A computational study of molecular diffusion and dynamic flow through carbon nanotubes

Molecular dynamics simulations are used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature. The interatomic forces in the simulations are calculated using a classical, reactive? empirical bond-order hydrocarbon potential coupled to Lennard-Jones potentials. The simulations show 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 36 Angstrom. In addition, molecules moving at thermal velocities are predicted to diffuse from areas of high density to areas of low density through the nanotubes. Normal-mode molecular thermal diffusion is predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene are predicted to diffuse by normal mode, single-file mode, or at a rate that is 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 are between 16 and 22 Angstrom, ethane and ethylene are predicted to follow a helical diffusion path that depends on 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 are also considered.