Structural and energetic signatures of SO2 and CH4 incorporation in imidazolium-based ionic liquids.

Abstract

A systematic molecular dynamics study of sulfur dioxide (SO2) and methane (CH4) incorporation in 24 imidazolium-based ionic liquids is presented, spanning different anion families and cation alkyl chain lengths. The two gases represent limiting cases of solvation behavior in ionic liquids: SO2 exhibits strong electrostatic interactions and Lewis amphoteric character, whereas CH4 is governed primarily by weak London dispersion forces. Radial distribution functions, coordination numbers, cluster-size distributions, and interaction energies were analyzed to elucidate the molecular organization of the mixtures. The results show that neither SO2 nor CH4 substantially disrupts the polar network of the ionic liquids. SO2 partitions between the polar and nonpolar regions of the ionic liquid, leading to reduced self-association and more gas distributions, whereas CH4 preferentially occupies alkyl-rich regions and exhibits only weak self-association with minimal dependence on cation chain length. Interaction energy decomposition highlights the dominant role of electrostatics in SO2 solvation and the free volume-driven, weakly interacting nature of CH4 incorporation. These findings provide a molecular-level framework for rationalizing incorporation trends and for guiding the design of ionic liquids for gas separation, highlighting how polarity, anion charge distribution, and cation alkyl chain length can be tuned to enhance specific SO2 stabilization while controlling nonspecific CH4 accommodation in alkyl-rich free-volume domains.