On the High-Pressure Solubilities of Carbon Dioxide in Several Ionic Liquids

The solubility of carbon dioxide in nine ionic liquids (ILs), at 298 K, 313 K, and 333 K and pressures up to 9 MPa, is presented. The solubility of CO2 in the selected ILs increases with increasing pressure and decreases with increasing temperature, as expected. The effect of several different anions on the solubility of CO2 with a common 1-ethyl-3-methyl­imidazolium cation is studied in this work. The anions compared were hydrogen sulfate ([emim][HSO4]), methylsulfate ([emim][MeSO4]), methane sulfonate ([emim][MeSO3]), thiocyanate ([emim][SCN]), and diethylphosphate ([emim][DEP]). The results for 1-ethyl-3-methyl­imidazolium hydrogen sulfate are particularly interesting and can be explained based on the stronger attractive interactions between the IL and CO2, which is supported by quantum calculations. Other ILs investigated were ethyl(tributyl)phosphonium diethylphosphate ([P2444][DEP]), 1-hexyl-3-methyl­imidazolium trifluoromethane sulfonate ([hmim][OTf]), 1-(2-hydroxyethyl)-3-methyl­imidazolium trifluoroacetate ([OHemim][TFA]), and trihexyl tetradecyl­phosphonium bis(trifluoro­methyl­sulfonyl)imide ([P66614][Tf2N]). Using the new data for the [emim]+ ILs, in concert with data from the literature, these ILs were selected primarily to study the effect on the solubility of changing the cation. Of particular note is the high solubility of CO2 in the trihexyltetra­decyl­phosphonium based IL compared with its imidazolium equivalent. A brief discussion is presented to explain the observed solubility results.