Colloidal electrolyte friction: the effect of finite-sized electrolyte ions

Abstract

The electro-hydrodynamic coupling of electrolyte ions and surface-dissociated counterions,i.e., microions, to the motion of a suspended colloidal macroion leads to an additionalcontribution to the colloidal friction coefficient. On the basis of the primitive model and thegeneralized Smoluchowski diffusion equation, a simplified mode–mode coupling scheme(MCS) is developed for quantifying the effect of electrolyte friction on the tracer diffusionof a macroion. In this scheme, far-field hydrodynamic interactions between all ionic speciesare considered. The influence of the finite size of the microions is accounted for by usingmean spherical approximation expressions of static pair correlation functions forunequal sizes. The present paper extends earlier work of one of the authors toinclude the effects of finite-sized and hydrodynamically interacting microions. Ourtheoretical results are used to test the relevance of finite size effects in suspensions ofnano-sized particles such as charged globular micelles. Significant finite size effectsare only observed for macroion–microion size ratios typically smaller than 10.