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

Abstract

Liquid–liquid phase separation occurs at room temperature when mixing an excess of benzene with solid viologen bistriflimide salts with various alkyl side-chain lengths. A liquid phase composed of (almost) pure benzene is above the other sponge-like liquid phase with salt absorbed in benzene. Nuclear magnetic resonance experiments indicate that the mole ratio of benzene/salt in the sponge-like phase remains unchanged upon varying the amounts of (nonexcessive) salt or benzene. Moreover, the benzene/viologen salt mole ratio in the sponge-like phase increases linearly with respect to the side-chain length of the cation. Similarly, when an excess of viologen salt is added in benzene, a sponge-like liquid phase composed of salt absorbed by benzene is observed in equilibrium with some solid viologen salt neither dissolved nor absorbed by the solvent. The mole ratio of the sponge-like liquid phase again increases linearly with side-chain length, while it remains independent of the relative amount of benzene and viologen salt as long as the latter is in excess. Finally, when appropriate amounts of benzene and viologen salt are mixed, a single sponge-like liquid phase is observed at an intermediate composition between the lower and upper limits. Molecular dynamics simulations reveal that because of their dual ionic and organic nature, when absorbed in benzene, the studied salts form nanoscale segregated liquid structures, akin ionic liquids, with a continuous polar network composed of anions and cationic charged groups, along with nonpolar domains composed of alkyl cationic side chains. Benzene molecules are preferentially absorbed inside the nonpolar region, which effectively expands the nonpolar region to be sponge-like and consequently liquidizes the viologen salt. The linearity of the benzene/salt ratio in the upper and lower phase boundaries comes from the fact that the effective volume of the nonpolar region for accommodating benzene molecules grows linearly with cationic alkyl side-chain length. The occurrence of the above phenomena is attributed to the nonpolar feature of benzene molecules, and there is no evidence of π–π or ion−π interaction between the ions and benzene molecules. Moreover, the diffusion of benzene in the sponge-like phase is found to be close to that in n-alkanes, supporting the idea of nanoscale segregation of polar and nonpolar regions in the sponge-like phase. The revealed mechanism is anticipated to be general for understanding liquid–liquid phase separation observed in mixtures of organic salts (ionic liquids) having relatively long alkyl chains with small organic molecules.

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

“…Another possible future route to investigate the benzene solubility in ILs involves changing the cationic alkyl side-chain length. Our previous study demonstrates that in the benzene/IL mixtures where the cations possess a relatively long alkyl side chain, the benzene molecules solvated in nonpolar domains composed of alkyl side chains dominantly contribute to benzene solubility, and the π-stacking feature is negligible. Another experiment also indicates that when the alkyl chain is long enough, the liquid phase–phase separation can disappear at a certain temperature, and the temperature dependence of solubility can vary with alkyl side-chain length.…”

Phase behaviors of ionic liquids attributed to the dual ionic and organic nature

“…[133] performed both NMR experiments and all-atom MD simulations on a set of mixtures of benzene and viologen bistriflimide salts [CmbpCn)2][CF3SO2)2N] with m and n being the numbers of carbon atoms on the two alkyl side chains. The viologen salts are crystal at room temperature but also exhibit ILC and NSL phases at higher temperatures, as common ILs do [133].…”

Exploring the Aggregation Behavior of Extractant Molecules in Ionic Liquids: A Coupled Polarizable Molecular Dynamics and SAXS Study

“…It is widely recognized that extraction mechanisms in conventional diluents rely heavily on the self-assembly of the extractant molecules in supramolecular aggregates, − and some studies have shown that different kind of solutes and surfactants can self-assemble into aggregates of different shapes in ILs. − Additionally, some studies revealed that enhanced extraction efficiency in ILs without ion exchange can be attributed to variations in structural properties. ,, Consequently, it is crucial to further investigate the relationship between structure and extraction efficiency in ILs, and to do so, a thorough understanding of the physicochemical interactions between the extractant and solvent molecules is essential.…”