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The phenomenon of ionic liquid (IL) nanoconfinement within a copolymer/IL membrane reportedly enhances membrane selectivity, solubility, and transport in gas separations. Also, the copolymer/IL membrane morphology has been found to affect IL stability at high transmembrane pressures. In this work, a combined mesoscopic dynamics simulation and hybrid grand canonical Monte Carlo/molecular dynamics (GCMC-MD) simulations were carried out to investigate the morphologies, as well as CO2/CH4 gas diffusivities, solubilities, and selectivities of polystyrene-b-poly(ethylene oxide) (PS-b-PEO)/1-Ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) and PS-b-PEO/1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) membranes. The latter simulations focused on nanoconfined ILs in the copolymer/IL phase boundaries at 2.5 and 5 nm confinement lengths. The investigated systems were four nanoconfined ILs, i.e., PS/[EMIM][SCN]/PEO (the IL forming a separate microphase, denoted IL-Micro), PS/[EMIM][Tf2N]/PEO, PS/[EMIM][SCN]-PEO/PS (the IL distributed in the PEO phase, denoted IL-PEO), and PS/[EMIM][Tf2N]-PEO/PS, and five control systems, i.e., PS/PEO/PS, bulk PS, bulk PEO, bulk [EMIM][SCN], and bulk [EMIM][Tf2N]. Based on the mesoscopic dynamics simulation results, the dominant membrane morphologies at IL loadings of <50 vol % were lamellar or cylindrical (favorable for both IL stability at high transmembrane pressures if the bedding planes are horizontal, i.e. at 90° to the nominal direction of the transmembrane pressure gradient) with the IL-PEO or IL-Micro phases. Also, there was an overall 50% match between the observed PS-b-PEO/[EMIM][SCN] and PS-b-PEO/[EMIM][Tf2N] membrane morphologies. Based on the MD simulation results, both CO2 and CH4 diffusivities were the smallest in the bulk PS (control) and highest in the PS/[EMIM][Tf2N]/PEO system (IL-Micro between the PS and PEO phases) at both confinement lengths. The CO2 diffusivities were, on average, larger when the confinement length increased to 5 nm. The GCMC-MD results indicated that the CO2 solubility in the IL-Micro phases was higher than in the corresponding bulk ILs at both confinement lengths, with the PS/[EMIM][Tf2N]/PEO system exhibiting the highest CO2 solubility, followed by the PS/[EMIM][SCN]/PEO system. Additionally, the permselectivities of the nanoconfined IL systems were, on average, 40–50% larger than those of the bulk systems, with the highest permselectivity observed for PS/[EMIM][Tf2N]/PEO at the confinement length of 5 nm. Overall, the IL nanoconfinement between the PS and PEO phases (IL-Micro) leads to significant improvements in the CO2/CH4 permselectivities, suggesting that strategies to create nanoconfined IL morphologies in the copolymer/IL membranes are very promising for optimizing the membrane gas separation performance.
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This content will become publicly available on August 17, 2027
CO2/CH4 Transport and Pressure-CoupledIonic 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.
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- Award ID(s):
- 2031021
- PAR ID:
- 10710595
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of PhysicalChemistry C
- Volume:
- 130
- Issue:
- 34
- ISSN:
- 1932-7447
- Page Range / eLocation ID:
- 12093–12110
- Subject(s) / Keyword(s):
- membrane ionic liquid molecular dynamics simulation ion gating gas separation
- Format(s):
- Medium: X Other: pdf
- Sponsoring Org:
- National Science Foundation
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