Molecular dynamics simulations of diffusion in mesoporous glass

The effect of gas-solid intrapore potential and surface roughness on diffus ion in a single capillary was investigated by molecular dynamics simulation s. Calculations were carried out for nitrogen and isobutane under free mole cular flow conditions in pores of diameter 4-14 Angstrom at temperatures of 200-800 K. The gases were treated as Lennard-Jones atoms and the pore surf ace was taken as cylindrical, exerting a 9-3 potential. No energy transfer was considered between the gas and solid, but interaction with the roughene d pore wall provided the scattering required for diffusive transport. Two e ffects of the gas-solid potential were examined in some detail. One is the enhanced intrapore gas concentration which increases the flux, and the othe r is the bending of the molecular trajectories which decreases the flux. In pores of radius 20 Angstrom, both effects were significant for temperature s as high as 500 K and were enhanced as the temperature decreased. For nitr ogen, the two effects partially canceled each other over the temperature ra nge examined, resulting in a temperature dependence similar to that of Knud sen diffusion. For isobutane, the partitioning effect dominated the path cu rvature effect at temperatures as high as 500 K.