Short-time rotational diffusion in monodisperse charge-stabilized colloidal suspensions

Abstract

We investigate the combined effects of electrostatic interactions and hydrodynamic interactions (HI) on the short-time rotational self-diffusion coefficient D s r in charge-stabilized suspensions. We calculate D s r as a function of volume fraction φ for various effective particle charges and various amounts of added electrolyte. The influence of HI is taken into account by a series expansion of the two-body mobility tensors. At sufficiently small φ this is an excellent approximation due to the strong electrostatic repulsion. For larger φ, we also consider the leading hydrodynamic three-body contribution. Our calculations show that the influence of the HI on D s r is less pronounced for charged particles than for uncharged ones. Salt-free suspensions are particularly weakly influenced by HI. For these strongly correlated systems we obtain the interesting result D s r = D 0 r (1 − a r φ 2) for small φ. Here D 0 r denotes the Stokesian rotational diffusion coefficient, and a r is a positive parameter which is found to be nearly independent of the particle charge. The quadratic φ-dependence can be well explained in terms of an effective hard sphere model. Experimental verification of our theoretical results for D s r is possible using depolarized dynamic light scattering from dispersion of optically anisotropic spherical particles.