The effect of gas-solid intrapore potential and surface roughness on diffusion in a single capillary was investigated by molecular dynamics simulations. Calculations were carried out for nitrogen and isobutane under free molecular flow conditions in pores of diameter 4 to 14Å at 200 to 800K. The gases were treated as Lennard-Jones atoms and the pore surface was taken as cylindrical, exerting a 9-3 potential. No energy transfer was considered between the gas and solid, but interaction with the roughened pore wall provided the scattering required for diffusive transport. Two effects of the gas-solid potential were examined in some detail. One was the enhanced intrapore gas concentration which increased the flux, and the other was the bending of the molecular trajectories which decreased the flux. In pores of radius 20Å, both effects were significant for temperatures as high as 500K and were enhanced as the temperature decreased. For nitrogen, the two effects partially cancelled each other over the temperature range examined, resulting in a temperature dependence similar to that of Knudsen diffusion. For isobutane, the partitioning effect dominated the path curvature effect at temperatures as high as 500 K.

Molecular Dynamics Simulations of Diffusion in Mesoporous Glass. Fernandes, N.E., Gavalas, G.R.: Industrial and Engineering Chemistry Research, 1999, 38[3], 723-30