Influence of Double-Layer Overlap on the Electrophoretic Mobility and DC Conductivity of a Concentrated Suspension of Spherical Particles

Abstract

In this paper, the specific effect of the overlapping of electrical double layers on the electrophoretic mobility and the electrical (DC) conductivity of concentrated suspensions of spherical colloidal particles is considered. Previous work has dealt with the study of such physical quantities for general conditions, including arbitrary zeta potentials, particle volume fractions, double-layer thicknesses (overlapping of double layers was allowed), and ionic properties of the solution. However, the large number of variables involved screened to some extent the specific role and, in many cases, the extraordinary magnitude of the overlapping contribution by itself. This is the reason that we are concerned here with the analysis of the latter effect in more detail. Thus, we present an extensive set of numerical data of both the electrophoretic mobility and the electrical conductivity versus the zeta potential and particle volume fraction for different electrolyte concentrations, specially in the region where overlapping effects are more relevant. Numerical results corresponding to the same theoretical conditions but where the overlapping correction is neglected are also shown for comparison. The treatment is based on the use of a cell model to account for hydrodynamic and electrical interactions between particles. The results show that neglecting double-layer overlapping corrections leads in most cases to an important underestimation of the true values of both the electrophoretic mobility and the electrical conductivity. The differences between both treatments are more pronounced the larger the double layer thickness and/or particle volume fraction are. Concerning DC conductivity, the overlap of double layers might help in interpreting discrepancies generally found between experimental data and standard theoretical values, even in suspensions with particle volume fractions that are typically considered to be dilute.