Electrodynamics of the AgI/solution interface

Abstract

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