Mass effect on thermodiffusion using molecular dynamics

The scope of this study is to improve the understanding of the thermal diffusion process on a microscopic scale by studying the mass effect on thermal diffusion factors. To achieve such a goal, non-equilibrium molecular dynamics simulations are performed on binary mixtures of simple Lennard-Jones spheres for a large range of thermodynamic states. Mixtures for which only the mass between species differs, up to mass ratios of 50, are analysed (isotope-like mixtures). In equimolar mixtures, it is shown that the link between the thermal diffusion factors and the ratio between the difference in masses and the sum of masses holds approximately for all states studied. In addition, it is found that this link strongly depends on density but weakly on temperature. In nonequimolar mixtures, results indicate that the effect of the mass ratio between species depends on the molar fraction. Using the data computed, a simple density-dependent correlation is proposed to quantify the mass effect in Lennard-Jones binary mixtures. Finally, it is shown that, taking into account only the mass effect, this correlation is able to provide a reasonable estimation of thermodiffusion in n-pentane/n-decane mixtures, which underlines the intrinsic weakness of some of the usual thermodynamic models predicting thermodiffusion.