Influence of Anion Species on Liquid-Liquid Phase Separation in [EMIm⁺][X^-]/Benzene Mixtures

When a molar excess of benzene is mixed with an ionic liquid (IL), liquid-liquid phase separation may appear with a pure liquid phase almost composed of only benzene molecules separated from a liquid clathrate phase with benzene molecules dissolved in the IL. Our previous study (J. Phys. Chem. B, 124, 7929, 2020) on long-chain IL/benzene systems has concluded that benzene molecules, as planar nonpolar ones, majorly dissolve in the IL nonpolar domains consisting of cationic alkyl side chains. Nevertheless, the above mechanism is inadequate for explaining the experimental observations that benzene can also dissolve in IL systems with very short alkyl side chains. In this study, by molecular dynamics simulation of the [EMIm⁺][X^-]/benzene mixtures with X^- being Cl^-, NO₃^-, PF₆^-, or Tf₂N^-, we still observe liquid-liquid phase separation of the pure benzene phase from a liquid clathrate (mixed IL/benzene) phase where benzene molecules are almost equally distributed near imidazolium rings through π-stacking or near alkyl side chains. The anion species strongly influences the solubility of benzene and the ratio of the two liquid phases via the alteration of anionic charge density, which tunes the strength of the electrostatic interaction among ions and thus the probability of benzene molecules interacting with both imidazolium rings and alkyl side chains: a larger anionic charge density corresponds to a lower solubility of benzene.