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.