Abstract
This thesis describes an experimental study on the electrokinetic and electrical properties of concentrated polystyrene and silver iodide dispersions. The purpose of the study is to obtain information on the structure of the electrical double layer at the solid-liquid interface. Special attention is paid to the various polarization phenomena that may interfere in electrokinetic and conductance measurements on condensed systems.After a general introduction in chapter 1, in chapter 2 we discuss the preparation and characterization of the polystyrene latices. The geometry of these colloids is well-defined and easy to control, their surface properties. on the contrary, are not. The number and nature of the surface groups may change upon characterization, due to contact with a solution of high ionic strength as well as during storage. In order to preclude any effect of these phenomena on the electrokinetic experiments, after preparation the latices have been treated in a special way.In chapter 3 the afore-mentioned polarization phenomena are studied with a four-electrode cell. The characteristics of this cell are investigated for both reversible and irreversible electrodes. In both cases electrode polarization phenomena considerably interfere with the streaming current measurements. Particularly disturbing are the polarizations that occur in the dispersed medium itself. The four- electrode method, however, enables correction for both forms of polarization. As a result. the electrokinetic data thus obtained are independent of the material of the measuring electrodes. When using reversible electrodes, another complication can be that electrolysis processes at the current-carrying electrodes disturb the system under investigation. When irreversible (platinum black) electrodes are incorporated in the cell, such processes do not occur. Another important advantage of the latter electrodes is that the four-electrode method is then most accurate and , moreover, generally applicable. Two independent experimental procedures (streaming potential and streaming current method) yield identical results, confirming the reliability of the four-electrode technique.Chapter 4 deals with the choice of the theoretical models that have been applied to infer double layer parameters from electrokinetic and conductance data. Zeta potentials are calculated by means of capillary models. In these the effect of double layer interaction has been taken into account, but not that of polarization of the double layer. For the inference of surface conductivities both capillary and cell models are considered. The latter take account of double layer and concentration polarization. The chapter concludes with a theoretical consideration of the relation between zeta potential and surface conductivity.In chapter 5 the electrokinetic techniques described in chapter 3 are applied to polystyrene plugs. The results show the slipping plane to shift away from the solid surface upon decreasing the ionic strength. Considerable conductance takes place in the corresponding fixed layer. The relaxation phenomena, observed in the streaming current measurements, are also studied by measuring frequency-spectra of the plug conductance. In the low-frequency range dispersions occur, being the more pronounced the lower the salt concentration. At high salt concentrations the dispersions probably stem from processes taking place at the measuring electrodes. Below concentrations of 10 -2M relaxation processes in the plug itself contribute increasingly to the dispersions. These intrinsic processes as well as the electrokinetic data are accounted for by assuming the surface layer of polystyrene particles to consist of protruding, partly mobile polymer chains. The thickness of this "hairy" layer and, consequently, the position of the shear plane is controlled by the salt concentration.In chapter 6 the techniques developed are used to investigate the influence of adsorbed charged species (tetraalkylammonium (TAA +) ions) on the structure of the polystyrene-solution interface. To this end, adsorption isotherms are also determined. When the surface is (partially) covered with TAA +ions, the position of the slipping plane is independent of ionic strength. Between this plane and the solid surface under these circumstances only very little or no conductance takes place any more. The polarization phenomena in the plug practically disappear when TAA +ions adsorb. The conclusion is that TAA +adsorption largely eliminates the hairiness of the polystyrene particles.Chapter 7 describes the measurements on silver iodide plugs. These have been performed as a function of ionic strength and surface potential. Intrinsic polarizations do also occur in plugs of this material, but they are much less pronounced than those observed in polystyrene plugs. No satisfactory explanation for these polarization phenomena can be offered. The experiments reveal a number of peculiar features. For high salt concentrations the electrokinetically displaceable charge density considerably exceeds the surface charge densities reported in literature. This inconsistency is ascribed to the fact that the latter data is based on an improper value of the specific surface area. Upon decreasing the ionic strength, for this system the slipping plane also shifts outwardly. This phenomenon is interpreted in terms of water structuring due to influences of the silver iodide surface. The concept of a stagnant layer can also account for the results obtained at variable surface potential. Procedures to calculate the thickness of the adhering layer are indicated, but definite conclusions cannot be drawn, because of uncertainties as to the extent of specific adsorption. The layer behind the slipping plane substantially contributes to the conductance of the system. To a small extent this stems from ionic transfer in the solid phase.It can be concluded that the electrokinetic and electrical techniques described in this thesis are useful tools to obtain insight into double layer structures. They have provided detailed information on the structure of the interface in aqueous polystyrene and silver iodide dispersions.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution | |
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Award date | 7 Nov 1984 |
Place of Publication | Wageningen |
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DOIs | |
Publication status | Published - 7 Nov 1984 |
Keywords
- aromatic compounds
- colloids
- dispersion
- iodide
- polymers
- polystyrenes
- silver
- electrochemistry
- electrokinetic potential