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

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 additional contribution to the colloidal friction coefficient. On the basis of the primitive model and the generalized Smoluchowski diffusion equation, a simplified mode-mode coupling scheme (MCS) is developed for quantifying the effect of electrolyte friction on the tracer diffusion of a macroion. In this scheme, far-field hydrodynamic interactions between all ionic species are considered. The influence of the finite size of the microions is accounted for by using mean spherical approximation expressions of static pair correlation functions for unequal sizes. The present paper extends earlier work of one of the authors to include the effects of finite-sized and hydrodynamically interacting microions. Our theoretical results are used to test the relevance of finite size effects in suspensions of nano-sized particles such as charged globular micelles. Significant finite size effects are only observed for macroion-microion size ratios typically smaller than 10.