  We investigate the adsorption and diffusion behaviors of CO2, CH4, and N2 in
interfacial systems composed of a polymer of intrinsic microporosity (PIM-1)
and amorphous silica using grand canonical Monte Carlo (GCMC) and molecular
dynamics (MD) simulations. We build model systems of mixed matrix membranes
(MMMs) with PIM-1 chains sandwiched between silica surfaces. Gas adsorption
analysis using GCMC simulations shows that gas molecules are preferentially
adsorbed in microcavities distributed near silica surfaces, resulting in an
increase in the solubility coefficients of CO2, CH4, and N2 compared to bulk
PIM-1. In contrast, diffusion coefficients obtained from MD simulations and
then calibrated using the dual-mode sorption model show different tendencies
depending on gas species: CO2 diffusivity decreases in MMMs compared to PIM-1,
whereas CH4 and N2 diffusivities increase. These differences are attributed to
competing effects of silica surfaces: the emergence of larger pores as a result
of chain packing disruption, which enhances gas diffusion, and a
quadrupole-dipole interaction between gas molecules and silica surface hydroxyl
groups, which retards gas diffusion. The former has a greater impact on CH4 and
N2 diffusivities, whereas the latter has a greater impact on CO2 diffusivity
due to the strong quadrupole-dipole interaction between CO2 and surface
hydroxyls. These findings add to our understanding of gas adsorption and
diffusion behaviors in the vicinity of PIM-1/silica interfaces, which are
unobtainable in experimental studies.
