(110e) Diffusion Studies of Water and N-Alkane Molecules in Carbon Nanotubes

Previous simultions have predicted that the transport rates of light gases in nanotubes are orders of magnitube faster than in any other known porous materials, e.g., zeolites.[1] Nanotube membranes are thus predicted to have faster fluxes than crystalline zeolite membranes.

Hinds and coworkers have synthesized multi-walled carbon nanotube/polymer composite membranes.[2] Their experimental work has demonstrated that the transport rates of gases and liquids through nanotube membranes are much faster than can be expected from Knudsen or hydrodynamic (Haagen-Poiseuille) flow. Hinds et al. attribute the extraordinarily high fluxes to the nearly frictionless nature of the carbon nanotube walls, thus verifying the predictions made from simulations.

Simulations have been carried out for complex molecules, such as water and alkanes, diffusing through carbon nantoubes. However, all previous simulations of complex molecules in nanotubes have only reported self diffusivities. Three types of diffusivities are important in both experiments and simulations, namely, self, corrected, and tranport diffusivities. The transport diffusivity, also known as the Fickien diffusivity, is most useful for real systems, such as pressure driven flow across a membrane. We have calculated all three diffusivities for water, n-hexane, and n-heptane in different SWNTs using equilibrium molecular dynamics simulations. We examine the effects of the nanotube radius, helicity, loading, and temperature on the diffusivities.

Barriers to entry into nanotubes may dominate over diffusion through nanotubes. We have used simulations to probe the mechanism of alkane diffusion from the nanotube surface into the internal sites. Various alkane coverages and SWNT bundle configurations have been considered.

[1] Anastasios I. Skoulidas, David M. Ackerman, J. Karl Johnson, and David S. Sholl "Rapid Transport of Gases in Carbon Nanotubes", Physical Review Letters, 89, 185901 (2002).

[2] Mainak Majumder,Nitin Chopra, Rodney Andrews, and Bruce J.Hinds "Enhanced flow in carbon nanotubes", Nature, 438, 44, (2005).