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Ionic liquids have been suggested as replacement solvents in reactions and separations since they have negligible vapor pressure; thus, they would reduce fugitive emissions that are common when organic solvents are used in these applications. To fully utilize ionic liquids in reactions and separations, a fundamental understanding of the factors that govern the phase behavior of ionic liquids with other common liquids is necessary. In this work, we present a systematic study of the impact of different factors on the phase behavior of imidazolium-based ionic liquids with alcohols. All systems examined showed upper critical solution temperature (UCST) behavior, with low solubility of the ionic liquid in the alcohol and high solubility of the alcohol in the ionic liquid. An increase in the alkyl chain length of the alcohol resulted in an increase in the UCST. Branching of the alcohol resulted in a higher solubility of the alcohol in the ionic-liquid-rich phase. By increasing the alkyl chain length on the cation, the UCST decreased, while replacement of the hydrogen at the C2 position of the ring with a methyl group resulted in an increase in the UCST. The choice of anion was shown to have a large impact on the UCST of the system. The relative alcohol affinity for the different anions observed was (CN)2N > CF3SO3 > (CF3SO2)2N > BF4 > PF6.
Crosthwaite et al. (Sat,) studied this question.