High-Pressure Phase Behavior of Ionic Liquid/CO2 Systems

This work presents the high-pressure phase behavior of CO 2 with six ionic liquids: 1- n -butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]), 1- n -octyl-3-methylimidazolium hexafluorophosphate ([C 8 -mim][PF 6 ]), 1- n -octyl-3-methylimidazolium tetrafluoroborate ([C 8 -mim][BF 4 ]), 1- n -butyl-3-methylimidazolium nitrate ([bmim][NO 3 ]), 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO 4 ]), and N -butylpyridinium tetrafluoroborate ([ N -bupy][BF 4 ]). We explored the effect of systematically changing the anionic and cationic components of the ionic liquid on the CO 2 −ionic liquid phase behavior. For all of the ionic liquids tested, large quantities of CO 2 dissolved in the ionic liquid phase, but no appreciable amount of ionic liquid solubilized in the CO 2 phase. In addition, the liquid phase volume expansion with the introduction of even large amounts of CO 2 is negligible, in dramatic contrast to the large volume expansion observed for neutral organic liquids. Our results seek to elucidate the underlying physical mechanisms of this highly unusual phase behavior.