Interfacial Resistance and Length-Dependent Transport Diffusivities in Carbon Nanotubes

We investigate the transport diffusion of methane at 300 K in a series of short (10, 10) carbon nanotubes with length of up to 100 nm, using a novel equilibrium molecular dynamics simulation (EMD) method. The calculated transport diffusivities for methane in the short CNTs were validated by gravity-driven nonequilibrium molecular dynamics (NEMD) simulations. Because of the dominant interfacial resistance, which collectively accounts for the entrance and exit interfacial barriers, the transport diffusivities of methane in the finite CNTs are generally 2-3 orders of magnitude lower than those in an infinitely long CNT and depend significantly on the tube length. The EMD simulations show that interfacial resistance is the major source of resistance to transport, and that the ratio of the interfacial resistance to the, overall resistance decreases with increase in the length of the CNTs. Significant correlation between the motion of methane near the interfaces and that deep inside the CNT was found, showing that the interfacial region is not limited to a narrow range at the entrance and exit, but extends to more than SO nm inside the CNT. We find that high net flux in the NEMD simulations increases the heat release/supply at the entrance/exit region, in accord with the exothermic/endothermic adsorption/desorption processes occurring at the interface. As a result, a temperature gradient is generated inside the CNT, which in turn enhances the diffusion of methane. Nevertheless, good agreement between NEMD and EMD results is obtained. The generalized EMD method proposed in this work for determining the interfacial resistance is readily applied to any nanoporous material.