Solution Thermodynamics of Imidazolium-Based Ionic Liquids and Water

Abstract

This work presents the vapor−liquid equilibrium and the liquid−liquid equilibrium phase behavior and associated thermodynamic properties of water with three ionic liquids: 1- n -butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]), 1- n -octyl-3-methylimidazolium hexafluorophosphate ([C 8 mim][PF 6 ]), and 1- n -octyl-3-methylimidazolium tetrafluoroborate ([C 8 mim][BF 4 ]). Although water stable, these compounds are hygroscopic, so the uptake of water vapor is an important issue. Due to the negligible volatility of ionic liquids, we were able to measure vapor−liquid equilibrium using a gravimetric microbalance, which was designed to measure adsorption on solids. The Henry's law constants range from 0.033 to 0.45 bar, with infinite dilution activity coefficients ranging from slight positive deviations from Raoult's law to as high as 8.62. The enthalpies and entropies of absorption are similar to those for the absorption of water into alcohols. In addition, we present water/ionic liquid liquid−liquid equilibria, which is important if water is used as a solvent to extract solutes from ionic liquids. In particular, dissolution of the ionic liquid in the aqueous phase could represent a wastewater treatment challenge. As one possible means for removing ionic liquids from water, we show that activated carbon is effective for this separation.