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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This work presents the solubility of nine different gases in 1-n-butyl-3-methylimidazolium hexafluorophosphate. The gases considered include carbon dioxide, ethylene, ethane, methane, argon, oxygen, carbon monoxide, hydrogen, and nitrogen. We also report the associated Henry's constants and enthalpies and entropies of absorption. We found carbon dioxide to have the highest solubility and strongest interactions with the ionic liquid, followed by ethylene and ethane. Argon and oxygen had very low solubilities and immeasurably weak interactions. Carbon monoxide, hydrogen, and nitrogen all had solubilities below the detection limit of our apparatus. Our results suggest that the mass transfer of gases into ionic liquids likely will be an important issue for reactions involving these gases. We also determined that ionic liquids show good potential for use as a gas-separation medium.

This work presents the vapor−liquid equilibrium and the liquid−liquid equilibrium phase behavior and associated thermodynamic properties of water with three ionic liquids:  1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]), and 1-n-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]). Although water stable, these compounds are hygroscopic, so the uptake of water vapor is an important issue. Due to the negligible volatility of ionic liquids, we were able to measure vapor−liquid equilibrium using a gravimetric microbalance, which was designed to measure adsorption on solids. The Henry's law constants range from 0.033 to 0.45 bar, with infinite dilution activity coefficients ranging from slight positive deviations from Raoult's law to as high as 8.62. The enthalpies and entropies of absorption are similar to those for the absorption of water into alcohols. In addition, we present water/ionic liquid liquid−liquid equilibria, which is important if water is used as a solvent to extract solutes from ionic liquids. In particular, dissolution of the ionic liquid in the aqueous phase could represent a wastewater treatment challenge. As one possible means for removing ionic liquids from water, we show that activated carbon is effective for this separation.

We report the results of a molecular dynamics study of the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6 ], a widely studied ionic liquid. An all-atom force field is developed using a combination of density functional theory calculations and CHARMM 22 parameter values. Molecular dynamics simulations are carried out in the isothermal-isobaric ensemble at three different temperatures. Quantities computed include infrared frequencies, molar volumes, volume expansivities, isothermal compressibililties, self-diffusivities, cation-anion exchange rates, rotational dynamics, and radial distribution functions. Computed thermodynamic properties are in good agreement with available experimental values.