An investigation of the characteristics of Maxwell-Stefan diffusivities of binary mixtures in silica nanopores

Diffusion of pure components (hydrogen (H-2) argon (Ar), krypton (Kr), methane (C1), ethane (C2), propane (C3), n-butane (nC4), and n-hexane (nC6)) in silica nanopores with diameters of 1, 1.5, 2, 3, 4, 5.8, 7.6, and 10 nm were investigated using molecular dynamics (MD). The Maxwell-Stefan (M-S) diffusivity (D-i,D-s) and self-diffusivities (D-i,D-self,D-s) were determined for pore loadings ranging to 10 molecules nm(-3). The MD simulations show that zero-loading diffusivity D-i,D-s(0) is consistently lower, by up to a factor of 10, than the values anticipated by the classical Knudsen formula: the differences increase with increasing adsorption strength. Only when the adsorption is negligible does the D-i(0) approach the Knudsen diffusivity value. MD simulations of diffusion in binary mixtures C1-H-2, C1-Ar, C1-C2, C1-C3, C1-nC4, C1-nC6, C2-nC4, C2-nC6, and nC4-nC6 in the different pores were also performed to determine the three parameters D-1.s, D-2.s, and D-12, arising in the M-S formulation for binary mixture diffusion. The D-i,D-s in the mixture were found to be practically the same as the values obtained for unary diffusion, when compared at the same total pore loading. Also, the Di, of any component was practically the same, irrespective of the partner molecules in the mixture. Furthermore the intermolecular species interaction parameter D-12, could be identified with the binary M-S diffusivity in a fluid mixture at the same concentration as within the silica nanopore. The obtained results underline the overwhelming advantages of the M-S theory for mixture diffusion in nanopores. Our study underlines the limitations of the commonly used dusty-gas approach to pore diffusion in which Knudsen and surface diffusion mechanisms are considered to be additive