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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    Strength, structure and stability of polyelectrolyte complex coacervates
    Spruijt, E. - \ 2012
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): Jasper van der Gucht. - S.l. : s.n. - ISBN 9789461733542 - 291
    elektrolyten - bindingssterkte - elektrische dubbellaag - chemische structuur - moleculaire structuur - oppervlaktespanning - electrolytes - bond strength - electrical double layer - chemical structure - molecular conformation - surface tension - cum laude
    cum laude graduation (with distinction)
    Interfacial properties of water-in-water emulsions and their effect on dynamical behavior
    Scholten, E. - \ 2006
    Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis. - Wageningen : - ISBN 9789085043669 - 148
    emulsies - grensvlak - oppervlaktespanning - dynamica - gelatine - dextraan - arabische gom - emulsions - interface - surface tension - dynamics - gelatin - dextran - gum arabic
    Keywords: biopolymer mixtures, water-in-water emulsions, phase separation, interfaces, tension, bending rigidity, permeability, droplet deformation, morphology.

    The objective of this work was to investigate interfacial properties of biopolymer-based water-in-water emulsions, and to determine the effect of these interfacial properties on the kinetics of phase separation and the deformation behavior of emulsions droplets in shear flow. Since the experimental determination of interfacial properties, such as interfacial thickness and bending rigidity is difficult, we have developed a model that determines these parameters from the experimentally accessible interfacial tension and the interaction potential of the dissolved biopolymers. From the results we could conclude that the thickness of these water/water interfaces is much larger than for oil/water interfaces. The bending rigidities for these interfaces were found to be very large compared to those of water/oil interfaces. The permeability of these interfaces was tested with the spinning drop and the droplet relaxation method. These water/water interfaces were found to be permeable to all ingredients in the system at long time scales (spinning drop experiments) and permeable to water for short time scales (droplet relaxation after cessation of a flow field). This permeability was incorporated into the description of the droplet relaxation time, from which the interfacial tension and the permeability can be deduced simultaneously. Due to the permeability, both the spinning drop method and the droplet relaxation method (without contribution of permeability) cannot be used to measure the interfacial tension accurately. Furthermore, both bending rigidity and permeability were incorporated into the description of coarsening ofbicontinuousstructures during phase separation. We found four different regimes for coarsening depending on whether the process is dominated by interfacial tension, bending rigidity or permeability.
    Membrane emulsification: droplet formation and effects of interfacial tension
    Graaf, S. van der - \ 2006
    Wageningen University. Promotor(en): Remko Boom, co-promotor(en): Karin Schroen; Ruud van der Sman. - Wageningen : s.n. - ISBN 9789085043485 - 159
    emulgering - emulsies - membranen - druppels - druppelstudies - oppervlaktespanning - simulatiemodellen - computersimulatie - kunstmatige membranen - emulsification - emulsions - membranes - droplets - droplet studies - surface tension - simulation models - computer simulation - artificial membranes
    Membrane emulsification is a relatively new technique to produce emulsions. In this method the oil phase is pushed through a membrane, a sieve with very small holes, and forms droplets in the water phase at the other side of the membrane. The most important advantage of this technique is that all the formed droplets have the same size and the emulsion has therefore a better quality. In this research, the influence of surfactants on the droplet formation and detachment process has been studied with the help of experiments and computer simulations.The results showed that both the concentration surfactant and the velocity of the oil phase flowing through the membrane influence the droplet size.
    Emulsion droplet spreading at air/water interfaces: mechanisms and relevance to the whipping of cream
    Hotrum, N.E. - \ 2004
    Wageningen University. Promotor(en): Martien Cohen Stuart, co-promotor(en): G.A. van Aken; Ton van Vliet. - [S.I.] : S.n. - ISBN 9789085040255 - 123
    emulsies - strooien - oppervlaktespanning - druppels - geslagen room - slagroom - emulsions - spreading - surface tension - droplets - whipped cream - whipping cream
    Keywords:emulsion, spreading coefficient, surface tension, emulsifier, whipped cream, dairy foam, partial coalescence

    In this thesis, the interaction between emulsion droplets and expanding air/water interfaces was investigated. The objective was to deepen our knowledge concerning the physical processes that take place at the expanding air surfaces that form during aeration of emulsions. Emulsions can become aerated as a result of various processing operations, for example, stirring or pouring. Moreover, emulsions may be aerated with the intention of producing an aerated food product such as whipped cream or ice cream. Emulsion droplet/air interaction can have important consequences for emulsion stability. For example, emulsion droplet spreading at the air/water interface can initiate a collective oil spreading mechanism, resulting in the spreading of many oil droplets. This may lead to coalescence of the emulsion droplets. The tendency for an oil droplet to spread at an expanding air/water interface depends on the values of the dynamic interfacial tensions at the air/water, oil/water and oil/air interfaces. This can be expressed in terms of a dynamic spreading coefficient; when the spreading coefficient is positive, oil spreads out of the droplets. Experimental results confirmed that oil indeed only spreads out of emulsion droplets if the dynamic spreading coefficient is positive. The tendency for an emulsion droplet to spread at the air/water interface could be controlled by manipulating the surface expansion rate, the protein type and concentration, and type and concentration of emulsifier in the emulsion. The presence of crystalline fat, although relevant to the stability of emulsions exposed to shear, was not found to influence the spreading behaviour of emulsion droplets at the air/water interface. The results of the emulsion droplet spreading experiments lead to the development of a model that describes the whipping time of cream in terms of the proportion of the air bubble surface for which the spreading coefficient is positive. Experimental results for the whipping of model creams could be well explained by this model.
    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
    Interface dilation : the overflowing cylinder technique
    Bergink - Martens, D.J.M. - \ 1993
    Agricultural University. Promotor(en): A. Prins, co-promotor(en): H.J. Bos. - S.l. : Bergink-Martens - ISBN 9789054851288 - 151
    vloeistofmechanica - capillairen - oppervlaktespanning - hydrodynamica - vloeistoffen (liquids) - vloeistoffen (fluids) - grenslaag - oppervlakteverschijnselen - fluid mechanics - capillaries - surface tension - hydrodynamics - liquids - fluids - boundary layer - surface phenomena

    A pure steady-state dilation of a liquid interface, either liquid-air or water-oil, can be accomplished far from equilibrium by means of the overflowing cylinder technique. The resulting dynamic surface tension data correlate well with characteristic parameters of processes like foaming, emulsification, and the spreading of droplets and thin liquid layers.

    Fundamental knowledge of the physical mechanism of operation of the overflowing cylinder technique is obtained by analyzing the relation between interface dilation and underlying bulk flow. Upon the addition of a surfactant the interface velocity increases considerably, since the propulsion mechanism changes from driven by the bulk flow to surface tension gradient driven.

    The surface rheological behaviour of the expanding interface is studied for various surfactant solutions. Generally practical systems give rise to a major increase in surface tension during interface dilation. The results are discussed in terms of the transport of surfactant components.

    The present findings explain why the overflowing cylinder technique is such a useful too[ for studying many practical processes which imply interface dilation far from equilibrium. Meanwhile, however, they urge a reconsideration of the meaning of the surface dilational viscosity.

    Molecular structure and interfacial behaviour of polymers
    Lent, B. van - \ 1989
    Agricultural University. Promotor(en): G.J. Fleer; J.M.H.M. Scheutjens. - S.l. : van Lent - 104
    kunststoffen - industrie - oppervlakten - grensvlak - vloeistofmechanica - capillairen - oppervlaktespanning - polymeren - oppervlakteverschijnselen - grenslaag - moleculaire structuur - plastics - industry - surfaces - interface - fluid mechanics - capillaries - surface tension - polymers - surface phenomena - boundary layer - molecular conformation

    The aim of this study was to investigate the influence of the molecular structure on the interfacial behaviour of polymers. Theoretical models were developed for three different systems. All these models are based on the self-consistent field theory of Scheutjens and Fleer for the adsorption of homopolymers.

    This self-consistent field theory is a lattice model. All possible polymer conformations on the lattice are taken into account. The potential of a conformation is sum of the local potentials of the segments of the molecule. In each layer a mean field approximation is used to calculate the mixing energy. The volume fraction profile is determined by the segmental potentials and vice versa. A numerical method is used to solve the obtained set of equations.

    In chapter 2 the influence of association of block copolymers on adsorption is considered. In order to model spherical aggregates (micelles), the planar lattice, as used for modelling planar aggregates (membranes) and adsorption on flat surfaces, is replaced by a spherical lattice. The equilibrium solution concentration in a micellar solution is determined by a small system thermodynamics argument. The adsorption of diblock copolymers with long lyophobic and short lyophilic blocks shows strongly cooperative effects. A single molecular layer is present if the lyophobic block adsorbs. The adsorption isotherm shows an S-shape at the onset of adsorption. A strong increase of the adsorbed amount occurs near the cmc and above the cmc the adsorbed amount is almost constant. A bilayer at the surface can be formed if the lyophilic block adsorbs. Adsorption of the lyophilic blocks would expose the insoluble blocks to the solvent. Therefore, a second layer of molecules adsorbs with their lyophobic block towards the molecules attached to the surface. The influence of the interaction energies and the block sizes on these trends is described. The results obtained show good qualitative agreement with experimental results on surfactant adsorption.

    The adsorption of random copolymers from solution is described in chapter 3. Experimentally, random copolymers are usually very polydisperse, both in chain length and in primary structure. Random copolymers which are only polydisperse in primary structure are considered here. They can be prepared experimentally by random chemical modification of monomer units of monodisperse homopolymers. The sequence distribution of random copolymers is determined by the fractions of the segment types in the polymer and the correlation factors between them. For random copolymers consisting of two different segment types, a blockiness parameter B is defined. The extremes of this parameter are -1 and 1, where the lower limit depends on the fractions of the different segment types. A value of B = -1 represents an alternating copolymer, whereas B = 1 stands for a mixture of two homopolymers. The complete statistical sequence distribution is implemented into the theory. In the results section random copolymers with two different segment types are studied. Chains with a higher than average content of adsorbing segments are preferentially adsorbed from the bulk solution. Only in the first few layers near the surface this preferential effect plays a role. In the remainder of the profile the segment types are more randomly mixed. The adsorption behaviour of these random copolymers is remarkably different from the adsorption of diblock copolymers. In the latter case, the chains have their adsorbing segments mainly in the layers near the surface, whereas further away from the surface long dangling tails of nonadsorbing segments are found. Random copolymers cannot spacially separate their segments so easily. Much higher adsorbed amounts are found for diblock copolymers than for random copolymers with the same fraction of adsorbing segments. The adsorption of random copolymers is less than that of homopolymer of equal length and consisting of the same type of adsorbing segments. Only for very high adsorption energies the adsorbed amounts are essentially the same. An increase in the blockiness parameter of the chains gives an higher adsorbed amount, but it is always below the adsorbed amount of the homopolymer. Analytical expressions have been derived which relate the interaction parameters of purely random copolymer and homopolymer.

    In chapter 4 the interactions between surfaces coated with grafted polymer (also called hairy plates or soft surfaces) in the presence of nonadsorbing polymer is studied. The interaction free energy between the surfaces is obtained from the partition function. which is rederived for this more general case. For hard plates the interaction is fully determined by the osmotic pressure of the bulk solution and the depletion layer thickness. However. It turns out that In the case of soft surfaces the hairs have an attractive contribution to the free energy of interaction at a plate separation just below twice the hydrodynamic layer thickness of the grafted layer. The hairs mix mutually more easily than with free polymer. At a larger overlap of hairs the interaction becomes repulsive. In contrast with bare planar surfaces, the free energy of interaction between hairy surfaces shows a minimum as a function of the concentration of free polymer in the bulk solution. At a certain (very low) surface coverage the attraction is minimal. For even lower and for larger grafting densities the plates become more attractive. Increasing the repulsion between the hairs and free polymer makes the attraction stronger. The solvencies of grafted and free polymer have a less pronounced effect. Without free polymer, the interaction between the hairy surfaces becomes attractive if the solvency becomes worse than theta conditions.

    It can be concluded that the self-consistent field theory has been successfully extended to three rather complex but technologically relevant systems. In this way a better understanding of the behaviour of polymers near interfaces has been obtained.

    Interfacial thermodynamics and electrochemistry of protein partitioning in two-phase systems
    Fraaije, J.G.E.M. - \ 1987
    Agricultural University. Promotor(en): J. Lyklema. - S.l. : Fraaije - 131
    capillairen - vloeistofmechanica - eiwitten - oppervlaktespanning - thermische energie - thermodynamica - elektrochemie - grenslaag - oppervlakteverschijnselen - tweefasesystemen - capillaries - fluid mechanics - proteins - surface tension - thermal energy - thermodynamics - electrochemistry - boundary layer - surface phenomena - two-phase systems

    The subject of this thesis is protein partition between an aqueous salt solution and a surface or an apolair liquid and the concomitant co-partition of small ions. The extent of co-partitioning determines the charge regulation in the protein partitioning process.

    Chapters 2 and 3 deal with phenomenological relations between the partition coefficient of the protein and the extent of the co- partition. The method of analysis is illustrated by some worked-out examples, using data taken either from literature or from chapter 5. The examples include proton titration curves, ion exchange chromatography, adsorption on colloidal particles and solubilization in reverse micelles. An important conclusion is that the partition process is subject to a rule, similar to the principle of Le Chatelier for chemical equilibria: if upon protein partitioning ions are expelled into the water phase, an increase of the ionic concentrations results in a decrease of the protein partition coefficient and conversely.

    A theory which allows for the prediction and molecular interpretation of the charge regulations is presented in chapter 4. The model describes the electrochemistry of a protein molecule through site binding of ions on a rigid surface. Although this is a considerably simplified picture of a real protein molecule, some aspects of the theory may be of general validity. One of them is the notion of the electrochemical adaptability of a charged colloidal particle, as measured by its intrinsic capacitance. In the case of a high intrinsic capacitance, a change in electrostatic interactions results in a large charge regulation whilst the surface potential remains almost constant. On the other hand, if the intrinsic capacitance is low, the particle resists externally imposed shifts in charge but does adapt its surface potential.

    Chapter 5 contains an experimental study towards understanding the mechanism of charge-regulation in protein adsorption. The system consists of crystals of the insoluble salt silver iodide as the adsorbent and the protein Bovine Serum Albumin as the adsorbate. By using a combined iodide and proton titration technique, the charges of the surface and the protein can be measured independently. We find that a negative surface induces a positive shift in the charge of the adsorbed protein. Opposed to intuitive expectation, the reverse is not always true: when the charge of the protein charge is maximally
    positive, adsorption renders the silver iodide surface less negative.

    The anomalous charge regulation is explained in terms of the intrinsic capacitance of the adsorbed protein. The maximally positive protein cannot adapt its charge, and so the silver iodide surface is forced to adjust its charge completely to that of the protein. As the contact layer between adsorbed protein and the silver iodide crystal is electroneutral under almost all circumstances, the silver iodide surface must be as negative as the protein is positive. Hence, if the charge of the surface before adsorption is more negative than this value, adsorption of the protein is accompanied by a desorption of negative charge.

    The experimental results are well understood in view of the developed phenomenological theory and model analysis. Two thermodynamic relations are succesfully verified, indicating the internal consistency of the various experiments. Application of the model gives two independent estimates of the size of the adsorbed protein. It is concluded that the protein does not substantially modify its native structure upon adsorption.

    Ion adsorption on oxides : surface charge formation and cadmium binding on rutile and hematite
    Fokkink, L.G.J. - \ 1987
    Agricultural University. Promotor(en): J. Lyklema; A. de Keizer. - S.l. : Fokkink - 128
    adsorptie - sorptie - vloeistofmechanica - capillairen - oppervlaktespanning - cadmium - oxiden - grenslaag - oppervlakteverschijnselen - adsorption - sorption - fluid mechanics - capillaries - surface tension - cadmium - oxides - boundary layer - surface phenomena

    The adsorption of charge-determining (H +and OH -) and cadmium ions on rutile (TiO 2 ) and hematite (α-Fe 2 O 3 ) has been studied in different concentrations KNO 3 as a function of temperature and pH.

    Rutile and hematite show identical surface charge - pH behavior, except for the position of the point of zero charge (pH o): normalized with respect to pH oo ,pH) curves of both oxides are experimentally indistinguishable.

    Measurement of the surface charge density on these oxides at temperatures other than room temperature is well possible with the potentiometric titration technique, provided certain precautions are taken. The surface charge - pH relations of rutile and hematite have been studied as a function of temperature in the range 5A thermodynamic treatment has been put forward, that relates the observed shifting of (σ o ,pH) curves with temperature to the enthalpy of proton adsorption/desorption In the interfacial region. We showed that pH ois determined by the enthalpy of proton association with the surface rather than by the entropy. Entropy changes accompanying charge formation are aspecific in the sense that for hematite and rutile identical values for ΔS oare obtained. The adsorbing proton loses about half of its hydration water on adsorption.

    Enthalpies obtained by the thermodynamic analysis could be verified by direct microcalorimetric titrations of rutile and hematite suspensions. Calorimetric heats of charge formation are independent of the surface charge density on the oxides. This behavior Is shown to be typical for high capacitance interfaces, because in this case electrical and countercharge contributions to the enthalpy of charge formation practically compensate each other.

    The adsorption of cadmium ions on rutile and hematite has been measured using a potentiometric pH-stat technique. The influence of temperature on the adsorption is relatively small. The surface charge density of the ox ide, however, is a major factor determining the adsorbability of Cd 2+ions.

    On adsorption of cadmium Ions, hydroxyl Ions are depleted from the solution. This OH -co-adsorption could be explained in terms of 'surface charge adjustment'. Our surface charge adjustment theory is based on the constant potential principle: on specific adsorption of (cat)ions, the surface potential is assumed to maintain a constant (Nernst) value. Considering the specific characteristics of oxide - solution interfaces, our OH -co-adsorption data for the adsorption of cadmium onto rutile and hematite at different temperatures, and those taken from literature for other heavy metal ions onto Inorganic oxides, could be explained satisfactorily within the framework of the theory.

    Experimental adsorption isotherms of Cd 2+were analysed according to the Frumkin - Fowler - Guggenheim model, where chemical, electristatic and lateral interactions are taken into account. The chemical contribution to the Gibbs energy of adsorption was shown to be insensitive towards changes in surface charge density and electrolyte concentration, which is an Indication of the correctness of the model.

    The enthalpy of cadmium adsorption in the rutile-Cd(NO 3 ) 2 -KNO 3 system could be obtained from calorimetric acid/base titrations. The adsorption of Cd 2+ions on rutile was shown to be slightly endothermic, even though calorimetric measurements always showed exothermic overall heat effects. It was inferred that Cd 2+adsorption on rutile is driven by a gain in hydration entropy.

    A strong correlation between Cd 2+adsorption and the formation of Cd(OH) +complexes in aqueous solutions is observed. Both reactions are mainly driven by a gain in hydration entropy. This finding supports the Idea that surface groups on oxides resemble bulk OH -ions.

    The fact that the adsorpion of protons and cadmium Ions on oxides In this study were shown to be strongly related processes, has Important practical implications. With a minimum of experimentation It is now possible to predict the adsorption of heavy metal ions on oxides, using essentially easily obtainable and accurate potentiometric acid/base titration data.

    Electrodynamics of the AgI/solution interface
    Polder, R.B. - \ 1984
    Landbouwhogeschool Wageningen. Promotor(en): B.H. Bijsterbosch, co-promotor(en): H.P. van Leeuwen. - Wageningen : Polder - 77
    vloeistofmechanica - capillairen - oppervlaktespanning - elektromagnetisch veld - zilver - jodide - analytische methoden - elektrochemie - grenslaag - oppervlakteverschijnselen - elektroanalytische analyse - elektrodynamica - elektromagnetisme - fluid mechanics - capillaries - surface tension - electromagnetic field - silver - iodide - analytical methods - electrochemistry - boundary layer - surface phenomena - electroanalytical analysis - electrodynamics - electromagnetism

    The purpose of this study is to gain insight in electrodynamic processes in colloidal systems, that is, in the electrical currents that flow because of the movement of charged particles. There is a need for such insight, because the DUO theory describing the stability of electrocratic colloids cannot answer the following question: can interacting particles in the short time of Brownian encounter adjust their charge to the disequilibration resulting from the overlap of the double layers and thus keep their potential constant ? Or will the particles keep their charge constant during the interaction? The answer depends on the rate of various possible charge transfer processes. We have chosen for electrodes to investigate the dynamic phenomena of interest, and for AgI as the model substance.
    Chapter I offers a general introduction to the theme, and describes the outline of this study.
    The experimental technique used (the coulostatic impulse method) is based on the following experiment. A small departure from the equilibrium potential is instantaneously imposed on two identical electrodes. The overpotential relaxes and the decay, of which the precise shape contains information on the various processes, is recorded. In chapter II we describe the preparation of the AgI electrodes and the other materials, the setup and the procedure to convert the decay signal into an impedance spectrum.
    In chapter III an analysis of the impedance spectrum is made, and the possible components of the electrical equivalent circuit are discussed. It is shown that surface roughness of the electrode seriously complicates the mass transport impedance and this may ruin the analysis of the impedance spectrum. It is concluded that the combined analysis of two admittance functions, employing both the real and imaginary components, provides the best method: it allows to clearly recognize the effects of surface roughness, and the analysis can easily be automated. It is also shown that ion transfer through the interface is a rapid process, and that diffusion in the solution (mass transport) is the rate limiting step.
    In chapter IV experimental results are presented in terms of capacitances and Warburg (diffusional) coefficients under various conditions of potential and electrolyte concentration. The data refer to 'clean' electrodes, as well as to electrodes with an adsorbed polymer layer. The polymers used were PVA (M ca. 9 * 10 4 ) and PVP (M ca. 9 * 10 5 ). The results are compared with literature data. Generally, the agreement was satisfactory, and a tentative explanation for the trends in the capacitances was forwarded. The Warburg coefficients showed some deviation from the theoretically expected behaviour, the more so when polymer was adsorbed.
    Chapter V summarizes the literature on relaxation processes during interaction in colloids, and resumes the experimental information from the previous chapters. Particularly the assessment of ion transfer as a fast process calls for a reconsideration of the hitherto existing picture. Three possible relaxation routes (after ion transfer) of the excess charge on the particle are described and relaxation times for each are estimated. It is concluded that silver iodide particles, uncovered or covered with a polymer layer, can adjust their surface charge on the time scale of a collision by at least one of the transport processes described. The interaction thus takes place under conditions of constant potential .

    Thermisch en hygrisch gedrag van enkele bouwconstructies en -materialen
    Tol, J.C. - \ 1974
    Wageningen : IMAG (Publikatie / Instituut voor Mechanisatie, Arbeid en Gebouwen 1) - 16
    bouwconstructie - bouwnijverheid - bouwmaterialen - gebouwen - capillairen - chemische afbraak - civiele techniek - constructie - corrosie - effecten - vloeistofmechanica - warmte - hydraulica - vloeistoffen (liquids) - materialen - mechanica - daken - constructieonderdelen - oppervlaktespanning - theorie - thermische eigenschappen - thermodynamica - muren - grenslaag - oppervlakteverschijnselen - building construction - building industry - building materials - buildings - capillaries - chemical degradation - civil engineering - construction - corrosion - effects - fluid mechanics - heat - hydraulics - liquids - materials - mechanics - roofs - structural components - surface tension - theory - thermal properties - thermodynamics - walls - boundary layer - surface phenomena
    Specific ionic effects in free liquid films
    Bruil, H.G. - \ 1970
    Wageningen : Veenman (Mededelingen / Landbouwhogeschool Wageningen 70-9) - 69
    vloeistofmechanica - capillairen - oppervlaktespanning - dispersie - adsorptie - sorptie - vaste stoffen - grenslaag - oppervlakteverschijnselen - fluid mechanics - capillaries - surface tension - dispersion - adsorption - sorption - solids - boundary layer - surface phenomena
    Adsorption of polyvinylalcohol on paraffin-water interfaces and the properties of paraffin-in-water emulsions stabilized by polyvinylalcohol : an experimental study
    Lankveld, J.M.G. - \ 1970
    Wageningen University. Promotor(en): J. Lyklema. - Wageningen : Veenman - 114
    alkanen - koolwaterstoffen - alcoholen - oppervlakten - grensvlak - vloeistofmechanica - capillairen - oppervlaktespanning - adsorptie - sorptie - oppervlakteverschijnselen - grenslaag - alkanes - hydrocarbons - alcohols - surfaces - interface - fluid mechanics - capillaries - surface tension - adsorption - sorption - surface phenomena - boundary layer

    The adsorption of polyvinylalcohol (PVA) on the paraffin-water interface has been studied by interfacial tension measurements and by direct measurement of the adsorption with emulsions. The aim of the study was to provide more insight into the factors which influence the adsorption of polymers at phase interfaces. The interfacial properties of polymers, some of which find application as adhesives, flocculating agents and emulsion stabilizers, are of great practical importance. This helps to explain why much fundamental research, both theoretical and experimental, has been performed in recent years on polymer adsorption. Only a small proportion of this research, however, has been devoted to the adsorption of polymers at liquid- liquid interfaces, and so this was chosen as the subject for our investigation. To this end, the interface of a well defined model system has been studied by several different techniques.

    The interfacial tension was measured with the static Wilhelmy plate method as a function of the concentration of PVA (0.1-4000 ppm) and time (up to 30 h) for PVA samples differing in degree of hydrolysis, molecular weight and origin, Experiments were also done in a Langmuir trough with spread and adsorbed PVA monolayers at the paraffin-water interface. Interpretation of the interfacial tension measurements in terms of polymer adsorption is complicated by the irreversible nature of the latter. The irreversibility of polymer adsorption can be deduced from the fact that the measured interfacial tension depends not only on the concentration of PVA in the aqueous phase but also on the history of the interface. As a result, the Gibbs law of adsorption cannot be applied without further preface to polymers.

    Characteristic differences were noted between the interfacial activities of PVA of different degrees of hydrolysis. The discontinuity found in the plots of interfacial tension (γ) against the logarithm of polymer concentration ( cp ) for PVA with a low content of acetate groups (about 2 %) was absent from the corresponding plots for PVA with a higher acetate content (>12 %). The discontinuous nature of the former γ-log cp curves was explained in terms of kinetic factors, namely the relative rates of diffusion and unfolding of the polymer, which determine how the interface is occupied. Differences in the mechanism of interfacial occupation will be reflected by differences in the configuration of the adsorbed polymer molecules. This picture was confirmed by the time-dependence measurements,

    The molecular weight of the PVA was found to have no obvious effect on the interfacial tension, whereas the adsorption measured directly on the emulsion interface clearly increased with increasing molecular weight. One may conclude therefore that there is no well-defined relationship between the reduction in interfacial tension and the amount of polymer adsorbed. In this respect, the behaviour of polymer contrasts with that of low-molecular-weight surfactants.

    In the adsorption experiments with emulsions, the specific area of each emulsion was determined by measurement of the turbidity. The degree of dispersion went through a maximum with increasing PVA concentration when the acetate content of the latter was ≥12 %, This maximum corresponded to a discontinuity in the adsorption isotherm. The concentration at which the maximum was situated decreased with increasing molecular weight and with decreasing volume fractions of paraffin. An explanation for this was again sought in terms of kinetic factors influencing the formation of the stabilizing polymer layer during emulsification.

    The properties of the polymer layer of the emulsions were markedly different, depending on whether the emulsion had been prepared with a polymer concentration below or above that corresponding to the maximum specific area. The emulsion area was large when the amount of polymer per ml paraffin was kept low. A number of tentative experiments were done to enable the mechanism of emulsion formation to be studied more closely.

    All emulsions obtained were found to be extremely stable to coalescence right down to low polymer concentrations.

    Comparison of the results of the different measurements with the same system shows that the way in which the interface is formed has a great influence on its properties. This underlines the irreversible nature of the adsorption, which is one of the reasons why none of the existing theories is adequate to explain the measured effects quantitatively. By virtue of the many different measurements, however, this tudy serves as a basis for further theoretical and experimental work on the surface- active properties of polymers. In addition this investigation helped to show which factors determine the emulsifying capacity of polymers.

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