Exchange dynamics of molecules at the fluid-solid interface determining the diffusion rate in nanopores

Molecule exchange at the fluid-solid interfaces and diffusion in the confined space determine the performance of a variety of nanoporous-materials-based applications. We investigated the transport diffusion of methane in carbon nanotubes (CNTs) with and without considering the exchange dynamics of fluid molecules at the fluid-CNT interface, using molecular dynamics simulations. By excluding the fluid-CNT interfaces, superfast diffusion of methane is found in an infinite CNT having a diameter of 0.81 nm, which is more than 2 orders of magnitude higher than that in larger CNTs. However, the diffusion coefficients are reduced by 3-4 orders of magnitude in the finite CNTs due to the exchange of fluid molecules at the fluid-CNT interfaces. We find the exchange of fluid molecules at the fluid-solid interface induces the dominating interfacial resistance, which also leads to the transfer of ballistic diffusion of methane into Fickian diffusion in the small CNT. Molecules adsorbed on the CNT membrane surface experiencing the streamline line bending effect define a flange resistance, and these molecules further penetrating through the fluid-solid interfaces experiencing the phase change define the entrance-exit resistance, and these two sources together determine the interfacial resistance. It is the entrance-exit resistance makes the major contribution to determine the interfacial resistance in general cases, whose role is weakened by increasing CNT diameter and membrane porosity. (C) 2021 Elsevier B.V. All rights reserved.