Determination of stagnant layer conductivity in polystyrene suspensions: temperature effects

Abstract

In the classical theory of electrokinetic phenomena, it is admitted that the whole electrokinetic behavior of any colloidal system is fully determined by the zeta potential, zeta, of the interface. However, both experimental data and theoretical models have shown that this is an incomplete picture, as ions in the stagnant layer (the region between the solid surface and the slip plane--the plane where the equilibrium potential equals zeta) may respond to the field. In this paper, we aim at the evaluation of this contribution by the estimation of both K(SL)(sigma) (the surface conductivity of the stagnant layer) and K(d)(sigma) (the conductivity associated with the diffuse layer). This will be done by measuring the high-frequency dielectric dispersion (HFDD) in polystyrene suspensions; here "high-frequency" means the frequency interval where Maxwell-Wagner-O'Konski relaxation takes place (typically at MHz frequencies). Prior to any conclusions, a treatment of electrode polarization effects in the measurements was needed: we used two methods, and both led to similar results. Simulating the existence of surface conductivity by bulk conductivity, we reached the conclusion that no consistent explanation can be given for our HFDD and additional electrophoresis data based on the existence of diffuse-layer conductivity alone. We thus show how K(SL)(sigma) can be estimated and demonstrate that it can be explained by an ionic mobility very close to that characteristic of ions in the bulk solution. Such mobility, and hence also the values of K(SL)(sigma), increases with temperature as expected on simple physical grounds.