Gas Solubilities in Ionic Liquids

Bonnesen, in Physical Factors in Anion Separations (A. Bianchi, K. Bowman-James, 

42 NIST Database 46 Critically Selected Stability Constants of Metal Complexes Database, U.S. Department of Commerce, Gaithersburg, MD, 1998, ver. 5.0. 

Atwood, Liquid Clathrates , in Inclusion Compounds (J. L. Atwood, 

Joan P. Brennecke, Jennifer L. Anthony, and Edward J. Maginn 3.4.1 

A wide variety of physical properties are important in the evaluation of ionic liquids (ILs) for potential use in industrial processes. These include pure component properties such as density, isothermal compressibility, volume expansivity, viscosity, heat capacity, and thermal conductivity. However, a wide variety of mixture properties are also important, the most vital of these being the phase behavior of ionic liquids with other compounds. Knowledge of the phase behavior of ionic liquids with gases, liquids, and solids is necessary to assess the feasibility of their use for reactions, separations, and materials processing. Even from the limited data currently available, it is clear that the cation, the substituents on the cation, and the anion can be chosen to enhance or suppress the solubihty of ionic liquids in other compounds and the solubihty of other compounds in the ionic Hquids. For instance, an increase in alkyl chain length decreases the mutual solubihty with water, but some anions ([BF4] , for example) can increase mutual solubility with water (compared to [PF JT for instance) [1-3], While many mixture properties and many types of phase behavior are important, we focus here on the solubihty of gases in room temperature ILs. 

A third motivation for studying gas solubilities in ILs is the potential to use compressed gases or supercritical fluids to separate species from an IL mixture. As an example, we have shown that it is possible to recover a wide variety of solutes from ILs by supercritical C02 extraction [9]. An advantage of this technology is that the solutes can be removed quantitatively without any cross-contamination of the C02 with the IL. Such separations should be possible with a wide variety of other compressed gases, such as C2H6, C2H4, and SF(l. Clearly, the phase behavior of the gas in question with the IL is important for this application. 

Finally, a fourth motivation for exploring gas solubilities in ILs is that they can act as probes of the molecular interactions with the ILs. Information can be discerned on the importance of specific chemical interactions such as hydrogen bonding, as well as dipole-dipole, dipole-induced dipole, and dispersion forces. Of course, this information can be determined from the solubility of a series of carefully chosen liquids, as well. However, gases tend to be of the smallest size, and therefore the simplest molecules with which to probe molecular interactions. 

A second motivation for understanding gas solubilities in ILs is the possibility of using ILs for the storage of gases. In this way, they could essentially become liquid adsorbents. A novel application of this idea is discussed in the Applications section. 

---

Gas solubilities in ionic liquids and related measurement techniques... 

Anthony, J. L. et al.. Anion effects on gas solubility in ionic liquids, /. Phys. 

Anderson JL, Anthony JL, Brennecke JF, Maginn EJ (2008) Ionic liquids in synthesis, In Gas solubilities in ionic liquids, Vol. 1. Wiley New York 103-129... 

Brennecke, J.F., and Maginn, E.J. (2008) Gas solubilities in ionic liquids, in Ionic Liquids in Sythesis, Chapter 3.4, Wiley-VCH Verlag GmbH. 

Imidazolium ionic liquids are known to be a remarkable solvent for CO2 solubility ref It has been found that anion play the important role in determining the gas solubility in ionic liquids. It has been reported that the bistriflamide anion has the greatest affinity for CO2 experimentally compared to other anions, such as the halides, triflate ([OTf] ), hexafluorophosphate ([PFe] ), tetrafluoroborate ([BF4] ), dicyanamide ([N(CN)2] ) and ethanoate ([MeC02] ) Although fluorination effect are shown to have greatly enhanced the solubility of CO2, these class of ionic liquids are very expensive and are not environmental friendly, due its high stability and low reactivity... 

---