Solution thermodynamics of imidazolium-based ionic liquids and water

This work presents the vapor-liquid equilibrium and the liquid-liquid equil ibrium phase behavior and associated thermodynamic properties of water with three ionic liquids: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([b mim][PF6]), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C(8)mim][PF 6]), and 1-n-octyl-3-methylimidazolium tetrafluoroborate ([C(8)mim][BF4]). Although water stable, these compounds are hygroscopic, so the uptake of wa ter vapor is an important issue. Due to the negligible volatility of ionic liquids, we were able to measure vapor-liquid equilibrium using a gravimetr ic microbalance, which was designed to measure adsorption on solids. The He nry's law constants range from 0.033 to 0.45 bar, with infinite dilution ac tivity coefficients ranging from slight positive deviations from Raoult's l aw to as high as 8.62. The enthalpies and entropies of absorption are simil ar to those for the absorption of water into alcohols. In addition, we pres ent water/ionic liquid liquid-liquid equilibria, which is important if wate r 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 wa stewater treatment challenge. As one possible means for removing ionic liqu ids from water, we show that activated carbon is effective for this separat ion.