Molecular dynamics simulations were used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature (table 4). 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 affect both dynamic molecular flow and molecular diffusion.For example, 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 through the nanotubes. Normal-mode molecular thermal diffusion was predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene were predicted to diffuse by normal mode, single-file mode, or at a rate that was transitional between normal-mode and single-file diffusion over the time scales considered in the simulations, depending on 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 of atomic termination at the nanotube opening and pore-pore interactions within a nanotube bundle on the diffusion results were also considered.

A Computational Study of Molecular Diffusion and Dynamic Flow through Carbon Nanotubes. Mao, Z., Sinnott, S.B.: Journal of Physical Chemistry B, 2000, 104[19], 4618-24

Table 4

Diffusion of methane, ethane and ethylene in zig-zag nanotubes