CO2/N2 Selectivity in [emim][BF4] + [emim][TFSI] and [emim][DCA] + [emim][TFSI] Ionic Liquid Mixtures

Mixtures of ionic liquids (ILs) present intriguing possibilities for modifying both physical and chemical properties relative to the pure ILs. Here we investigate the solubility of CO₂ and N₂ in mixtures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TFSI]) with 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF₄]) at 313.2 K and 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]) at 313.2 and 333.2 K. CO₂ solubilities are measured at pressures up to 15 bar, while N₂ solubilities were measured up to 140 bar. Both gases dissolve in these ILs by physical dissolution. For both mixtures the Henry’s Law constants for CO₂ and N₂ in the mixtures are lower (i.e., higher solubility) than a mole fraction weighted linear combination of the Henry’s Law constants in the pure ILs. In fact, the Henry’s Law constants for both gases in the mixtures are adequately fit with a mole fraction weighted linear combination of the natural logarithm of the pure IL Henry’s Law constants. However, in no case is the solubility of either gas in the IL mixtures greater than or less than the solubilities in the two pure ILs. As a result, the anticipated CO₂/N₂ selectivity in the IL mixtures, based on pure gas solubility measurements in the IL mixtures, is always between the values of the CO₂/N₂ selectivity for the pure ILs.