The diffusivities of methane in single-walled carbon nanotubes were investigated at various temperatures and pressures by using classical molecular dynamics simulations, complemented with 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 single-walled carbon nanotubes decreased dramatically as the temperature falls, especially at subcritical temperatures and high loading of gas molecules when the adsorbed gas forms a solid-like structure. With the increase in pressure, the diffusion coefficient first declines rapidly and then exhibits a non-monotonic behavior due to the layering transitions of the adsorbed gas molecules as seen in the equilibrium density profiles. At a sub-critical temperature, the diffusion of methane in a fully loaded single-walled carbon nanotubes exhibited a solid-like behavior, and the value of the diffusion coefficient varied markedly with the nanotube diameter. At a supercritical temperature, however, the diffusion coefficient at high pressure reached a plateau, with the limiting value essentially independent of the nanotube size. For single-walled carbon nanotubes with the radius larger than approximately 2nm, capillary condensation occurred when the temperature was sufficiently low, following the layer-by-layer adsorption of gas molecules on the nanotube surface. For single-walled carbon 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. Cao, D., Wu, J.: Langmuir, 2004, 20[9], 3759-65