol molecules.

    d. The double layer capacitance decreases with increasing x EG . This is mainly due to an increase of the STERN layer thickness. At values of x EG ≥0.5 no further decrease in double layer capacitance was observed, from which it was concluded that the AgI, surface is entirely covered by EG molecules at these EG contents of the medium.

    In Chapter 5, the double layer data were combined with the information derived from colloidal stability in order to gain insight into the composition of the STERN layer. The following conclusions were drawn:

    - A large part of the counter charge resides in the non-diffuse part of the double layer. This could only be explained by assuming specific adsorption of K +and Ba 2+ions at negatively charged AgI surfaces.

    - The specific adsorption of Ba 2+ions is much stronger than that of K +ions. It has the consequence, that, because of the much lower concentration used in the case of Ba 2+, only a small part of the Ba 2+ions take part in the double layer interactions.

    - These pronounced effects of Ba(NO 3 ) 2 may have repercussions with respect to the applicability of the usual smeared-out double layer picture, a feature that deserved more attention.

    This study has shown that the influence of the STERN layer on colloidal stability should not be neglected. From the results in this study, it can be concluded that, in the case of KNO 3 and RbNO 3 in EG, at least 20-40 % of the stability changes are caused by STERN layer effects and only 60-80 % to changes of the dielectric constant and ionic association in the diffuse double layer.

    Adsorption of polyelectrolytes at liquid-liquid interfaces and its effect on emulsification
    Böhm, J.T.C. - \ 1974
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 110
    oleïnezuur - onverzadigde vetzuren - carbonzuren - acrylzuur - kunststoffen - industrie - emulsies - oliën - water - adsorptie - sorptie - chemie - colloïden - oppervlakten - macromoleculaire stoffen - oppervlaktechemie - oleic acid - unsaturated fatty acids - carboxylic acids - acrylic acid - plastics - industry - emulsions - oils - water - adsorption - sorption - chemistry - colloids - surfaces - macromolecular materials - surface chemistry

    In this study we have investigated the adsorption behaviour of a number of synthetic polyelectrolytes at the paraffin oil-water interface and the properties of paraffin oil-in-water emulsions stabilized by these polyelectrolytes.

    Polyacrylic acid (PAA), polymethacrylic acid (PMA) and the copolymers of the monomeric acids with their methyl esters (resp. PAA-pe and PMA-pe) are used. Most of the experiments have been performed with PMA-pe, whereas the other polyelectrolytes are mainly used for the sake of comparison. In chapter 2 the synthesis and some relevant bulk properties are described. These properties are determined by potentiometric titrations and by viscosimetry. In agreement with the literature it is found that PMA and PMA-pe undergo a reversible conformational transition as a function of pH. At low charge density these polyelectrolytes are characterized by a compact hypercoiled conformation (aconformation) and by increasing the charge density they unfold, resulting finally in an expanded conformation (b-conformation). This conformational transition is not observed for PAA and PAA-pe.

    Several techniques are used to obtain information about the mode of adsorption of these polyelectrolytes at the paraffin oil-water interface and about the properties of emulsions stabilized by them. At undisturbed interfaces the interfacial tension as a function of time γ( t ) has been determined from interfacial tension measurements with the Wilhelmy-plate technique. Adsorbed (section 3.2.) as well as spread (section 3.3.) monolayers are investigated. It is found that the attainment of the steady state of the interfacial tension γ(∞) is faster the more compact the molecules are (i.e. lower charge density, higher ionic strength, Ca 2+counterions) and the higher the polyelectrolyte concentration. These findings agree with the fact that the rate of reduction of the interfacial tension dγ/d t is determined by three processes, viz. diffusion of the polymer molecule to the interface with adsorption in its bulk conformation, reconformation and spreading of the arrived molecules. As all three processes usually occur simultaneously, the adsorption behaviour of polymers is very complex. For compact molecules diffusion leads to a rapid accumulation of segments at the interface and reconformation and spreading hardly take place. For more expanded molecules diffusion is slow and the rate of reduction of γtakes place mainly through reconformation. As at higher c p the supply by diffusion is faster, it is understandable that the steady state is attained faster according as c p is higher and reconformation and spreading are reduced.

    If spreading can take place to a high degree, the fraction of segments per molecule adsorbed in the first layer p , becomes relatively high. From a semiquantitative interpretation of the interfacial tension measurements it is indeed found that p increases the less compact the molecules are (section 3.5.). To be able to do these calculations also adsorption experiments at an undisturbed liquid-solid (L/S) interface have been performed (section 3.4.). The amount of adsorbed PMA-pe at the polystyrene latex of low surface charge has been determined and it is assumed that Γ at the oil-water interface and the latex surface does not differ.

    Additional information about the mode of adsorption has been obtained from experiments in which the conditions in the water phase were changed after adsorption (chapter 4). Especially changes in pH produced interesting results (so-called γ-pH cycles). It was found that the mode of adsorption determines the degree of (ir)reversibility of the adsorbed layer by increasing pH. The longer the average train length of the adsorbed molecule, the higher the degree of irreversibility. These findings agree with the mode of adsorption as described already in chapter 3. If reconformation and spreading can take place to a high degree (i.e. at high pH) the average train length is large and the molecules are irreversibly adsorbed. If diffusion is the main factor (i.e. at low pH) the average train length is small and desorption is possible.

    The properties of emulsions stabilized by these polyelectrolytes have been investigated by the specific interfacial area S (expressed in m 2per ml paraffin oil), the amount of adsorbed polyelectrolyte at the emulsified interface Γ e (expressed in mg per m 2) and the viscosity of the emulsions. Γ e has been obtained by a depletion method (section 5.3.) and S by turbidity measurements (section 5.4.). As these properties did not change with time after finishing emulsification, they enable us to characterize the adsorbed layer at an emulsified interface and they reflect the interesting but complex dynamic processes occurring during emulsification with polyelectrolytes as stabilizers. The results of these investigations are summarized in chapter 5 ( S , Γ e and the mechanism of emulsification) and in chapter 6 (rheology of emulsions and characterization of the adsorbed layer at an emulsified interface).

    The interfacial tension at undisturbed interfaces and the emulsion properties have been investigated as a function of a number of parameters. These parameters are polyelectrolyte concentration c p or polyelectrolyte supply c p ', degree of neutralization α(charge density) or pH, ionic strength, nature of the counterion (Na +, Ca 2+) and chemical constitution of the polyelectrolytes. Moreover, it was investigated whether the a- and b-conformation in bulk was reflected in the adsorbed layer, the emulsion properties and the behaviour during emulsification. We will here summarize the effects of these parameters on the interfacial tension (undisturbed interface) and on the emulsion properties (disturbed interface). The comparison between both conditions of the interface is not meant to predict emulsion properties from the adsorption behaviour at an undisturbed interface. We intend to find out which of the investigated parameters play a major role in the interfacial activity at undisturbed interfaces, the emulsification process and the properties of the emulsions once they are formed. To predict the emulsifying behaviour of a polyelectrolyte it is necessary to investigate its interfacial behaviour under dynamic conditions. Such measurements are available (section 3.6.). However, it appears that the theory for the interpretation of these results is not yet applicable to polymers.

    In the course of this study the parameters mentioned before have been investigated seperately for each emulsion property or for each technique to determine the interfacial properties at undisturbed interfaces. Therefore it seems worth while to summarize the results here systematically for each parameter.

    Polyelectrolyte concentration c P or polyelectrolyte supply c P '

    The steady-state value of the interfacial pressure for adsorbed layers always increases with c p , d Π(∞)/d c p >0 (see figs. 3.4. and 3.5.). For spread monolayers it is always found that d Π(∞)/d Γ sp >0 (see figs. 3.11.-3.14.). From the literature it is known that at L/S interfaces d Γ/d c p ≥0. Hence it is acceptable to assume dΓ ad /d c p ≥0 for the adsorption of polyelectrolytes onto undisturbed L/L interfaces. However, for adsorbed polymer layers an unambiguous relation between c p , Πand Γ ad cannot be given. In this study it is argued that Πis primarily related to the occupation of the first layer θ ad (see section 3.5.).

    At emulsified interfaces it is always found that dΓ e /d c P ' ≥0 (see section 5.5.2.). S as a function of c P ' is not univocal and depends on both c P ' and the flexibility of the polyelectrolyte (see below: degree of neutralization). In the region of low c P ' it is always found that S increases with c P ' and that the flexibility only affects the value of S . Above a given c P ' the relation S ( c P ') depends on the flexibility of the polyelectrolyte molecule (see section 5.6.). As the flexibility depends strongly on the degree of neutralization the relation between S and c P ' is a function of α.

    The viscosity of emulsions - restricted to the highly viscous emulsions with PMA-pe at α ≤0.30 as the stabilizer - increases with c P '. It is found that the viscosity is a function of the amount of PMA-pe adsorbed in loops in the a-conformation (see chapter 6).

    Degree of neutralization α(or pH)

    As shown in chapter 2 an increase of α gives an expansion of the coil in bulk. At an oil-water interface a higher charge density will reduce the interfacial activity and if adsorption takes place it will result in a thin layer. Moreover, a higher charge density of the adsorbed molecules will give and increased stability of the emulsion droplets by electrostatic repulsion.

    To summarize our findings for this parameter it is necessary to distinguish between esterified (PAA-pe and PMA-pe) and non-esterified (PAA and PMA) polyelectrolytes.

    In general the steady-state interfacial pressure is reduced at higher pH or α(see figs. 3.6.-3.9.). PAA and PMA are even not interfacially active at pH>6. From the Π(∞) values of PMA-pe it is deduced that the adsorption mechanism at high pH differs from that at low pH. The γ-pH cycles underline this conclusion (chapter 4). It appears that adsorption of PMA-pe at pH = 4 enables desorption upon pH increase. However, adsorption at pH = 9, followed by a reduction of the pH to 4, gives an adsorbed layer that is not desorbable upon pH increase. Adsorption at pH 4 takes place with short sequences of segments in trains, whereas at pH 9 these sequences are longer. Desorption of the long sequences is unlikely.

    Spread monolayers of PMA-pe do not confirm the important differences in the interfacial properties as a function of pH. The Π- A isotherms of PMA-pe are hardly dependent on the pH (figs. 3.12-3.14.).

    The differences between esterified and non-esterified polyelectrolytes are also reflected in the emulsifying and stabilizing capacity of these polyelectrolytes as a function of α. At α< 0.5 emulsions stabilized by any of the polyelectrolytes are stable to coalescence, whereas at α>0.5 emulsions prepared with PMA and PAA coalesce directly but emulsions prepared with PMA-pe and PAA-pe remain stable to coalescence (see fig. 5.3.). The obtained area of emulsions stabilized by PMA-pe and PAA-pe as a function of αreflects again the difference in adsorption mechanism between low and high pH.

    As the emulsions have been prepared under standard conditions (time of agitation, intensity of agitation and volume fraction of dispersed phase constant) the differences in S as a function of αand c P ' reflect the mechanism of emulsification. The main factor is the way in which the gradients of γalong the interfaces of newly created droplets are reduced. The supply by diffusion to the interface and the rate of reconformation and spreading reduce these gradients and it is the ratio of their contributions that determine the mechanism of emulsification. From earlier experiments with PVA as emulsifying agent it was found that for polymers with a relatively low flexibility S passes through a maximum as a function of c P '. For polymers with a relatively high flexibility S levels off with increasing c P '. Analogous results are now found for PMA-pe as a function of α. Especially at α ≥0.70 the flexibility is low and a maximum in S is observed. However, the higher αthe less pronounced the maximum is, because of additional factors such as the increased electrostatic repulsion and the reduced diffusion (see section 5.6.). At 0.30 ≤ α ≤0.50 S levels off with increasing c P '. At α ≤0.10 an enhanced coalescence without desorption results in an increasing S with c P ' and the leveling-off or the maximum is not reached in the region of c P ' investigated by us.

    Γ e decreases with increasing αfor all types of polyelectrolytes (see fig. 5.5.). From a comparison with the adsorption at the polystyrene latex surface it is concluded that emulsions stabilized by Na +PMA-pe at α< 0.30 and emulsions stabilized by Ca 2+PMA-pe over the whole region of αare characterized by a coalescence without desorption during emulsification (see section 5.5.4.).

    The viscosity of emulsions stabilized by PMA-pe decreases considerably with higher α. For the explanation see below the section on the conformational transition.

    Ionic strength (NaCI)

    A high ionic strength reduces the electrostatic repulsion on the polyelectrolyte chain and hence increases its flexibility. Moreover, the diffusion to the interface increases with increasing ionic strength. These effects will be more pronounced the higher the degree of neutralization. It is anticipated that Γand Π(∞) will increase with increasing ionic strength. The effect on the emulsion interface is not easy to predict. It depends on the sum effect of the increased diffusion and flexibility and the reduced electrostatic repulsion.

    The effect of ionic strength is only investigated for PMA-pe. In general the interfacial pressure increases with higher ionic strength (see fig. 3.4). It is notable that the ionic strength has no effect on the interfacial pressure at pH ~ 9, whereas it is still measurable at pH = 4. This again indicates the differences in adsorption mechanism as a function of pH.

    At the polystyrene latex surface it is indeed found that Γ increases with increasing ionic strength.

    S decreases and Γ e increases slightly with increasing ionic strength. The viscosity of PMA-pe stabilized emulsions at α< 0.30 does not depend on ionic strength.

    In general the effects of ionic strength are small compared with the more drastic effects for αand the nature of the counterion.

    Nature of the counterion (Na+, Ca 2+)

    It is anticipated that Ca 2+counterions considerably affect all investigated properties. The bivalent bonding of Ca 2+ions with dissociated carboxylic groups reduces the flexibility of the polyelectrolyte molecule.

    The interfacial pressure is increased and becomes much less a function of αand c p (see figs. 3.5. and 3.8).

    S is decreased and Γ e considerably increased in the presence of Ca 2+ ions. Moreover, S and Γ e are also hardly dependent on α. It is concluded that bivalent bonding is indeed responsible for this behaviour. From a comparison with the adsorption on polystyrene latex it is deduced that emulsions stabilized by PMA-pe in the presence of Ca 2+ ions undergo coalescence without desorption during emulsification (see section 5.5.4). The effect of Ca 2+ions on the viscosity of emulsions is not investigated.

    Chemical constitution of the polyelectrolytes

    A number of the results are already mentioned in the summary of the effect of the degree of neutralization. On account of bulk properties a distinction between PMA(-pe) and PAA(-pe) is obvious (see chapter 2). However, it appears that in the interfacial pressure, S and Γ e the difference between esterified and non-esterified polyelectrolytes is more pronounced. The interfacial activity of the esterified polyelectrolytes is higher than that of the non-esterified ones (see fig. 3.9). In general S is higher for the esterified polyelectrolytes, although at α< 0.30 the presence of the ester groups is responsible for an additional coalescence during emulsification, resulting in a lower S for emulsions stabilized by esterified polyelectrolytes (see fig. 5.3). Γ e is higher for the esterified polyelectrolytes than for the non-esterified ones. This effect is very obvious at α< 0.30.

    It is found that the viscosity of emulsions is strongly affected by the presence of the methyl groups in the main chain but not by the presence of esterified groups (see fig. 6.1.). It appears that the high viscosity of emulsions stabilized by PMA-pe at α< 0.30 is related to the occurrence of a hypercoiled conformation in the molecules.

    Conformational transition

    The occurrence of two different conformations in bulk is partly reflected in the interfacial and emulsion properties. S and Γ e are not affected by the presence of the hypercoiled conformation, but only by the presence of the ester groups. Moreover, the interfacial pressure as such does not depend on the conformation. However, the γ-pH cycles (chapter 4) allow us to locate a transition region in the conformation of the adsorbed layer, which is related to the occurrence of the a- and b-conformation in bulk. Adsorption of the PMA-pe molecules onto the interface from the solution in which the a-conformation exists, leads to reversibility of the adsorbed molecules upon increasing pH after adsorption. However, the molecules are irreversibly adsorbed if the b-conformation exists in solution. This difference in the conformation is only detectable for the esterified polyelectrolytes, since for PMA and PAA desorption always occurs upon increasing pH.

    The reversible adsorption of PMA-pe at low pH as compared to the irreversibility at high pH is interpreted by proposing an adsorption model in which the length of the trains plays a dominant role. Adsorption of PMA-pe at pH = 4 gives relatively short trains which are desorbable by increasing pH. However, adsorption from solutions in which the b-conformation exists gives relatively long trains and desorption is less probable by increasing pH.

    The occurrence of two different conformations in the adsorbed layer is indicated in the viscosity of emulsions stabilized by PMA-pe and/or PAA-pe. The viscosity is only very high for emulsions stabilized by PMA-pe at α ≤0.30. It appears to be caused by intermolecular foces of attraction between molecules adsorbed at different droplets and not by a bridging of the droplets by adsorption of one molecule at two or more droplets. These forces are of the same origin as those responsible for the stabilization of the hypercoiled a-conformation. From the viscosity of emulsions stabilized by PMA-pe at α= 0.10 in a mixture of water and methanol as the continuous phase it is concluded that the main stabilizing factor for the hypercoiled structure is hydrophobic bonding. This conclusion disagrees with some literature references in which v.d. Waals forces of attraction are held responsible.

    It appears that the prediction of emulsion properties from the interfacial tension measurements at undisturbed interfaces and from the bulk properties of the polyelectrolyte is a difficult problem, because of the very complex dynamic processes occurring during emulsification. However, this study demonstrates that a number of parameters exert an influence on one or more of the investigated emulsion properties and on the interfacial properties at undisturbed interfaces. It provides more insight into the factors which influence the behaviour of polyelectrolytes at disturbed and undistrubed liquid-liquid interfaces. It contributes to a better conception of the behaviour of proteins at interfaces in more practical systems.

    Oppervlakkig bezien
    Bijsterbosch, B.H. - \ 1972
    Wageningen : Veenman - 15
    oppervlakten - grensvlak - chemie - evenwicht - colloïden - adsorptie - oppervlakteverschijnselen - oppervlaktechemie - surfaces - interface - chemistry - equilibrium - colloids - adsorption - surface phenomena - surface chemistry
    Polymer adsorption and its effect on colloidal stability : a theoretical and experimental study on the polyvinyl alcohol-silver iodide system
    Fleer, G.J. - \ 1971
    Landbouwhogeschool Wageningen. Promotor(en): J. Lyklema. - Wageningen : Veenman - 144
    colloïden - industrie - kunststoffen - macromoleculaire stoffen - colloids - industry - plastics - macromolecular materials - cum laude

    The purpose of this study was to gain insight in the factors determining the stability of hydrophobic sols in the presence of polymers, with the emphasis on the destabilisation of sols by polymers and the role played by salts therein.

    In chapter 1. the practical importance of polymer stabilisation and destabilisation is shown by several examples, a.o. in industrial applications, in water purification and soil structure improvement. Thereafter the choice of the PVA-AgI system as a model for this study was explained. PVA has a simple structure and is uncharged and its concentration in solution may be readily determined. This is important for adsorption measurements. AgI provides a good model for the dispersed phase: the properties of the electrical double layer on AgI in the presence of salts and low molecular weight organic substances have been investigated extensively and the specific surface area can be determined easily. Moreover, with a combination of PVA and AgI one has the advantage of being able to acquire information on the properties of the first layer on the surface by a comparison with known data on the butanol-AgI and ethylene glycol-AgI systems.

    The characterisation of the materials used is described in chapter 2. The specific surface area of AgI was determined by three independent methods. The results of these methods agreed well with each other. The average radius of the AgI particles turned out to be about 500 Å. From viscosimetric measurements on PVA solutions the molecular weights and configurational parameters of PVA, such as the radii of gyration, the length of a statistical chain element and the linear expansion factors were determined. In addition, it was shown that the PVA used is essentially uncharged.

    In chapter 3. the measurement of the amount of PVA adsorbed per ml is treated. The adsorption isotherm shows a pronounced high affinity character. The maximum amount adsorbed is 1-1.5 mg/m 2, depending on the molecular weight and the degree of hydrolysis of the PVA. The maximum adsorption increases somewhat with increasing molecular weight; for PVA with 12% of acetate groups it is distinctly higher than for PVA which is nearly completely hydrolysed. At maximum adsorption one fourth of the segments at most can be in contact with the surface; the remaining parts of the molecule protrude into the solution in the form of loops and tails. From measurements of the adsorption as a function of time and from 'two- step' adsorption experiments it could be deduced that the adsorption of segments is reversible. However, desorption of whole polymer molecules is not measurable.

    In chapter 4. measurements are described to obtain the layer thickness and the coverage in the first layer on the surface by PVA. From protection measurements qualitative information was obtained about the layer thickness. The protective power appeared to be slightly dependent on the molecular weight and to depend somewhat more strongly on the degree of hydrolysis of the PVA. The thickness of the adsorbed layer was viscosimetrically determined as a function of the amount adsorbed. The maximum layer thickness is about 100 Å. By measuring the electrophoretic mobility of polymer covered particles the layer thickness was likewise estimated. These results are in good agreement with the viscosimetric results.

    The coverage in the first layer on the surface was estimated from the shift of the point of zero charge and from the change in the surface charge on adsorption of polymer, in comparison with the same properties of AgI in the presence of butanol and ethylene glycol. A reasonable estimation for the percentage of the surface which is occupied by PVA turned out to be 70% for amounts adsorbed of more than half the maximum.

    With the help of these data the distribution of segments in the adsorbed layer could be obtained. For amounts adsorbed between 0.5 and 1.0 mg/m 2a HOEVE distribution applies. In the first layer on the surface the polymer volume fraction is about 70%. At a distance equal to the thickness of the first layer a discontinuity occurs, the volume fraction dropping to 56 %, and in the remaining part of the adsorbed layer the segment distribution is exponential. If more than 1 mg/m 2is adsorbed possible end effects occur: due to the presence of long tails at the ends of a polymer molecule the thickness increases more strongly with the amount adsorbed than predicted from the HOEVE distribution.

    The model for the segment distribution is somewhat oversimplified: it appeared to be impossible to account for the differences between different molecular weights at a given amount adsorbed.

    Results with respect to the flocculation of AgI by PVA have been given in chapter 5. Flocculation was found to be optimal if a special method is used for the mixing of PVA and Agl. Most efficient flocculation is obtained if a given volume of sol with uncovered particles is added to an equal volume of a sol with nearly completely covered particles. This phenomenon could be easily explained on the bridging model: flocculation occurs because loops of the adsorbed layer of one particle attach to the other. In this way a network of AgI particles interconnected by polymer bridges is formed. For the explanation of the efficiency of the way of mixing irreversibility of the adsorption of the polymer molecules is essential.

    Another important condition for efficient flocculation is the presence of a small amount of electrolyte. On these grounds the flocculation should be referred to as sensitisation. The minimum salt concentrations which are needed for flocculation are in the ratio of about 100:10:1 for salts with univalent, bivalent and trivalent counterions, respectively. Critical flocculation concentrations measured after a fixed time of flocculation were found to depend on the sol concentration. From measurements of the initial rate of flocculation, and from experiments in which the flocculation time was adjusted to the sol concentration, it was shown that this dependence on the salt concentration has a kinetic origin. The flocculation by bridging was found to be a bimolecular process.

    The critical flocculation concentrations were found to depend only slightly on the molecular weight of the PVA. For a PVA with a higher acetate content the amount of electrolyte needed was found to be significantly lower.

    In chapter 6. an attempt has been made to interpret the flocculation theoretically. To that order the free energy of interaction between a covered and an uncovered particle has been calculated. On account of the complicated nature of the problem only an approach for flat surfaces has been considered.

    In addition to the VAN DER WAALS attraction and the double layer repulsion the contribution to the free energy of interaction due to the adsorbed polymer has to be calculated. This contribution was formally split up in two terms, the first being the adsorption attraction due to the gain in free energy on account of the adsorption of segments on the second particle. The second term is the configurational repulsion which is caused by the entropy loss if a loop becomes two bridges by the adsorption of the middle segments of the loop. The fundamental assumption used to evaluate these two terms is that the number of segments which, at a given interparticle separation H , adsorbs on the second particle equals the number of segments which, in the absence of the second particle, would lie beyond a distance H from the surface. Using the theories of HOEVE and HESSELINK and the distribution of segments derived in chapter 4. these two polymer contributions to the free energy of interaction could be obtained.

    It was found that the VAN DER WAALS attraction is negligibly small in comparison to the other terms. The total free energy of interaction has the following characteristics. At small salt concentrations a maximum occurs at large distances due to the double layer repulsion, whilst at distances of some tens of Ångströms a minimum is present, sufficiently deep for irreversible flocculation. The function of salt is to suppress the maximum at large distances by partial compression of the double layer, so that the particles can approach each other to a distance corresponding to the minimum in the free energy. The system will then flocculate.

    Although the magnitude of the polymer contribution, especially at small distances, is somewhat doubtful on account of various approximations made, the theory does give a good explanation for the amount of salt, with ions of different valencies, which is needed for flocculation. The theoretical predictions with respect to the effect of the amount of adsorbed polymer agree also with the experimental observations. From this it follows that the theory is essentially correct. Indications were obtained that a theory which is applicable to spherical particles would agree even better with the experiments. The development of such a theory would be a promising next step.

    In conclusion, this study firmly establishes the bridging model for flocculation by polymer. It appeared possible to interpret several aspects quantitatively. Especially the function of indifferent electrolytes emerged clearly.

    The influence of alcohols on the protoplasmic membrane and colloid models
    Saubert, G.G.P. - \ 1937
    Wageningen University. Promotor(en): E. Reinders; H.J.C. Tendeloo. - Amsterdam : Mulder - 90
    colloïden - modellen - plasmamembranen - alcoholen - colloids - models - plasma membranes - alcohols
    Experiments with colloid systems attempted to achieve a better understanding of biological processes. The influence of alcohols on phosphatid coacervates was compared with their influence upon the protoplasmic membrane of living cells of the alga Chara ceratophylla. Some similarity could be seen between the influences of methanol, ethanol and butanol. But the alcohols exerted their influence upon the living membrane at much lower concentration than they did on the phosphatid coacervates (ratio 1 : 10) and on the dielectric medium (1 : 10). Propanol, however, decreased the permeability of the living protoplast, in the opinion of the author by condensation of the membrane, but opened up the phosphatid coacervate. Very low concentrations of propanol could, however, increase the permeability of the protoplasmic membrane in a similar way as it increased the volume of the coacervate.

    In explaining the influence of alcohols on the permeability of the protoplasmic membrane the author considered: action on the carbon chains of the membrane components; action on the dielectric condition of the system; the structure of the system (amorphous or oriented). The use of phosphatid coacervates as a model did not mean that the protoplasmic membrane was thought to be an amorphous system. Alcohols would probably open up a protoplasmic membrane composed of oriented components.

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