  Carbon nanostructures are promising materials to improve the performance of
current gas separation membrane technologies. From the molecular modeling
perspective, an accurate description of the interfacial interactions is
mandatory to understand the gas selectivity in the context of greenhouse gases
applications. Most of the molecular dynamics simulations studies considered
available force fields with the standard Lorentz-Berthelot (LB) mixing rules to
describe the interaction among carbon dioxide (CO2), methane (CH4) and carbon
structures. We performed a systematic study in which we showed the LB
underestimates the fluid/solid interaction energies compared to the density
functional theory (DFT) calculation results. To improve the classical
description, we propose a new parametrization for the cross-terms of the
Lenard-Jones (LJ) potential by fitting DFT forces and energies. The effects of
the new parametrization on the gases adsorption within single-walled carbon
nanotubes (SWCNTs) with varying diameters, are investigated with Grand
Canonical Monte Carlo simulations. We observed considerable differences in the
CO2 and CH4 density within SWCNTs compared to those obtained with the standard
approach. Our study highlights the importance of going beyond the traditional
LB mixing rules in studies involving solid/fluid interfaces of confined
systems. The revised mixing terms enhanced fluid/carbon interface description
with excellent transferability ranging from SWCNTs to graphene.
