A study was made of the self-diffusion of simple gases inside single-walled carbon nanotubes at the zero-loading limit by molecular dynamics simulations. To show that the results reproduced the flexibility influence correctly, and to demonstrate that the observed effects were not artefacts of the thermostat, some systems were simulated with a flexible C nanotube. It was found that simulations using the LA-IFC thermostat resulted in effectively the same diffusivities (table 5). The host-framework flexibility influence was taken into account. In particular, the influences of nanotube size and temperature were studied. For the carbon-nanotube radius-dependent self-diffusivities, a maximum was observed, which resembled the so-called levitation effect. This occurred for pores having a radius comparable to the position of the interaction-energy minimum. Surprisingly, the temperature influence was not uniform for all pore sizes. Diffusivities were expected to increase with temperature. This effect was observed for carbon nanotubes distinctly larger than the guest molecules. Remarkably, for smaller pores, the self-diffusivities decrease with increasing temperature or exhibit a maximum in the temperature dependence. This effect was caused by the competing influences of collision-frequency and temperature.

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