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-diffu sion coefficient D-s(r) in charge-stabilized suspensions. We calculate D-s(r) as a function of volume fraction phi for various effective par ticle charges and various amounts of added electrolyte. The influence of HI is taken into account by a series expansion of the two-body mobi lity tensors. At sufficiently small phi this is an excellent approxima tion due to the strong electrostatic repulsion. For larger phi, we als o consider the leading hydrodynamic three-body contribution. Our calcu lations 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 correlate d systems we obtain the interesting result D-s(r) = D-0(r)(1 - a(r) ph i(2)) for small phi. Here D-0(r) denotes the Stokesian rotational diff usion coefficient, and a(r) is a positive parameter which is found to be nearly independent of the particle charge. The quadratic phi-depend ence can be well explained in terms of an effective hard sphere model. Experimental verification of our theoretical results for D-s(r) is po ssible using depolarized dynamic light scattering from dispersions of optically anisotropic spherical particles.