Staff Publications

Staff Publications

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

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

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

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

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    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.
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