Computer simulation and mode coupling theory study of the effects of specific solute-solvent interactions on diffusion: Crossover from a sub-slip to a super-stick limit of diffusion

In many experimental situations, the interaction potential between the tagg ed solute and the solvent molecules is often different from that between th e two solvent molecules. In such cases, the Stokes-Einstein relation attemp ts to describe the self-diffusion of the solute in terms of an effective hy drodynamic radius which, along with the hydrodynamic boundary condition (sl ip or stick), are varied to fit the experimental results. Extensive molecul ar dynamics (MD) simulations have been carried out to obtain the diffusion coefficient by varying interaction between the solute and the solvent. It i s found that when this interaction is more repulsive than that between solv ent-solvent, the diffusion can be significantly faster, leading to a comple te breakdown of the Stokes-Einstein relation. In the limit of strong attrac tive interaction, we recover a dynamic version of the solvent-berg picture. The diffusion coefficient of the solute is found to depend strongly and no nlinearly on the magnitude of this specific interaction. The velocity corre lation function also shows an interesting dependence on the sign and magnit ude of the specific interaction. Another potentially important observation is that the specific solute-solvent interaction can induce a crossover from a sliplike to a stick-like diffusion, if one still uses the hydrodynamic l anguage. Mode coupling theory analysis of the friction shows that the chang e in it originates largely from the modification of the binary component of the total friction. This is because the cage structure around the solute i s modified due to the specific solute-solvent interaction, which directly a ffects the binary dynamics. (C) 1999 American Institute of Physics. [S0021- 9606(99)51409- 8].