Microscopic interpretation of a pure chemical contribution to the Soret effect.

In a recent Letter by Köhler [Phys. Rev. Lett. 87, 055901 (2001)], it has been shown that the Soret effect or thermal diffusion can be split into three different contributions: mass, moment of inertia, and a so-called chemical effect, but only the chemical effect gives rise to a composition dependent contribution. As it is experimentally difficult to deal with the chemical contribution without changing the two others, it has not been studied accurately yet. Our Letter presents both equilibrium and nonequilibrium Molecular Dynamics in simple Lennard-Jones mixtures. By thoroughly changing the strength of direct and cross interaction energies between particles, we show that the composition dependence and the change of sign of the Soret coefficient is driven only by the nature of interactions between unlike particles and propose a microscopic interpretation of the Soret effect.