The diffusivities of methane in single-walled carbon nanotubes were investigated at various temperatures and pressures by using classical molecular dynamics simulations supplemented by grand canonical Monte Carlo simulations. The carbon atoms at the nanotubes were structured according to the (m,m) armchair arrangement, and the interactions between each methane molecule and all atoms of the confining surface were explicitly considered. It was found that the parallel self-diffusion coefficient of methane in an infinitely long defect-free nanotube decreased dramatically as the temperature fell; especially for sub-critical temperatures and high loadings of gas molecules when the adsorbed gas formed a solid-like structure. With increasing pressure, the diffusion coefficient first declined rapidly and then exhibited a non-monotonic behavior due to layering transitions of the adsorbed gas molecules, as seen in equilibrium density profiles. At a sub-critical temperature, the diffusion of methane in a fully loaded nanotube followed solid-like behavior and the value of the diffusion coefficient varied markedly with the nanotube diameter. At a super-critical temperature, the diffusion coefficient at high pressures reached a plateau: with the limiting value being essentially independent of the nanotube size. For nanotubes with a radius larger than about 2nm, capillary condensation occurred when the temperature was sufficiently low; following layer-by-layer adsorption of gas molecules on the nanotube surface. For nanotubes with a diameter less than about 2nm, no condensation was observed because the system became essentially one-dimensional.

Self-Diffusion of Methane in Single-Walled Carbon Nanotubes at Sub- and Supercritical Conditions. D.Cao, J.Wu: Langmuir, 2004, 20[9], 3759-65