Effects of Flange Adsorption Affinity and Membrane Porosity on Interfacial Resistance in Carbon Nanotube Membranes.

We have used nonequilibrium molecular dynamics simulations to investigate the transport diffusion of methane, at 300 K and pressures of up to 15 bar, in 30 nm-long (10, 10) carbon nanotubes (CNTs) held between two flanges mounted at the ends to represent the surface layers of an embedding matrix material. Strong interfacial resistance to the entry and exit of molecules is found in the 30 nm-long CNTs, which reduces their permeability by more than 2 orders of magnitude. Increasing the adsorption affinity and surface area of the flange reduces the interfacial resistance and consequently enhances the methane diffusivity in CNT membranes. Curved streamlines near the flange surface make a significant contribution to the permeability, even when the adsorption on the matrix surface is negligible. We propose a model to calculate the separate components of the interfacial resistance, the flange resistance, which increases with increase in the membrane porosity, and the entrance-exit resistance, which is independent of the membrane porosity. While the flange resistance accounts for the reduction of interfacial resistance with decrease in the membrane porosity, the entrance-exit resistance is responsible for the reduction of interfacial resistance with increase in the flange adsorption affinity. The flange resistivity demonstrates a complex dependency on the flange adsorption affinity, which is attributed to the competition between the enhanced adsorption and the enhanced migration time of the molecules on the flange. It is concluded that the embedding matrix adsorption affinity and membrane porosity separately play critical roles in determining the interfacial resistance and permeability in CNT membranes. Our simulation results can help reduce the interfacial resistance and improve the permeance in CNT membranes by appropriate choice of intertube spacing and flange material and are readily applied to all nanoporous membranes with a passive matrix.