Solubility of argon, krypton and xenon in ionic liquids

Abstract

This work presents the results of argon, krypton and xenon solubility measurements in a series of imidazolium-based ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide ([TFSI]-), 2-cyanopyrrolide ([2CNPyr]-) and hexafluorophosphate ([PF6]-) anions, as well as ILs with the trihexyltetradecylphosphonium ([P66614]+) cation paired with 2-cyanopyrrolide ([2CNPyr]-) and 1,2,4-triazolide ([4-triaz]-) anions. The solubility of Ar was measured at 295 K and pressures up to 9 MPa, Kr was also measured at 295 K and pressures up to 5 MPa, and Xe was measured at temperatures from 295 K to 333 K and pressures up to 0.3 MPa. Noble gases are simple nonpolar, spherical atoms with varying sizes and polarizabilities. While their separation, especially from air, is relevant because they have uses in providing inert atmospheres and in fluorescent lights, the main purpose of this study is to gain fundamental understanding of the influence of size and polarizability on gas solubility in ILs. As a result, the temperature effects on the dissolution of argon and xenon were investigated in order to determine enthalpies of dissolution. The solubilities of noble gases correlate strongly with the IL molar volume, indicating the importance of entropic interactions, because free volume generally correlates with molar volume. While van der Waals interactions are the dominant forces between smaller noble gas (like Ar) and the IL, Xe has sufficiently high polarizability that both van der Waals and induced dipole-dipole interactions contribute to its dissolution.