Liquid–Liquid Phase Separation of Viologen Bistriflimide/Benzene Mixtures: Role of the Dual Ionic and Organic Nature of Ionic Liquids

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.

Xenon Diffusion in Ionic Liquids with Blurred Nanodomain Separation

The dynamics of xenon gas, loaded in a series of 1-alkyl-3-methylimidazolium based ionic liquids, probes the formation of increasingly blurred polar/apolar nanodomains as a function of the anion type and the cation chain length. Exploiting ¹²⁹ Xe NMR spectroscopy techniques, like Pulse Gradient Spin Echo (PGSE) and inversion recovery (IR), the diffusion motion and relaxation times are determined for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C_n C₁ im][TFSI]. A correlation between the ILs nano-structure and both xenon diffusivity and relaxation times, as well as chemical shifts, is outlined. Interestingly, comparison with previous results of the same properties in the homologous imidazolium chlorides and hexafluorophospate shows an opposite trend with the alkyl chain length. Classical molecular dynamics (MD) simulations are used to calculate the xenon and cation and anion diffusion coefficients in the same systems, including imidazolium cations with longer chains (n=4, 6, 8 … 20). An almost quantitative agreement with the experiments validates the MD simulations and, at the same time, provides the necessary structural and dynamic microscopic insights on the nano-segregation and diffusion of xenon in bistriflimide, chloride and hexafluorphosphate salts allowing to observe and rationalize the shaping effect of the cation in the nanostructure.