Molecular-dynamics investigation of tracer diffusion in a simple liquid

Abstract

Extensive molecular-dynamics simulations have been 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 are observed, in qualitative agreement with experimental observations. It was also observed that as tracer and solvent become increasingly dissimilar, their respective dynamics become decoupled. We suggest that such decouplings can 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.