Influence of adsorption on the diffusion selectivity for mixture permeation across mesoporous membranes

Molecular dynamics (MD) simulations were carried out to determine the self-diffusivities, D-1,D-self, and D-2,D-self for a variety of binary mixtures: methane (C1)-ethane (C2), Cl-propane (C3), C1-n-butane (nC4), C1-n-hexane (nC6), C2-nC4, C2-nC6, Ar-C1, Ar-C2, Ar-C3, Ar-nC4, Ar-nC6, and Ar - Kr in a cylindrical silica mesopores. The diffusion selectivity, defined by (D-1,D-self/D-2,D-self) was found to be significantly different from the Knudsen selectivity, root M-2/M-1, where M-i is the molar mass of species i. For mixtures in which component 2 is more strongly adsorbed than component 1, (D-1,D-self/D-2,D-self)/root M-2/M-1 has values in the range 1.5-4; the departures from the Knudsen selectivity increased with increasing differences in adsorption strengths of the constituent species.