Long time self-diffusion in suspensions of highly charged colloids: A comparison between pulsed field gradient NMR and Brownian dynamics

The long time self-diffusion of highly charged colloidal particles consisti ng of polybutylacrylate was studied employing pulsed field gradient NMR (PF GNMR) and Brownian dynamics (BD) simulations. The potential parameters for the Yukawa interaction potential used in the BD simulations were derived fr om the static structure factor measured by means of light scattering. The l ong time self-diffusion coefficients resulting from NMR and BD were compare d for different ionic strengths, which cause a screening of the interaction potential due to stray ions. Increasing the ionic strength of the colloida l suspensions, we observed a phase transition from a crystalline to a liqui dlike structure. Whereas at high salt concentrations both long time diffusi on coefficients agree within a few percent, at intermediate ionic strengths the values measured via PFGNMR are by a factor of 2 higher than those pred icted by the BD simulation. This deviation is explained by hydrodynamic int eractions which are expected to enhance long time self-diffusion and which are not taken into account in our BD algorithm. (C) 2001 American Institute of Physics.