Extensive molecular-dynamics simulations were carried out for a model tracer-solvent system made up of 100 solvent molecules and 8 tracer molecules interacting through truncated Lennard-Jones potentials. The influence of the size ratio between solute and solvent, of their mass ratio, and of the solvent viscosity on the diffusivity of a small tracer was investigated. Positive deviations from a Stokes-Einstein behavior were observed, in qualitative agreement with experimental observations. It was also observed that as tracer and solvent became increasingly dissimilar, their respective dynamics became decoupled. It was suggested that such decoupling could be interpreted by writing the mobility of the tracer as the sum of two terms, the first one arising from a coupling between tracer dynamics and hydrodynamic modes of the solvent, and the second one describing jump motion in a locally nearly frozen environment.

Molecular-Dynamics Investigation of Tracer Diffusion in a Simple Liquid. Ould-Kaddour, F., Barrat, J.L.: Physical Review A, 1992, 45[4], 2308-14