Connection between partial pressure, volatility, and the Soret effect elucidated using simulations of non-ideal supercritical fluid mixtures

Building on recent simulation work, it is demonstrated using molecular-dynamics (MD) simulations of two-component fluid mixtures that the chemical contribution to the Soret effect in two-component non-ideal fluid mixtures arises due to differences in how the partial pressures of the components respond to temperature and density gradients. Further insight is obtained by reviewing the connection between activity and deviations from Raoult's law in the measurement of the vapor pressure of a liquid mixture. A new parameter $\gamma_{s}^{S}$, defined in a manner similar to the activity coefficient, is used to characterize differences deviations from ``ideal'' behavior. It is then shown that the difference $\gamma_{2}^{S}-\gamma_{1}^{S}$ is predictive of the sign of the Soret coefficient and is correlated to its magnitude. We hence connect the Soret effect to the relative volatility of the components of a fluid mixture, with the more volatile component enriched in the low-density, high-temperature region, and the less volatile component enriched in the high-density, low-temperature region. Because $\gamma_{s}^{S}$ is closely connected to the activity coefficient, this suggests the possibility that measurement of partial vapor pressures might be used to indirectly determine the Soret coefficient. It is proposed that the insight obtained here is quite general and should be applicable to a wide range of materials systems. An attempt is made to understand how these results might apply to other materials systems including interstitials in solids and multicomponent solids with interdiffusion occurring via a vacancy mechanism.