A molecular dynamics study of thermal diffusion in a porous medium

It is the main aim of this work to identify the microscopic origin of the perturbations brought about by a porous environment on the thermal diffusion (Soret effect) of a liquid mixture. For this purpose, nonequilibrium molecular dynamics simulations are carried out on model systems representing hydrocarbons in a porous medium. In keeping with previous simulations, a simplified model, i.e., Lennard-Jones spheres, is used for representing a methane decane mixture, while the porosity is modeled by the inclusion of quasi-harmonic solids of various sizes and shapes. The model parameters are chosen to yield the proper order of magnitude for a silicate and its interactions with the alkanes. The model was first validated by investigating the equilibrium properties of the system. Then the thermal influence of the porous medium was evaluated and the adsorbing behaviour of the alkanes on the pores was characterized. It is found that the Soret coefficient of the equimolar mixture studied here is lowered by about 30% at 75% porosity. We find also that this reduction is strongly dependent on the structure of the porous medium.