Effects of solute-solvent and solvent-solvent attractive interactions on solute diffusion

A study is reported on the diffusion process of a solute molecule in a Lennard-Jones-like liquid near the triple point by a molecular dynamics simulation. Systematic changes were made to the strength of the solute-solvent or solvent-solvent attractive interaction in order to elucidate its effects on the diffusion coefficient. When the solute-solvent attractive interaction is enhanced, the diffusion coefficient of the solute becomes much smaller than that predicted by the Stokes-Einstein relationship with a stick boundary condition. The generalized friction coefficient on the solute molecule was investigated, and the attractive force between solute and solvent is found to be the main cause for the enhancement of the friction. When the attractive interaction between solvent molecules is weakened, the diffusion coefficient of a solute does not change, whereas that of a solvent does. Compared with the shear viscosity of the solvent, the diffusion coefficient of the solute breaks the Stokes-Einstein relationship, whereas the Stokes-Einstein relationship appears to hold in the case of the solvent molecule.