Distinct Diffusion in Binary Mixtures Confined in Slit Graphite Pores

Abstract

Distinct diffusion coefficients in the barycentric reference frame for equimolar binary mixtures confined in slit-shaped graphite pores have been evaluated using equilibrium molecular dynamics simulations. In all mixtures the ratio of the interaction energies and mass ratios are similar to those of a Xe-Ar mixture with $\sigma_{22}/\sigma_{11}$ varied between 1 and 1.4. When $\sigma_{22}/\sigma_{11}$ = 1.0 both species adsorb in similar layers in the pore and the distinct diffusion is positive. As $\sigma_{22}/\sigma_{11}$ is increased, the distinct diffusion is found to become progressively more negative. This increasing associative tendency as $\sigma_{22}/\sigma_{11}$ is increased is predominantly due to the formation of distinct layers by individual species. In-plane pair correlation functions also reveal the increased association between unlike species with increasing $\sigma_{22}/\sigma_{11}.$ The Xe-Ar mixture is observed to be associative, with negative values of the distinct diffusion when the fluid forms two layers in the pore. At smaller pore widths, where only a single layer is accommodated, the mixture is weakly dissociative. Simulations with a non-Lorentz- Berthelot mixture were carried out to illustrate the effect of the interaction between unlike species.