The solubility of carbon dioxide in a series of imidazolium-based room-temperature ionic liquids has been determined using a quartz crystal microbalance. Henry's constants were obtained for CO2 in nine different ionic liquids:  1-methyl-3-propylimidazolium bis[trifluoromethylsulfonyl]amide (C3mimTf2N), 1-n-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide (C4mimTf2N), C4mimTf2N with polyethylenimine, 1-n-hexyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide (C6mimTf2N), 1-methyl-3-n-octylimidazolium bis[trifluoromethylsulfonyl]amide (C8mimTf2N), 1-methyl-3-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)imidazolium bis[trifluoromethylsulfonyl]amide (C8F13mimTf2N), 1,4-dibutyl-3-phenylimidazolium bis[trifluoromethylsulfonyl]amide, 1-butyl-3-phenylimidazolium bis[trifluoromethylsulfonyl]amide, and 1-methyl-3-propylimidazolium hexafluorophosphate (C3mimPF6). All results were obtained at 25 °C with CO2 pressures at or less than 1 bar. A comparison of results helps in understanding the role of chemical structure on the separation capabilities of these materials. Notable among the results is a significantly greater measured CO2 solubility in the ionic liquid having the fluorine-substituted cation as compared to the corresponding ionic liquid with a nonfluorinated cation. CO2 solubility was found to be lower in the ionic liquid containing PF6 - than for the corresponding liquid with Tf2N- anion. Addition of an imine polymer to the ionic liquid did not significantly change CO2 solubility characteristics. The presence of water had a minor effect on CO2 solubility for C8mimTf2N for environments with a relative humidity of 40% or less. Henry's constant for CO2 in C3mimPF6 obtained in this study is in good agreement with previously reported Henry's constant values for CO2 in C4mimPF6 that were obtained by other means. This study not only provides some important thermodynamic property information for these unique materials but also illustrates the utility of using a quartz crystal microbalance to obtain this information.

In this paper, results from an experimental investigation of carbon dioxide (CO 2 ) solubility and diffusivity in the ionic liquids 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim], and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]) are reported. A transient thin liquid film method was developed, which enables one to determine the Henry's law constant and the diffusivity at low pressure simultaneously. Measurements were performed at temperatures in the range of 283-323 K. Henry's law constants were in the range of 25.5-84 bar and were in general agreement with measurements reported by other researchers for these and similar ionic liquids. The entropies and enthalpies of absorption were determined to increase as gas solubility decreased. The measured CO 2 diffusion coefficients in the five ionic liquids were ∼10 -6 cm 2 /s, which is an order of magnitude smaller than the coefficients for CO 2 diffusion in traditional organic solvents. In contrast to the gas solubility results, measured diffusion coefficients were determined to be dependent on the ionic liquid cation as well as the anion. In addition, CO 2 diffusion coefficients were considerably more sensitive to temperature than were CO 2 solubilities in these ionic liquids. Results were used to develop a correlation relating CO 2 diffusion to ionic liquid properties and system temperature.

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