Diffusion of charged spheres in electroviscous media

Using the technique of dynamic light scattering we have measured the diffusion of polystyrene latex spheres (PLS) in dilute and semidilute solutions of fully sulfonated sodium neutralized polystyrene (PSS-Na) under different conditions of ionic strength. Capillary viscometer measurements of the PSS-Na solutions in which the PLS were suspended have been performed. In the semidilute concentration regime a good agreement between the normalized diffusion coefficient D/D₀ of the PLS and the inverse normalized viscosity η₀/η of the suspension was found (D/D₀ = η₀/η)- This agreement breaks down for the dilute regime and a strong deviation from the Stokes-Einstein relation is revealed. We have found that these deviations from the Stokes-Einstein relation depend strongly on the ionic strength of the suspension indicating that the diffusion of the PLS in PSS-Na solutions is influenced by the repulsive interactions between negatively charged sulfonate groups on both the polystyrene latex spheres and the PSS-Na rodlike molecules.