Liquid−Liquid Equilibrium in Mixtures of the Ionic Liquid 1-n-Butyl-3-methylimidazolium Hexafluorophosphate and an Alkanol

Abstract

The liquid−liquid phase equilibrium of mixtures of the room temperature ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], and three single alkanols (ethanol, 1-propanol, and 1-butanol) was investigated over the entire composition range at ambient pressure. The experiments were conducted from 262 K to the vicinity of the critical solution temperature of the binary mixture (at maximum 362 K) by two different methods, namely, synthetic cloud-point measurements and analytical UV spectroscopy. The cloud-point method was mainly applied for the [bmim][PF6]-rich liquid, whereas UV spectroscopy was used to determine the very small concentrations of [bmim][PF6] in the alkanols, since under these conditions the cloud-point method is no longer applicable. All three systems show an upper critical solution temperature. With increasing chain length of the alcohol, that temperature rises and simultaneously the biphasic region becomes larger. Inspired by recent publications, the liquid−liquid equilibrium of these three binary systems was predicted by applying the COSMO-RS method. Calculations resulted in predictions of a miscibility gap, but the calculated miscibility gap strongly differs from the experimental results. A far better representation of the experimental data was accomplished via a UNIQUAC-based correlation.