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

We investigate the combined effects of electrostatic interactions and hydrodynamic interactions (HI) on the short-time rotational self-diffusion coefficient Dsr in charge-stabilized suspensions. We calculate Dsr 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 Dsr 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 Dsr = D0r(1 − arφ2) for small φ. Here D0r denotes the Stokesian rotational diffusion coefficient, and ar 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 Dsr is possible using depolarized dynamic light scattering from dispersion of optically anisotropic spherical particles.