CO2/CH4 Transport and Pressure-Coupled Ionic Liquid (IL) Gating in Nanoconfined Single and Mixed IL Membranes

Abstract

Nonequilibrium molecular dynamics (NEMD) simulations were employed to investigate pressure-driven CO2/CH4 transport in nanoconfined ionic liquid (IL) membranes composed of [EMIM][Tf2N], [EMIM][Cl], and their equimolar mixture, i.e., [EMIM][(Tf2N)0.5Cl0.5], confined between nanoporous graphene (Gr) walls. Across all systems, CO2 flux exceeds that of CH4, with CO2 permeance values on the order of 105 GPU, reflecting intrinsic transport under nanoconfinement, and CO2 transport increasing by ∼25–40% when pressure rises from 10 to 20 bar. A key mechanistic finding in this work is the identification of a pressure-coupled IL gating mechanism that influences molecular transport under nanoconfinement. Spatial probability mapping reveals that ions preferentially accumulate at Gr pore entrances, dynamically modulating accessible transport pathways. This gating behavior is strongly dependent on anion chemistry. Specifically, [Cl]− produces anion-dominated gating, [Tf2N]− leads to cation-mediated gating, and mixed-anion systems generate heterogeneous, partially open pore environments. Importantly, the gating effect evolves with applied pressure and is consistent with the observed nonlinear permeation scaling and pressure-dependent diffusivity trends. The pressure-dependent ion occupancy patterns observed at the pore rims are consistent with the corresponding trends in gas transport properties, indicating that interfacial ion organization, in addition to bulk IL properties, plays an important role in governing selectivity and permeability under IL nanoconfinement. These findings provide molecular-level insights into the relationship between IL composition, interfacial structure, and transport behavior in the systems investigated, offering a foundation for the future design of nanoconfined IL membranes.