The role of the intermolecular potential on the dynamics of ethylene confined in cylindrical nanopores

Abstract

The influence of the molecular force field upon the transport properties of ethylene is studied by molecular dynamics simulations, addressing both the bulk fluid and the confined phases within pristine single-walled carbon nanotubes. Five different molecular models with different degrees of coarse-graining were selected, spanning from a simple isotropic Lennard–Jones sphere to a fully detailed all-atom description. The fluid was probed for its self-diffusion coefficient under isochoric (0.026 ≤ ρ (mol L−1) ≤ 15.751) and isothermal (220 ≤ T (K) ≤ 340) conditions, both in the sub- and supercritical regions (Tbulkc = 282.4 K). Although the particular details of each potential model are seen to be nearly irrelevant to the bulk fluid dynamics, they are crucial to correctly describe the inhomogeneous system. The most important aspects affecting fluid transport are the existence/absence of explicit electrostatic contributions and the molecular shape. The effect of temperature on the confined fluid self-diffusion is described by the Arrhenius law, D = Aexp(−Ea/RT), and the nonlinear density dependencies of the activation energy (Ea/R) and pre-exponential factor (A) have been fitted here to empirical equations. In spite of the quasi one-dimensional confining nature of SWCNTs, the isothermal results (T = 300 K) obtained for the bulk and confined systems collapse onto a unique master curve, D = D0ρλ, suggesting that the self-diffusion coefficient of a confined fluid can be estimated from its molecular density, an easily accessible property.