Molecular dynamics simulations were used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature. The interatomic forces in the simulations were calculated using a classical reactive empirical bond-order hydrocarbon potential coupled to Lennard-Jones potentials. The simulations showed that the intermolecular and molecule-nanotube interactions strongly affected both dynamic molecular flow and molecular diffusion. Molecules with initial hyperthermal velocities slowed to thermal velocities in nanotubes with diameters less than 36Å. In addition, molecules moving at thermal velocities were predicted to diffuse from areas of high density to areas of low density via the nanotubes. Normal-mode molecular thermal diffusion was predicted for methane across nearly all of the nanotube diameters considered. In contrast, ethane and ethylene were predicted to diffuse via normal-mode single-file routes, or at a rate that was transitional between normal-mode and single-file diffusion over the time-scales considered in the simulations; depending upon the diameter of the nanotube. When the nanotube diameters were between 16 and 22Å, ethane and ethylene were predicted to follow a helical diffusion path that depended upon the helical symmetry of the nanotube. The effects, upon the diffusion results, of atomic termination at the nanotube opening and pore-pore interactions within a nanotube bundle were also considered.

A Computational Study of Molecular Diffusion and Dynamic Flow through Carbon Nanotubes. Z.Mao, S.B.Sinnott: Journal of Physical Chemistry B, 2000,