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

Diffusion of pure components (hydrogen (H₂), 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 (?_i,s) and self-diffusivities (D_i,self,s) were determined for pore loadings ranging to 10 molecules nm⁻³. The MD simulations show that zero-loading diffusivity ?_i,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 ?_i(0) approach the Knudsen diffusivity value.MD simulations of diffusion in binary mixtures C1-H₂, 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 ?_1,s, ?_2,s, and ?₁₂, arising in the M-S formulation for binary mixture diffusion. The ?_i,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 ?_i,s of any component was practically the same, irrespective of the partner molecules in the mixture. Furthermore the intermolecular species interaction parameter ?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.