Electrokinetic investigations on the system polystyrene/aqueous electrolyte solution : verification of model theories on dilute and concentrated dispersions

This thesis presents a systematic experimental and theoretical study on electrokinetic and electroconducting properties of disperse systems. The increasing interest in transport processes through charged porous systems has recently brought about a corresponding growth of models and theories since real systems are extremely complex. Monodisperse polystyrene latex seems to constitute a good geometrical model system to theoretically and experimentally assess the influence of the interfacial properties of the dispersed particles and the overall properties of the dispersed system.The preparation and characterization of the polystyrene latices are dealt with in chapter 2. The concentrated dispersions (plugs), with a volume fraction of the solid in the order of 0.6, were obtained by centrifugation of the latex samples as described in chapter 3.Chapter 3 recalls the basic principles of the theory of irreversible thermodynamics which are relevant in connection with the determination and mutual dependency of electrokinetic quantities. All possible forms of polarization, viz. concentration polarization at the electrodes and/or at the plug surfaces, ohmic polarization and intrinsic polarization of the plug, are studied with a 4-electrode equipment. The main features of these polarization phenomena are:(i) concentration polarization at the plug-solution interface (negative adsorption) may cause transient streaming potential or an apparent nonlinear electro-osmotic flow-current relationship, when this cause is not properly recognized, the results can, e.g., erroneously be interpreted in terms of special transitions in the water structure at the interface.(ii) the retardation behaviour of the streaming current is caused by concentration polarization at the current-processing electrodes and/or is a result of a slow intrinsic relaxation process.(iii) the current-voltage characteristics may be obscured by induced concentration polarization processes (transport number effect) but also by slow intrinsic polarization processes.The employment of this new technique has led to the correct measurement of the pertinent phenomenological coefficients. Furthermore, it enables to obtain the time-dependency of a possibly present slow intrinsic polarization process and the transport number in the plug.A theoretical and experimental study also shows how to evaluate the phenomenological coefficients in case various forms of polarization take place simultaneously. The 4-electrode technique also enables to determine the concentration charge at the low pressure side of a plug during per meation. A quantitative expression has been derived which interrelates the ionic strenght outside the plug, the electrolyte concentration in the porous phase and the volume flow.Finally, the time-dependency of the depletion process is used to calcu late effective charge densities (dynamic negative adsorption method) which appear to be completely consistent with the electrokinetic results (chapter 4).Chapter 4 summarizes and discusses the basic geometric models. The electrokinetic and hydrodynamic data are then presented and discussed in relation to these various models. Special attention is given to the electroviscous effect and the concept of anomalous conduction. To that purpose theoretical predictions for the cell and the capillary model of Levine are made suitable or are extended to the concept of anomalous conduction.The main features of the electrodynamic and hydrodynamic results are:- the severe restriction to low potentials causes the cell model to be less usefull than Levine's capillary model;- a mutual comparison of various theoretical models demonstrates the effect of polarization in concentrated dispersions; when surface conduction is not negligible, porous systems are even polarized un der streaming current conditions, which may lead to an apparent lyotropic sequence in the streaming current;- our latex plugs show a strong deviation from 'model behaviour'. The experimental data are interpreted in terms of an outward shift of the shear surface when decreasing the electrolyte concentration, coinciding with a change in the double layer structure. The boundary layer thicknesses evaluated from the hydrodynamic and from the electroviscous retardation results both show the same concentration dependency. The difference between the absolute values is related to the structure of the double layer.Chapter 5 is concerned with the conversion of experimental data on the conduction of concentrated and dilute dispersions into double layer characteristics. A very clear view on the applicability on the various theoretical formulae is obtained. The conductivity of dilute dispersions can quite reasonably be described by an equation that allows for concentration polarization according to the Dukhin - Semenikhin theory, provided that the experimental conditions are chosen such that the particles are fully polarized. For concentrated systems, the concentration polarization mechanism is suppressed by short-circuiting of the interacting double layers near the contact points of the particles. This keeps the conductivity finite as the ionic strength approaches zero; consequently all polarization models finally underestimate the conductivity of a concentrated dispersion upon decreasing ionic strength, especially when the concentration polarization according to Dukhin - Semenikhin is incorpora ted. The 'classical' Bruggeman equation proves to be most usefull for the evaluation of double layer properties. The influence of the decrease in concentration polarization upon concentrating a particle dispersion is also noticed from the shift of the isoconductivity point to higher electrolyte concentrations.Also in chapter 5, the combined information on the influence of the na ture of the counter ions, the temperature and the surface charge density on the surface conductance is used to analyse the structure of the electrical double layer of our model system. The main conclusions are:- the counter charge effective in the electric conduction remains more or less constant upon variation of the ionic strength whereas the 'hydrodynamically immobilized' part of the counter charge increases sharply upon diluting the electrolyte;- the hydrodynamic and electroviscous data indicate a shift of the shear plane upon decreasing ionic strength;- contacting polystyrene with a medium of high ionic strength causes an increase in the titration charge density (charging phenomenon);- a alkali-specificity is noticed for the surface conductance and the activation energy of conductance.The 'hairy layer' model (Δ-layer) is used to explain these peculiar findings. Arguments are given that protruding polystyrene chains with terminal charges are present and that they are distributed in patches. Increase of ionic strength causes the thickness of the hairy patches to shrink; the shear surface enveloping the end-groups of the most protruded chains shifts inwardly and the counter ions originally accommodated be tween the headgroups become electrokinetically 'visible'. Furthermore, ion-paired groups in the innermost regions of the boundary layer are ac tivated and contribute to the surface conductance. Irreversible structural rearrangements in the Δ-layer are responsible for the fact that upon subsequent dilution of the electrolyte a larger fraction of the end-groups remains active.The concentration dependent surface diffusion of ions, the amount of specific adsorption and the observed alkali-specificity are strongly related to the shrinkage of the Δ-layer and the evoked permittivity change within that layer.In chapter 6 the effect of concentration polarization on electrophoresis under conditions of anomalous conduction is verified with the theory of Dukhin and Semenikhin. Several procedures are discussed to obtain information about the boundary layer thickness and the average mobility of the ions inside the shear surface. The results are in line with the concept of the concentration dependent boundary layer thickness formulated in the preceding chapters. In passing from Li +to Cs +the average ionic mobility in that layer seems to decrease.Chapter 7 presents an account of the dielectric dispersion occurring on a time-scale of seconds. The slow relaxation process is closely related with the 'hairy boundary' layer. The effect of salting out the hairy layer on the relaxation time and the conductivity increment confirms the proposed hairy layer model.In conclusion, this study shows, that electrokinetic investigations on concentrated dispersed systems, in combination with electrophoresis, are very useful in studying electrokinetic transport processes. Admittedly, our latex system, due to its simple geometry of dispersed spheres, is an idealized model for systems that are as complex as porous plugs or membranes. However, the presented results are of considerable help in visualizing the immensely complex transport processes and the present shortcomings in their theoretical description.