The goal of this study was to analyse how the thermal diffusion process was dependent on molecular parameters when describing a fluid mixture. To estimate the associated transport coefficient, which was the thermal diffusion factor αT, a non-equilibrium molecular dynamics algorithm was applied on equimolar binary mixtures of Lennard-Jones particles in supercritical conditions. Firstly, it was shown that this model was able to correctly estimate αT for simple alkane mixtures, provided there were a sufficient number of particles and long enough simulations. Then, using various mixing rules, the separate influences of the mass, the moment of inertia, the atomic diameter and the interaction strength were studied. Results indicated that the molar fraction of the component, having the smallest mass and moment of inertia as well as the biggest radius and the strongest potential, tended to increase in the hot area. Elsewhere, simulations for various cross-interaction parameters showed that αT was extremely sensitive to the intermolecular pair potential between unlike particles. Finally, results on methane/normal alkane mixtures indicated that a simple sum between the separate contributions provided a reliable evaluation of αT only when the molecular parameter ratios between the two components were close to 1.

Thermal Diffusion Sensitivity to the Molecular Parameters of a Binary Equimolar Mixture, a Non-Equilibrium Molecular Dynamics Approach. Galliéro, G., Duguay, B., Caltagirone, J.P., Montel, F.: Fluid Phase Equilibria, 2003, 208[1-2], 171-88