Predicting the Soret Coefficient of Molecular Binary Mixtures

Thermodiffusion of binary mixtures in an inhomogeneous temperature field is due to scattering phenomena and the effects of interparticle interactions. Accordingly, when describing the Soret coefficient, it is customary to distinguish between the kinetic and the chemical contributions. Predicting thermodiffusion even in simple binary mixtures is challenging because of numerous factors that influence these contributions. Here, we present a comprehensive theoretical study of both contributions and make predictions of the Soret coefficient for 141 equimolar binary mixtures composed of 24 nonassociating and non- or weakly polar simple liquids. The chemical contribution is an equilibrium property related to the gradient of a partial pressure of one of the mixture components. It is successfully described within the framework of the perturbed-chain statistical associating fluid theory, characterizing the pure liquids by only three physically significant parameters: the segment number, the hard-core segment diameter, and the segment–segment interaction parameter. The nonequilibrium kinetic contribution is approximated by the relation associated with hydrodynamic fluctuations. The sum of the chemical and kinetic contributions determines the Soret coefficient of the mixture. Both contributions are found for the case of dilute mixtures. The equimolar value of the Soret coefficient is calculated as the arithmetic mean of its limiting values. The predicted values of the Soret coefficient are compared with the experimental data available for 113 mixtures. Overall, the theoretical and experimental results are in good agreement with each other. For 70 mixtures, the difference of data does not exceed 0.002 K–1. The best agreement between the data occurs for the alkanes. The maximum deviations of the results are observed for mixtures with 1-methylnaphthalene, bromonaphthalene, and cyclohexane. We relate these deviations to the plate-like shape of their molecules, which is not taken into account when deriving the kinetic contribution.