The self-diffusion of simple gases inside single-walled carbon nanotubes at the zero-loading limit was studied by means of molecular dynamics simulations. The effect of host-framework flexibility was taken into account. In particular, the influences of nanotube size and temperature were studied. For carbon-nanotube radius-dependent self-diffusivities, a maximum was observed which resembled the so-called levitation effect. This occurred for pores having a radius that was comparable to the position of the interaction-energy minimum. The influence of temperature was not uniform across various pore sizes. The diffusivities were expected to increase with temperature. This effect was observed for carbon nanotubes that were distinctly larger than the guest molecules. For smaller pores, the self-diffusivities decreased with increasing temperature, or exhibited a maximum in the temperature dependence. This effect was attributed to the competing influences of collision frequency and temperature.

Temperature and Size Effects on Diffusion in Carbon Nanotubes. S.Jakobtorweihen, F.J.Keil, B.Smit: Journal of Physical Chemistry B, 2006, 110[33], 16332-6