Brownian Motion in Polydisperse Charged Colloidal Suspensions

Abstract

The effect of combined intrinsic size and charge polydispersity on the tracer diffusion in bimodal suspensions of strongly charged macroparticles is investigated both theoretically and experimentally. The measurable wavenumber-dependent mean squared displacement and the partial mean squared displacements are calculated based on a bimodal Schulz distribution for the particle sizes and charges, using both a single relaxation time approximation for the memory function (SEXP) and the multicomponent Brownian Dynamics (BD) simulation technique. It is found that intrinsic polydispersity has a significant effect on tracer diffusion. The partial static structure functions, which are required as an input for the single relaxation time approximation, are obtained by using the multicomponent hypernetted chain approximation. The predictions of the SEXP approximation and BD simulations are compared with data from large-wave-number dynamic light scattering (DLS) experiments on binary suspensions of strongly interacting polystyrene spheres. Good agreement between SEXP, BD, and DLS data is found within the experimentally scanned time window. However, at intermediate to large times, it is found that the mean squared displacements calculated from the SEXP approximation deviate significantly from results obtained from BD simulations, especially for systems with substantial polydispersity in macroion sizes and charges. We also discuss the range of validity for a simple extension of the substitutional model applicable to intrinsically polydisperse bimodal suspensions.