Staff Publications

Staff Publications

  • external user (warningwarning)
  • Log in as
  • language uk
  • About

    '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.

    We have a manual that explains all the features 

    Current refinement(s):

    Records 1 - 50 / 60

    • help
    • print

      Print search results

    • export

      Export search results

    Check title to add to marked list
    Crystals, glasses and gels : synthesis and phase behavior of soft colloids
    Appel, Jeroen - \ 2017
    Wageningen University. Promotor(en): Frans Leermakers, co-promotor(en): Joris Sprakel. - Wageningen : Wageningen University - ISBN 9789463430104 - 139
    colloids - crystals - gels - phases - physics - colloidal properties - physical chemistry - colloïden - kristallen - gels - fasen (chemie) - fysica - colloïdale eigenschappen - fysische chemie

    Colloidal suspensions are an experimental model system for studying structural and mechanical properties of soft materials. These properties are manifested differently in colloidal solid-like phases such as crystals, glasses and gels. To further understand relations between structural and mechanical properties, it is necessary to develop well-defined colloids and employ techniques such as microscopy and rheology to study the structure and mechanics of their suspensions. This thesis presents five experimental chapters dealing with the synthesis and characterization of colloids and their suspensions. The first part of the thesis describes facile synthesis methods for latex, conjugated polymer and microgel colloids. In the second part, measurements of crystal-to-glass and glass-to-gel phase transformations in dense suspensions of microgel particles are presented.

    Natural nanoparticles in soils and their role in organic-mineral interactions and cooloid-facilitated transport
    Regelink, I.C. - \ 2014
    Wageningen University. Promotor(en): Rob Comans, co-promotor(en): Liping Weng. - Wageningen : Wageningen University - ISBN 9789462571501 - 221
    bodem - bodemchemie - interacties - colloïden - transportprocessen - deeltjes - soil - soil chemistry - interactions - colloids - transport processes - particles

    Mineral nanoparticles are naturally present in the soil and play an important role in several soil processes. This thesis uses a combination of novel analytical techniques, among which Field-Flow-Fractionation, to study nanoparticles in soil and water samples. The results show that nanoparticles can be as small as a few nanometer only and play an important role in the transport of phosphorus and trace metals in the environment. Furthermore, Fe-(hydr)oxide nanoparticles play an important role in sequestration of organic matter and phosphate in soils. The adsorption interactions between phosphorus and organic matter have important implications for the predictions of phosphorus-fertility status of the soil because phosphorus becomes more soluble in soils rich in organic matter. Moreover, this thesis shows that Fe-(hydr)oxide nanoparticles form strong aggregates with organic matter and thereby improve aggregate stability and water retention in soils.

    Groepsgedrag op de nanoschaal
    Gucht, J. van der - \ 2014
    Wageningen : Wageningen University, Wageningen UR - ISBN 9789461739711 - 23
    nanotechnologie - colloïden - groepsgedrag - polymeren - nanotechnology - colloids - group behaviour - polymers
    Organic monolayers and fluoropolymer brushes : functionalization, stability and tribology
    Bhairamadgi, N.S. - \ 2014
    Wageningen University. Promotor(en): Han Zuilhof; Cees van Rijn. - Wageningen : Wageningen University - ISBN 9789461739636 - 178
    unimoleculaire films - organische verbindingen - organische fluorverbindingen - colloïden - polymeren - adhesie - frictie - oppervlakteverschijnselen - unimolecular films - organic compounds - organofluorine compounds - colloids - polymers - adhesion - friction - surface phenomena

    This thesis deals with the adhesion and friction properties of densely grafted and covalently bound fluoropolymer brushes on silicon surfaces with varying thickness and fluorine content. A novel surface-functionalizing method is described using the thiol-yne click (TYC) reaction. The TYC reaction is highly useful for the attachment of functional (bio-)molecules and immobilization of radical initiators onto a surface with high density. Next, the hydrolytic and thermal stability of 24 different types of monolayers on Si(111), Si(100), SiC, SiN, SiO2, CrN, ITO, PAO, Au and stainless steel surfaces was evaluated. Subsequently, based on this outcome, highly stable fluorinated polymers are described as obtained using surface-initiated atom transfer radical polymerization (SI-ATRP) reactions. The effects of thickness and fluorine content on tribological properties of these layers were studied. The adhesion and friction properties were investigated using colloidal probe atomic force microscopy under dry and ambient conditions. The solvent-free lubricating properties of obtained fluoropolymer brushes have been characterized in detail, and demonstrate their potential for e.g., MEMS/NEMS devices.

    Towards colloidal size control by precipitation
    Lebouille, J.G.J.L. - \ 2014
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers; R. Tuinier. - Wageningen : Wageningen University - ISBN 9789461738677 - 151
    micellen - colloïden - nanotechnologie - deeltjesgrootte - deeltjesgrootteverdeling - micelles - colloids - nanotechnology - particle size - particle size distribution

    Many active ingredients like drugs, preservatives and vitamins are hydrophobic. In most applications for food and pharma, however, they need to be functional in aqueous environments. In order to facilitate their usage in aqueous environments one needs a way to enable the dispersion of hydrophobic compounds into submicron particles in water in a controlled manner. We investigated the stabilization by surfactants and encapsulation into micelles of hydrophobic compounds using the nanoprecipitation method. The research described in this thesis is about building more understanding of the nanoprecipitation method in relation to the relevant physical chemical parameters. The theoretical results led to predictions that were compared to experimental data. For water-soluble surfactants as stabilizers in the nanoprecipitation process a new theory was developed to relate the process parameters to the final particle size. For non-water-soluble surfactants self-consistent field theory was used in order to unravel the structure-function relationship between used copolymer chemistry and the form and morphology of the obtained particles, spherical micelles and their size.

    We analyzed new and existing experiments on the nanoprecipitation method using water-soluble surfactants as stabilizers in a systematic manner. These were interpreted in terms of a new theory that links the process and material properties to the final particle size. The nanoprecipitation procedure consists of quenching a polymer solution from a good to a poor solvent containing surfactant solution. Three characteristic time scales can be identified which affect the final particle size. First, the mixing time (τmix) was identified; the time needed to mix the polymer solution (polymer in good solvent) into the surfactant solution (poor solvent). Second, the coalescence time (τcls) was identified; the time needed for the collapsed polymer chains to coalesce into bigger droplets and subsequently to harden out into particles with long term storage stability. Last, the protection time (τpro) was identified; the time that the surfactant molecules need to completely cover the coalescing droplets and by this stop the coalescence of the collapsed polymer chains/droplets. The two latter characteristic times are intrinsic properties of the used solvents, surfactants and polymers and cannot be changed without addition of extra/new molecules. However, the mixing time is the only parameter which can be changed without modifying the material properties of the system. The mixing time can be easily varied by the method of mixing the good and the poor solvent. Using a pipette to mix the two solutions will result in a 'slow' mixing time regime and utilizing for instance an impingent jet mixer can result in a 'fast' mixing regime. For both mixing regimes a clear analytical expression could be derived enabling more efficient experimentation in order to obtain a specific final particle size. For the 'slow' mixing regime the relation between final particle size ()was found only to be dependent of the used polymer concentration (cmp) as ~cmpThe practical interpretation of this analytical expression is rather simple; an eight times higher polymer concentration will result in a two times bigger final particle size. For the 'fast' mixing regime the analytical expression can be interpreted also in an easy way; the faster the mixing the smaller the final particle size. Below a certain value for the mixing time the final particle size attains a plateau value; even faster mixing will not further decrease the final particle size. When using water-soluble surfactants the release of the cargo, which in practice often takes place after significant dilution, is expected to be fast. In order to increase the release of the encapsulated compound(s) in time we incorporated the surfactant functionality into a non-water soluble triblock copolymer. This results, even upon huge dilution, in an extended release profile in time.

    We employed self-consistent field theory for non-water-soluble surfactants in order to relate the (block copolymer) surfactant chemistry to the size and composition of the resulting spherical equilibrium micelles. The surfactants, triblock copolymers synthesized via ring-opening polymerization, were employed in the nanoprecipitation process in order to make spherical micelles. The theoretical predictions were compared to the experimental results and it was concluded that self-consistent field theory is an accurate theoretical tool to predict the size of spherical micelles given a certain chemistry and composition of the copolymers and the conditions required to form these micelles.

    We experimentally studied whether hydrophobic compounds (polymers, different active ingredients or a mixture of the two) were added in order to verify whether these spherical micelles could be loaded by these compounds. We investigated the encapsulation behavior of these micelles for hydrophobic compounds both theoretically and experimentally and considered the influence of the size for the micelles. From both the theoretical predictions and the experimentally obtained data it followed that these micelles can be used for encapsulation of hydrophobic compounds. Moreover, the theoretical predictions matched with the experimentally obtained data. It was concluded that self-consistent field predictions can be used to predict the size and stability of spherical micelles with encapsulated hydrophobic compounds.

    Tuning size and loading is mandatory for passive targeting applications because the particle size mainly determines the biologic faith. In order to enable active targeting, utilizing a targeting moiety and (specific) receptor interaction is needed while maintaining the stealthy nature of the spherical particles. We performed a theoretical self-consistent field study on spherical block copolymer micelles to investigate whether it is feasible to hide the targeting moiety within the micellar corona while maintaining receptor interaction. We determined the maximum interaction distance wherefrom targeting moiety receptor connection can be established and the required energy barrier at different distances. The outcome of these calculations was used to design a (theoretical) optimized system for active targeting.

    We used self-consistent field theory to calculate the size, loading and targeting capability of triblock copolymer based micelles enabling both passive and active targeting and verified our calculation results experimentally. Although the active targeting predictions were not verified experimentally we established a design for passive and active targeting micellar applications for, for instance, drug delivery applications while maintaining the stealthy nature of these micelles.

    Colloids at liquid interfaces: dynamics and organization
    Ershov, D.S. - \ 2014
    Wageningen University. Promotor(en): Jasper van der Gucht, co-promotor(en): Martien Cohen Stuart. - Wageningen : Wageningen University - ISBN 9789461738943 - 127
    colloïden - oppervlaktechemie - grensvlak - oppervlakteverschijnselen - capillairen - vloeistoffen (liquids) - colloids - surface chemistry - interface - surface phenomena - capillaries - liquids

    This thesis deals with spherical microparticles trapped at liquid interfaces. It focuses on two aspects of their behavior: firstly, the effect of the curvature of a liquid interface on interparticle interactions and their organization; secondly, the mobility of particles at visco-elastic interfaces.

    In Chapter 2of this thesis we showed that it is possible to induce capillary interactions between spherical microparticles with homogeneous surface chemistry by tailoring the curvature of the liquid interface. If the interfacial curvature is anisotropic, the constraint of constant contact angle along the contact line can only be satisfied if the interface is deformed locally. These deformations create excess surface area, which changes when two particles approach each other. This leads to a change in the surface free energy, which manifests itself as a capillary interaction between the particles.

    To study the effect of curvature on the interactions between particles, we created oil-water interfaces of different shape (ellipsoid, dumbbell, torus and squares) and added spherical negatively charged particles that adsorbed at these interfaces. On all these interfaces, we observed quadrupolar capillary interactions that organized the particles into square lattices. The order of this organization increased with increasing curvature anisotropy, indicating that capillary interactions are stronger as well. By contrast, on flat interfaces or on spherical droplets with homogeneous curvature, no attractive interaction was observed and only at very high surface coverage did the particles order in a hexagonal lattice, as a result of repulsive interactions.

    In Chapter 3we studied the interface deformations around particles at curved interfaces and the resulting capillary interactions theoretically. We used the finite element method to solve the Young-Laplace equation for the shape of the interface around a particle and calculated the interaction potential between the particles numerically.

    The main finding of these calculations is that for an anisotropically curved interface, with two different local principal curvatures, the particle deforms the interface in two ways simultaneously: concave deformation along one principal direction and convex – along the other, thus creating a deformation field with quadrupolar symmetry. Two particles with such deformations interact favorably only if the overlapping deformations are similar (concave-concave, convex-convex), which occurs when they approach each other along one of the two principal directions. Since the two local principal directions are always perpendicular, particles interacting along them will tend to arrange into a square pattern.

    As a consequence of the quadrupolar deformation field, two particles approaching each other along a line forming 45 degrees with the principal axes will repel each other (which is confirmed by our observations), because in this case the deformation fields overlap with four different “petals” (2 pairs of concave-convex), and the excessive surface area doesn’t reduce upon approaching, but increases. A system of two particles oriented at an angle with respect to the principal axis is therefore subjected to a torque rotating the axis of the system so that it gets aligned with one of the two principal directions. The torque magnitude reaches its maximum when the system’s axis is at an angle of 45 degrees with respect to the principal direction and decreases to 0 when the axis is aligned with one of the principal directions.

    The family of interaction potentials we obtained allows for calculating the minimum deviatoric curvature needed to initialize capillary interactions strong enough to compete with thermal energy, so that a stable organization can be expected. The calculated value was very close to the deviatoric curvature where ordering was observed experimentally in Chapter 2.

    In Chapter 4we studied the mobility of 3 mm polystyrene particles in a monolayer of 1.5 mm core-shell microparticles deposited at flat air-water interfaces; all the particles present in the system were stabilized by negative charges.

    In this exploratory chapter we made an attempt to characterize the mechanical properties of such monolayers by analyzing the mobility of the larger tracer particles in the monolayer. With increasing particle density of the monolayer, we observed that the mean-square displacement of the tracer particles was reduced, which can be interpreted as an increase of the viscosity of the monolayer. At very high densities the motion of the particles became subdiffusive and confined, pointing at elasticity of the monolayer. We also studied correlated movements between neighboring particles in an attempt to do two-point interfacial microrheology. A comparison between the one-point and two-point methods shows clear indications of heterogeneous dynamics of the tracer particles. Our results therefore call for a further development of two-point microrheology at interfaces.

    In Chapter 5we used tracer particles to study the properties of thin cross-linked actin networks deposited at the surface of oil droplets. These networks are a model system for the intracellular actin cortex. We used the generalized Stokes-Einstein relation to extract the complex frequency-dependent shear modulus of such networks from the movement of the added tracer particles. We studied the effects of the length of actin filaments and the cross-linker concentration on the mechanical properties of these layers.

    The advantage of this system is that actin networks are freely accessible from the water phase, and therefore can be subjected to in-situ addition of cross-linkers, enzymes or other chemicals of interest. Using this, we managed to show strong stiffening after addition of myosin motor proteins and ATP, which we ascribed to contraction of the actin-myosin network.

    Heteroaggregation and sedimentation rates for nanomaterials in natural waters
    Quik, J.T.K. ; Velzeboer, I. ; Wouterse, M. ; Koelmans, A.A. ; Meent, D. van de - \ 2014
    Water Research 48 (2014)1. - ISSN 0043-1354 - p. 269 - 279.
    sedimentatie - zwevende deeltjes - aggregatie - nanotechnologie - emissie - schatting - colloïden - waterstroming - zeewater - oppervlaktewaterkwaliteit - sedimentation - suspended solids - aggregation - nanotechnology - emission - estimation - colloids - water flow - sea water - surface water quality - engineered nanomaterials - silver nanoparticles - carbon nanotubes - manufactured nanoparticles - aggregation kinetics - aquatic environments - ceo2 nanoparticles - organic-matter - fate - exposure
    Exposure modeling of engineered nanomaterials requires input parameters such as sedimentation rates and heteroaggregation rates. Here, we estimate these rates using quiescent settling experiments under environmentally relevant conditions. We investigated 4 different nanomaterials (C60, CeO2, SiO2-Ag and PVP-Ag) in 6 different water types ranging from a small stream to seawater. In the presence of natural colloids, sedimentation rates ranged from 0.0001md-1 for SiO2-Ag to 0.14md-1 for C60. The apparent rates of heteroaggregation between nanomaterials and natural colloids were estimated using a novel method that separates heteroaggregation from homoaggregation using a simplified Smoluchowski-based aggregation-settling equation applied to data from unfiltered and filtered waters. The heteroaggregation rates ranged between 0.007 and 0.6Lmg-1 day-1, with the highest values observed in seawater. We argue that such system specific parameters are key to the development of dedicated water quality models for ENMs.
    Structure of binary mixed polymer Langmuir layers
    Bernardini, C. - \ 2012
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers. - S.l. : s.n. - ISBN 9789461732149 - 200
    polymeren - colloïden - colloïdale eigenschappen - oppervlakteverschijnselen - polymers - colloids - colloidal properties - surface phenomena

    The possibility of preparing 2D stable emulsions through mixing of homopolymers in a Langmuir monolayer is the core topic of this thesis. While colloid science has achieved well established results in the study of bulk dispersed systems, accounts on properties of mixed monomolecular films are fewer, and seldom systematic. The aim of this investigation is to contribute to a deeper understanding of the subject, in order to explore opportunities to apply the acquired knowledge to the fabrication of technologically relevant materials. In particular, this study focused on a possibly applicable, innovative strategy for the manipulation of the morphology and the patterning of mixed Langmuir monolayers: the possibility to stabilize and control a dispersion of homopolymers through the addition of a lineactant (the equivalent of a surfactant in three dimensional systems), able to adsorb preferentially at the interfacial contact line of polymer domains, thereby lowering the interfacial energy (line tension) in the system and favoring an effective dispersion of one component into the other.

    The state of the art of the preparation and investigation of 2D colloids is the subject of Chapter 2, which is a comprehensive review on several systems able to yield phase–separated Langmuir monolayers, and includes a general definition of the concept of a 2D colloid, the most relevant instrumental techniques and experimental tools available, a summary of several systems suitable for preparing 2D colloid dispersions, an introduction to the concept of lineactant, and several examples, both experimental and theoretical, in which compounds acting as lineactants have been investigated. This review clearly shows that the polymer–based mixtures are a poorly explored subject, when compared to amphiphiles of natural origin, and so the rest of the thesis has been devoted to the investigation of polymer–based Langmuir monolayers.

    This investigation has been carried out with two parallel approaches: classical experiments at the Langmuir trough and morphological characterization of the Langmuir monolayers with the Brewster Angle Microscope have been performed, along with Self–Consistent Field modeling of the same systems. The setup of the SCF model and comparison of SCF calculation with experimental data from the reference experiments are dealt with in Chapter 3. Surface pressure isotherms at the air/water interface were reproduced for four different polymers, poly–l–lactic acid (PLLA), poly (dimethylsiloxane) (PDMS), poly (methyl methacrylate) (PMMA), and poly (isobutylene) (PiB). The polymers are all insoluble in water, but display a different degree of amphiphilicity; therefore the four isotherms differed strongly. The polymers were described through a SCF model on a united atom level, taking the side groups on the monomer level into account. In line with experiments, the model shown that PiB spread in a monolayer which smoothly thickened at a very low surface pressure and area/monomer value. The monolayer made of PMMA had an autophobic behavior: a PMMA liquid did not spread on top of the monolayer of PMMA at the air/water interface. A thicker PMMA layer only formed after the collapse of the film at a relatively high pressure. The isotherm of PDMS had regions with extreme compressibility which were linked to a layering transition. Finally, PLLA wetted the water surface and spread homogeneously at larger areas per monomer. The classical SCF approach features only short–range, nearest–neighbor interactions. For the correct positioning of the layering and for the thickening of the polymer films, a power–law van der Waals contribution was taken into account in this model. Two–gradient SCF computations were performed to model the interface between two coexistent PDMS films at the layering transition, and an estimation of the length of their interfacial contact was obtained, together with the associated line tension value. The SF–SCF molecularly detailed modeling of PLLA, PDMS, PMMA, and PiB monolayers, spread at the air/water surface, has proven to be consistent with experimental data: the incorporation in the model of a detailed molecular description of the monomeric features of the four compounds examined has been crucial to reproducing the features of the adsorption and pressure/area isotherms.

    In Chapter 4, the same approach was applied to the description of polymer mixtures spread at the air/water interface. The aim of this chapter was to analyze topics such as 2D phase separation and partitioning in mixed polymeric Langmuir monolayers. Two of the four polymers studied in Chapter 3 were selected in order to obtain a mixed Langmuir monolayer. A system consisting of water–insoluble, spreadable, fluid–like polymers was prepared. The polymers were polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA), combined, in some cases, with a minority of PDMS–b–PMMA copolymer. Both Langmuir trough pressure/area isotherm measurements and Brewster angle microscopy (BAM) observations were performed, and complemented with molecularly detailed self–consistent field (SCF) calculations. It was shown that PDMS undergoes a layering transition that is difficult to detect by BAM. Addition of PMMA enhanced contrast in BAM, showing a two–phase system: if this consisted of separate two–dimensional (2D) PMMA and PDMS phases, a PDMS–PMMA diblock should accumulate at the phase boundary. However, the diblock copolymer of PDMS–PMMA failed to show the expected “lineactant” behavior, i.e., failed to accumulate at the phase boundary. The calculations pointed to a non-trivial arrangement of the polymer chains at the interface: in mixtures of the two homopolymers, in a rather wide composition ratio, a vertical (with respect to the air/water interfacial plane) configuration was found, with PMMA sitting preferably at the PDMS/water interface of the thicker PDMS film, during the PDMS layering phase transition. This also explained why the diblock copolymer was not a lineactant. Both PMMA and PDMS–b–PMMA were depleted from the thin–thick PDMS film interface, and the line tension between the phases consequently increased in the binary mixtures, as well as in the ternary ones. The results shown in this chapter proved that gaining an accurate control over thin film structures at the microscopic level is a far from trivial task, and the acquisition of fundamental knowledge is necessary in order to interpret experimental data in an appropriate way.

    As a consequence, in Chapter 5 an investigation based solely on SCF modeling was carried out, in order to analyze which polymer blends could have the possibility to undergo lateral phase separation in two dimensions. Specifically, the model system investigated consisted of water–supported Langmuir monolayers, obtained from binary polyalkyl methacrylate mixtures (PXMA, where X stands for any of the type of ester side groups used: M, methyl–; E, ethyl–; B, butyl–; H, hexyl–; O, octyl–; L, lauryl–methacrylate). In particular, the conditions which determined demixing and phase separation in the two–dimensional system were addressed, showing that a sufficient chain length mismatch in the ester side group moieties is able to drive the polymer demixing. When the difference in length of the alkyl chain of the ester moieties on the two types of polymers was progressively reduced, from 11 carbon atoms (PMMA/PLMA) to 4 carbons only (POMA/PLMA), the demixing tendency was also reduced; it vanished, indeed, for POMA/PLMA. In the latter case the polymer/subphase interactions affected more the distribution of the polymer coils in the blend monolayer: mixing of the two polymers was observed, but also a partial layering along the vertical direction.

    Lineactancy was also considered, by selecting the mixture in which phase separation was best achieved: a third component, namely a symmetrical diblock copolymer of the type PLMA–b–PMMA, was added to a PMMA/PLMA blended monolayer. Adsorption of the diblock copolymer was observed exclusively at the contact line between the two homopolymer domains, together with a concomitant lowering of the line tension. The line tension varied with chemical potential of the diblock copolymer according to the Gibbs’ law, which demonstrated that PLMA–b–PMMA indeed acted as a lineactant (the two–dimensional analog of a surfactant) in the model system made of a binary demixed PMMA/PLMA Langmuir monolayer.

    In conclusion, the requirements needed to achieve polymer blend demixing in a Langmuir monolayer are the following: spreadable, insoluble polymers, with the same amphiphilicity degree, combined to a certain chemical mismatch of the side moieties are necessary in order to cause lateral demixing at the air/water interface. The polyalkyl methacrylate example investigated in the chapter represented a suitable model system, since the methacrylate backbone guarantees that the different polymers have the same affinity towards the water subphase, while the different ester moieties drive the occurrence of lateral demixing. The dependency of the lateral demixing on the difference in length between the two ester side groups chosen was demonstrated. A rather complex interplay of forces regulates the distribution of the polymer coils in the monolayer: subtle alterations of this complex balance might favor the dewetting of the mixture in a single domain, together with the layering of the blended polymers along the direction normal to the air/water interface, as well as accumulation of one polymer at the domain edge, instead of the occurrence of the lateral phase separation. Furthermore, the possibility to control emulsification of two–dimensional demixed polymer blends was proven. This was achieved by use of a diblock copolymer, which acted as a lineactant by adsorbing at the contact line of the polymer domains. The calculations demonstrated the possibility to extend the lineactant concept, first elaborated in the context of lipid membrane investigations, to the field of study of polymer thin films.

    Smart microgels for controlled uptake and release
    Li, Y. - \ 2011
    Wageningen University. Promotor(en): Martien Cohen Stuart; Willem Norde, co-promotor(en): Mieke Kleijn. - [S.l.] : S.n. - ISBN 9789085859994 - 173
    gels - zetmeel - colloïden - lysozym - gecontroleerde afgifte - gels - starch - colloids - lysozyme - controlled release

    This dissertation describes a systematic study on oxidized starch microgel particles. It begins with the preparation and characterization of oxidized starch gels in terms of some important physical-chemical properties, with the aim to select an optimum gel for further investigation of protein uptake. The gel with the highest degree of oxidation DO100% is chosen for lysozyme uptake because of its high protein uptake capacity and low swelling capacity. In addition, DO30% gels have been used in many experiments, since DO30% starch allows for preparation of well-defined spherical microgel particles and because it is enzymatically degradable. The two main aspects of interest are the protein binding affinity and protein saturation. Neutral pH and low salt concentration are found to be the optimum protein uptake conditions for high protein saturation. For more detailed studies, spherical microgels with a narrow size distribution have been made by optimizing the preparation process. The mobility of lysozyme molecules inside those microgel particles has been investigated. The main conclusion is that high salt and high pH increase the mobility of lysozyme in the gel particles. It implies that high pH and high salt concentration are potential triggers for lysozyme release from the gel. Subsequently, the kinetics of protein release by high pH and high salt concentration is presented. For the aim of application, the antimicrobial activity of lysozyme containing starch gel particles against some bacterial strains is determined. Finally, the deposition of poly-lysine/poly-glutamic acid complex layer around microgel surface is used to stabilize the microgel particle and optimize our system.

    Colloidal-scale self-assembly of microcapsules for food
    Rossier Miranda, F.J. - \ 2010
    Wageningen University. Promotor(en): Remko Boom; Karin Schroen. - [S.l. : S.n. - ISBN 9789085857853 - 152
    colloïden - inkapseling in microcapsules - zelf-assemblage - levensmiddelenfysica - colloids - microencapsulation - self assembly - food physics
    Microencapsulation is the technique of enclosing valuable or delicate materials in small containers for targeted delivery. These containers may consist of a core and a thin shell surrounding it. The core, with a size ranging from a few to hundreds of micrometers, contains the active material, and serves as template for the microcapsule. The shell, with a thickness of no more than a few hundred nanometers, lends mechanical, physical, or chemical protection, and the means to release the active materials in response to a well defined trigger.
    Microencapsulation is used in a number of industries, e.g. pharmaceutics and food. In the case of medicines, microencapsulation is important for the delivery of drugs at the correct location, in the right dose, and at the right moment, which otherwise would have to be overdosed to reach the desired therapeutic effects. In the case of food, microencapsulation may mask ingredients that would cause off-taste and protect sensitive or volatile materials like flavors, aromas, and antioxidants. In addition, microencapsulation will enable new product concepts, such as products with new sensory experiences (fizzy products from encapsulated CO2) and, more importantly, healthier foods including nutraceuticals or probiotics.
    The microencapsulation methods that are used nowadays in industry on a fairly large scale allow limited control over the uniformity of the microcapsule size and geometry, and the thickness of the shell, while the harsh operation conditions pose restrictions on the type of ingredients that can be used. This poses a challenge to improve production methods and materials. Besides, specifically for food, the materials should additionally be food grade and inexpensive.
    In this thesis, microcapsules (1-20 micron) are constructed via self-assembly; i.e. using driving forces already available in nature. From the routes using self-assembly proposed in literature for microencapsulation two called our special attention, both starting from an oil droplet as template which determines the size of the microcapsule. The first route is that of colloidosome preparation, in which colloidal particles adsorb irreversibly and organize at the oil-water interface, creating a sturdy shell with pores defined by the interstices between the particles. The second route is electrostatic layer-by-layer adsorption: polyelectrolytes of opposite charge are sequentially adsorbed on a charged template, creating a thin film of which the thickness can be controlled with precision in the order of few nanometers. We have combined the two methods by using larger building blocks to construct stronger shells with better defined characteristics, using less adsorption cycles and based on food grade materials. These microcapsules are designed to protect an eventual encapsulated material from low-pH conditions and deliver their contents in response to a change in pH, having in mind applications that would require delivery through the stomach into the small intestine.
    In Chapter 2, the available techniques to produce colloidosomes are reviewed. Microcapsules can be assembled from both, oil-in-water or water-in-oil emulsions, which translates in the potential ability to encapsulate hydrophilic and hydrophobic materials. Since the size of the microcapsules is defined by the droplet used as template the construction of almost perfectly monodisperse microcapsules is within reach, given the current developments in emulsification technology. The choice of size, geometry, and origin of the colloidal particles to be used to assemble the shell, and the means to lock them together, gives control over the targeting and release behavior of the colloidosomes. The requirement that always needs to be met is that particles should have affinity for the both oil and water at the interface, imposing a restriction on the choice of the to-be-used materials. Further, the methods usually applied to lock the colloidal particles to form the shell (e.g. heat up the microcapsule to sinter particles together) were seen as a drawback for the encapsulation of delicate materials such as foods, for which many active components are heat sensitive or volatile. Without significant sintering of the particles the capsule does not have protective properties. The next chapters are therefore devoted to new microencapsulation techniques that complement the colloidosome and the layer-by-layer adsorption routes.
    In Chapter 3, the first results are presented on microcapsules prepared by sequential electrostatic adsorption of protein fibrils, with approximate dimensions of 1 µm in length and 4 nm in width, and high methoxyl pectin. The encapsulation procedure was carried out at low pH (3.5), which allowed us to use positively charged oil droplets stabilized with whey protein isolate as templates. Confocal scanning laser microscopy showed that the fibrils adsorbed as an open structure on a layer of pectin in quantities much larger than a monolayer. This was confirmed from the shell thickness determined by scanning electron microscopy. The fibrils remained trapped at the surface after the adsorption of another layer of pectin. It could be shown indirectly that the inclusion of fibrils in the shell had a tremendous impact on the mechanical strength when compared to microcapsules made of layers of protein and pectin only. Since the microcapsules were assembled at low pH, under the action of pH- and ionic strength-dependent electrostatic interactions, we envision that these capsules would survive low pH, and increase their permeability, or totally disorganize, when exposed to a neutral pH. This is further elucidated in the next chapter.
    In Chapter 4 extensive characterization of the fibril-reinforced microcapsules is reported. Each layer (of fibrils or pectin) added approximately 30 nm to the total thickness, which is considerably more than monolayer coverage, as measured by reflectometry. The microcapsules keep their integrity when exposed to pH below 5.2 (showing slow dissolution at pH 2), but disintegrate at pH 7 or higher. The response is non linear for increasing number of layers, as it was the case for the mechanical strength of the microcapsules. While microcapsules with 7 or less layers had a similar Young modulus, microcapsules with 8 or more layers had twice that strength, around 0.6 GPa, comparable with the strength of polymeric microcapsules that are chemically cross-linked. This was related to the defects present in the shells as observed with scanning electron microscope: capsules with 8 or more layers had smooth and defect-free shells, which resulted in high pH stability.
    The available food grade materials allowed the definition of a second system (to be assembled also at low pH) presented in Chapter 5. A single layer of charged silica particles was adsorbed on sequential layers of whey protein and high methoxyl pectin, therewith reducing the number of adsorption steps. Those pre-adsorbed layers stabilized the adsorbed hydrophilic silica particles that, otherwise, would make a poor colloidosome structure. In this way a highly porous but strong structure that could be easily loaded was obtained. After loading the pores can be closed through the adsorption of additional layers of protein and pectin.
    The drawback of the two microencapsulation systems described above is that polyelectrolytes needed to be used in excess, and intermediate washing steps to rinse out the non-adsorbed materials were essential. To overcome this we went a step ahead in Chapter 6 using a microfluidic device to carry out layer-by-layer adsorption of up to ten layers of protein fibrils and pectin in continuous mode. The design of the chip is simple and does not require complex infrastructure around it, since it relies only on the right balance of the hydraulic resistances of different sections of the microfluidic circuit to control the dose of the materials for the microcapsules’ assembly. Although issues like surface modification of the chip for long-run operation, and the scale-up of the process to industrially-interesting volumes are still a challenge, we feel that this is an important step forward toward controlled microcapsule formation.
    The integration of knowledge on self-assembly, of which some examples can be found in this thesis, combined with the search for new food-grade materials that can act synergistically to assemble a smarter and multifunctional shell, and better design of microfluidics for tight control of this process are key to mature microcapsule formation into a real tool for the food industry. Chapter 7 discusses further requirements for the production of reinforced layer-by-layer microcapsules using microfluidics, and closes this thesis with a general discussion of the results, in the light of possible future developments in the area of microencapsulation.
    Physics of associative polymers : bridging time and length scales
    Sprakel, J.H.B. - \ 2009
    Wageningen University. Promotor(en): Martien Cohen Stuart; Frans Leermakers, co-promotor(en): Jasper van der Gucht. - S.l. : s.n. - ISBN 9789085853657 - 237
    polymeren - colloïden - micellen - reologische eigenschappen - oppervlakteverschijnselen - polymers - colloids - micelles - rheological properties - surface phenomena - cum laude
    cum laude graduation (with distinction)
    Wetting phenomena and interactions in phase-separate colloid-polymer mixtures
    Wijting, W.K. - \ 2004
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): N.A.M. Besseling. - [S.I.] : S.n. - ISBN 9789085040187 - 81
    colloïden - polymeren - mengsels - bevochtigen - bevochtigbaarheid - colloids - polymers - mixtures - wetting - wettability

    In this last chapter I will review and integrate the findings of the previous chapters and give suggestions for further research on this topic.

    In chapter 2 measurements of depletion interactions by means of colloidal probe atomic force microscopy (CP-AFM) are described. We found that the behaviour of the range of the depletion interaction is roughly in agreement with predictions of Fleer et aI. The strength of the depletion interaction is not in agreement with of predictions of Tuinier et al.2 and Louis3 and Bolhuis.4 It is much smaller. We explain this by the fact that the segment-surface interaction for polydimethylsiloxane (PDMS) and a stearylated silica surface is non zero. Comparison of our experimental values with values obtained by Scheutjens Fleer Self Consisted Field theory for different values of Xs leads to a reduced segment surface interaction parameter of 0.41. This corresponds to a decrease to about one order of magnitude of the maximal depletion interaction (see figure 2.13). This strong weakening is in agreement with experimental findings that significantly more PDMS is needed to reach the colloidal liquid-gas binodal than predicted, even if non ideality of the polymers is taken into account.5· 6

    In chapter 3 we present observations of wetting phenomena in depletion interaction driven, phase-separated colloidal dispersions. These dispersions consist of stearyl coated silica spheres in cyclohexane with PDMS as non-adsorbing polymer. The dynamic contact angle was determined by moving a thin fibre upward or downward through the colloidal liquid gas interface and drawing tangents to the meniscus at the fibre in the three phase contact line. The dynamic contact angle was measured as a function of the velocity of the fibre. From these dynamic contact angles the static contact angle was determined by interpolation to zero velocity. We observed no noticeable hysteresis. The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. We were able to locate a Cahn wetting transition7 from partial to complete wetting upon approaching the critical point. By this we have tested successfully the well known Cahn prediction regarding wetting transitions for coloid-polyrner mixtures.

    In the same way we determined the contact angle of the colloidal liquid-gas interface at a soft solid substrate for a series of compositions. A soft solid substrate was obtained by heating a glass fibre in a melt of PDMS. In this way we manipulated the colloidal particle-substrate interaction. Upon approach to the critical point, a transition occurs from partial to complete drying. The region where the wetting transition occurs was compared with that of density functional calculations of Brader et al. and Monte Carlo simulations of Dijkstra et al.8' 9

    Effect of synthetic iron colloids on the microbiological NH4+ removal process during groundwater purification
    Wolthoorn, A. ; Temminghoff, E.J.M. ; Riemsdijk, W.H. van - \ 2004
    Water Research 38 (2004)7. - ISSN 0043-1354 - p. 1884 - 1892.
    ammonium - ijzer - grondwater - waterzuivering - verwijdering - drinkwater - beluchting - colloïden - grondwaterwinning - ammonium - iron - groundwater - water treatment - removal - drinking water - aeration - colloids - groundwater extraction - transient-behavior - drinking-water - transport - nitrification - phosphate - column - mobilization - nitrosomonas - adsorption - sediments
    Subsurface aeration is used to oxidise Fe in situ in groundwater that is used to make drinking water potable. In a groundwater system with pH>7 subsurface aeration results in non-mobile Fe precipitate and mobile Fe colloids. Since originally the goal of subsurface aeration is to remove iron in situ, the formation of non-mobile iron precipitate, which facilitates the metal's removal, is the desired result. In addition to this intended effect, subsurface aeration may also strongly enhance the microbiological removal of ammonium (NH4+) in the purification station. Mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH4+ removal process. Therefore, the objective of this study was to assess whether synthetic iron colloids could improve the NH4+ removal process. The effect of synthetic iron colloids on the NH4+ removal process was studied using an artificial purification set-up on a laboratory scale
    Subsurface aeration is used to oxidise Fe in situ in groundwater that is used to make drinking water potable. In a groundwater system with pH > 7 subsurface aeration results in non-mobile Fe precipitate and mobile Fe colloids. Since originally the goal of subsurface aeration is to remove iron in situ, the formation of non-mobile iron precipitate, which facilitates the metal's removal, is the desired result. In addition to this intended effect, subsurface aeration may also strongly enhance the microbiological removal of ammonium (NH4+) in the purification station. Mobile iron colloids 4 could be the link between subsurface aeration and the positive effect on the NH4+ removal process. Therefore, the objective of this study was to assess whether synthetic iron colloids could improve the NH4+ removal process. The effect of synthetic iron colloids on the NH4+ removal process was studied using an artificial purification set-up on a laboratory scale. Columns that purified groundwater with or without added synthetic iron colloids were set up in duplicate. The results showed that the NH4+ removal was significantly (alpha = 0.05) increased in columns treated with the synthetic iron colloids. Cumulative after 4 months about 10% more NH4+ was nitrified in the columns that was treated with the groundwater containing synthetic iron colloids. The results support the hypothesis that mobile iron colloids could be the link between subsurface aeration and the positive effect on the NH4+ removal process. (C) 2004 Elsevier Ltd. All rights reserved.
    Faradaic and adsorption-mediated depolarization of electric double layers in colloids
    Duval, J.F.L. - \ 2003
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Herman van Leeuwen. - Wageningen : s.n. - ISBN 9789058089403 - 238
    colloïden - elektrokinetische potentiaal - oppervlakteverschijnselen - oppervlaktechemie - colloids - electrokinetic potential - surface phenomena - surface chemistry - cum laude
    cum laude graduation (with distinction)
    Deformation and fracture behavior of simulated particle gels
    Rzepiela, A.A. - \ 2003
    Wageningen University. Promotor(en): Johan Grasman, co-promotor(en): Joost van Opheusden; Ton van Vliet. - [S.I.] : S.n. - ISBN 9789058088031 - 109
    gels - colloïden - beweging - afschuifkracht - gels - colloids - movement - shear

    In this PhD project rheological properties of model particle gels are investigated using Brownian Dynamics (BD) simulations. Particle gels are systems of colloidal particles that form weakly bonded percolating networks interpenetrated by a suspending fluid. They are characterized as soft, deformable, elastic solids. Examples in the food domain are yogurt and cheese, in which the particles are casein micelles.

    Aggregation kinetics is discussed for a range of volume fractions. The model is set up to mimic aspects of aggregation of casein micelles in renneted milk. Smoluchowski classical theory is tested by comparing predictions with the BD simulation model. At low volume fractions aggregation rates were found close to the Smoluchowski rates, but they increase sharply at higher concentrations. Only when a large fraction of the particles has already clustered the quasi-stationary solution as used in Smoluchowski theory is attained.

    Oscillatory shear tests have been used to study small deformation characteristics of model particle gels. Continuous shear and tensile deformation tests have been used to study large deformation and fracture properties. Two different techniques of shear deformation were employed, namely affine and non-affine deformation, the second being novel in simulation studies of gels. In the affine method strain is applied with a homogenous profile. In the non-affine method strain is applied on the surface of the gel structure and can be transmitted through the network to the bulk. Also two different dynamic descriptions of the model are discussed, one with high energy dissipation, the high damping limit, and one with low energy dissipation, the inertia model.

    The small oscillatory shear study showed that the affine technique is insufficient for studying particle gels. The resulting rheology does not depend on details of the network while in real experiments it does. In the non-affine mode, deformation of the network is inhomogeneous depending on network structure and time scale of strain propagation. Moreover a frequency dependent transition can be observed from bulk loading to surface loading. Under continuous shear deformation, regardless of the model used, the particle gels were observed to fracture into lumps that compactified due to local reorganization. Fractal properties of the gels were irreversibly lost at large deformation. Under tensile deformation, within the parameter ranges used, the particle gels were found to be notch insensitive. They exhibit ductile fracture behavior, which was due to global material failure rather than crack propagation.

    Subsurface aeration of anaerobic groundwater : iron colloid formation and the nitrification process
    Wolthoorn, A. - \ 2003
    Wageningen University. Promotor(en): Willem van Riemsdijk, co-promotor(en): Erwin Temminghoff. - Wageningen : Wageningen Universiteit - ISBN 9789058088550 - 134
    grondwater - beluchting - anaërobe omstandigheden - nitrificatie - ijzer - colloïden - watervoorziening - grondwaterwinning - groundwater - aeration - anaerobic conditions - nitrification - iron - colloids - water supply - groundwater extraction
    Keywords: Iron, anaerobic groundwater, groundwater purification, heterogeneous oxidation, iron colloid formation, electron microscopy, nitrification In anaerobic groundwater iron and ammonium can be found in relatively high concentrations. These substances need to be removed when groundwater is used for the production of drinking water. Subsurface aeration can be applied to remove iron before the groundwater reaches the purification plant. The primary goal of subsurface aeration is to oxidise iron in-situ. As a side effect subsurface aeration can strongly enhance the microbiological removal of ammonium (i.e. nitrification) in sand filters. It is recognized that subsurface aeration could be a practical tool to enhance the nitrification process. Until now, subsurface aeration and the nitrification process were not specifically considered as related processes. It is hypothesized that mobile iron colloids may be the link between subsurface aeration and the positive effect on the nitrification process. To gain insight into the processes that can explain the effects of subsurface aeration the fate of iron after the application of subsurface aeration was studied. The potentially mobile iron colloids are of particular interest. A method is developed that could be used to study the effects of subsurface aeration of an anaerobic groundwater well under well-defined laboratory conditions. The first issue was whether mobile iron colloids could be formed as a result of subsurface aeration. At a pH > 7 the oxidation of Fe2+ is a heterogeneous oxidation process. The heterogeneous oxidation was described using a model with a homogeneous and an autocatalytic oxidation rate constant. The results of this study showed that the application of subsurface aeration of a groundwater system with a pH higher than 7 leads to the formation of iron colloids. A field experiment was performed to assess whether mobile iron colloids could be detected in an aerated groundwater well. From this field experiment it followed that a subsurface aerated well contained more iron colloids than a groundwater well that was not aerated. The iron colloids from the field were analysed using both chemical analysis and electron microscopy. The characteristics of the iron colloids from the field were used to prepare a synthetic analogue. The effect of the synthetic iron colloids on the nitrification process was studied by building a purification set up on a laboratory scale. In conclusion the results of this study strongly support the hypothesis that mobile iron colloids may be the link between subsurface aeration and the positive effect on the nitrification process.
    Food Colloids, Biopolymers and Materials
    Dickinson, E. ; Vliet, T. van - \ 2003
    Cambridge UK : Royal Society of Chemistry (Special publication / Royal Society of Chemistry no. 284) - ISBN 9780854048717 - 416
    voedingsmiddelen - colloïden - chemische samenstelling - chemie - foods - colloids - chemical composition - chemistry
    Colloids and interfaces in life sciences
    Norde, W. - \ 2003
    New York; Basel : Marcel Dekker - ISBN 9780824709969 - 433
    colloïden - colloïdale eigenschappen - grensvlak - oppervlaktespanning - emulsies - schuim - reologische eigenschappen - studieboeken - oppervlaktechemie - colloids - colloidal properties - interface - surface tension - emulsions - foams - rheological properties - textbooks - surface chemistry
    Surface forces studied with colloidal probe atomic force microscopy
    Giesbers, M. - \ 2001
    Wageningen University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer; J.M. Kleijn. - S.l. : S.n. - ISBN 9789058083586 - 135
    colloïden - krachten - colloids - forces

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

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

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

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

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

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

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

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

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

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

    Fundamentals of interfacial and colloid science Vol III: Liquid-fluid interfaces
    Lyklema, J. - \ 2000
    San Diego : Academic Press - ISBN 9780124605237 - 785
    colloïden - colloïdale eigenschappen - grensvlak - oppervlakte-interacties - oppervlaktechemie - oppervlakteverschijnselen - colloids - colloidal properties - interface - surface interactions - surface chemistry - surface phenomena
    This volume deals with various aspects of surface tensions and interfacial tensions. Together with the phenomenon of adsorption (enrichment of molecules at interfaces), these tensions constitute the basic characteristics of interfaces. The authors try to keep the treatment systematic and deductive. Recurrent features are that each chapter begins, as much as possible, with the general thermodynamic and/or statistical thermodynamic foundations and the various phenomena are presented in order of increasing complexity. The requirement that the work be both a reference and a textbook is reflected in its being comprehesive as far as the fundamentals are concerned and in its didactic style.
    Brushes and soap : grafted polymers and their interactions with nanocolloids
    Currie, E.P.K. - \ 2000
    Agricultural University. Promotor(en): M.A. Cohen Stuart; G.J. Fleer. - S.l. : s.n. - ISBN 9789080347069 - 209
    borstels - polymeren - colloïden - oppervlaktespanningsverlagende stoffen - brushes - polymers - colloids - surfactants - cum laude

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    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.

    Check title to add to marked list
    << previous | next >>

    Show 20 50 100 records per page

    Please log in to use this service. Login as Wageningen University & Research user or guest user in upper right hand corner of this page.