Soret coefficient of isotopic Lennard-Jones mixtures and the Ar-Kr system as determined by equilibrium molecular-dynamics calculations

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Abstract
The Soret coefficient (thermal diffusion coefficient) characterizing the coupling between heat and mass transport in more-component systems has been obtained by equilibrium molecular dynamics. Two-component liquid systems of the Lennard-Jones type were considered: three isotopic mixtures and Ar-Kr. Computations 20 times longer than the ones used for a ‘‘usual’’ transport coefficient, as the thermal conductivity, were necessary to obtain this cross coefficient with modest accuracy. The analysis of the Green-Kubo integrand shows that, opposite to the thermal conductivity, the thermal diffusion depends strongly on the terms containing the partial enthalpies of the components. For the isotopic mixtures, it was found that the Soret coefficient grows with the mass ratio in a way consistent with experimental results. For the Ar-Kr system, direct comparisons with nonequilibrium molecular-dynamics data indicated good agreement. As experimental Soret coefficients for Ar-Kr are lacking, we cannot compare our results with data of the real mixture. The order of magnitude found agrees, however, with that experimentally observed for binary liquid mixtures.