  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.
