Ionic liquids (ILs) are salts that are liquid at low temperatures, usually including the region around room temperature. They are under intense investigation, especially as replacement solvents for reactions and separations, since they exhibit negligible vapor pressure and would not, therefore, contribute to air pollution. Clearly, basic thermophysical properties are vital for design and evaluation for these applications. We present density as a function of temperature, melting temperatures, glass-transition temperatures, decomposition temperatures, and heat capacities as a function of temperature for a series of 13 of the popular imidazolium-based ILs. The ionic liquids investigated here are 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium tris(trifluoromethylsulfonyl)methide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide, 2,3-dimethyl-1-ethylimidazolium bis(trifluoromethylsulfonyl)imide, 2,3dimethyl-1-propylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, and 1-butyl-2,3-dimethylimidazolium hexafluorophosphate. The properties follow quite reasonable trends. For instance, density decreases as the length of the alkyl chain on the cation increases. For a given cation, the density increases as the molecular weight of the anion increases for the anions studied here. Many of the ILs tend to subcool easily, forming glasses at very low temperatures rather than exhibiting crystallization or melting transitions. The thermal stability increases with increasing anion size, and heat capacities increase with temperature and increasing number of atoms in the IL.

Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ionic liquids. CO2 absorption isotherms at 10, 25, and 50 degrees C are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO2, while there is little difference in CO2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO2 about hexafluorophosphate anions, but only small differences in CO2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.

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Previously we have shown that supercritical carbon dioxide can be used to extract organics from ionic liquids (ILs). Subsequently, ionic liquids/carbon dioxide biphasic solutions have been used for a variety of homogeneously catalyzed reactions. Therefore, an understanding of the phase behavior of carbon dioxide with ionic liquids is needed to design extraction and reaction processes necessary for these applications. We present measurements of the solubility of carbon dioxide in 10 different imidazolium-based ionic liquids at 25, 40, and 60 °C and pressures to 150 bar. As expected, the solubility increases with increasing pressure and decreases with increasing temperature for all the ILs investigated. To investigate the influence of the anion, seven of the ILs studied have 1-butyl-3-methylimidazolium ([bmim]) as the cation. The anions are dicyanamide ([DCA]), nitrate ([NO3]), tetrafluoroborate ([BF4]), hexafluorophosphate ([PF6]), trifuoromethanesulfonate ([TfO]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), and tris(trifluoromethylsulfonyl)methide ([methide]). The other ILs considered in the study, chosen to investigate the influence of varying number and length of alkyl chains on the cation, include 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]), 2,3-dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl)imide ([hmmim][Tf2N]), and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([omim][Tf2N]). Results show that the solubility of carbon dioxide is strongly dependent on the choice of anion. In particular, CO2 solubility is greater in ILs with anions, such as [Tf2N] and [methide], which contain fluoroalkyl groups. Also, we observe that an increase in the alkyl chain length on the cation increases the CO2 solubility marginally.

This work presents the results of solubility measurements for a series of gases in 1-n-butyl-3-methyl imidazolium tetrafluoroborate and 1-n-butyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) imide. The gases considered include benzene, carbon dioxide, nitrous oxide, ethylene, ethane, oxygen, and carbon monoxide. Carbon dioxide and oxygen solubilities are also reported in methyl-tributylammonium bis(trifluoromethylsulfonyl) imide, butyl-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide, and tri-isobutyl-methyl phosphonium p-toluenesulfonate. We report the associated Henry's constants and enthalpies and entropies of absorption. In general, benzene, followed by carbon dioxide and nitrous oxide, have the highest solubilities and strongest interactions with the ionic liquids, followed by ethylene and ethane. Oxygen had very low solubilities and weak interactions. Carbon monoxide had a solubility below the detection limit of our apparatus. Ionic liquids with the bis(trifluoromethylsulfonyl) imide anion had the largest affinity for CO2, regardless of whether the cation was imidazolium, pyrrolidinium, or tetraalkylammonium. These results suggest that the nature of the anion has the most significant influence on the gas solubilities.