Correlations of Low-Pressure Carbon Dioxide and Hydrocarbon Solubilities in Imidazolium-, Phosphonium-, and Ammonium-Based Room-Temperature Ionic Liquids. Part 2. Using Activation Energy of Viscosity

Abstract

This paper provides insights into the dependence of gas solubility on the viscosities of room-temperature ionic liquids (RTILs). Hildebrand solubility parameters are estimated from the activation energy of viscosity via a proportionality constant. The solubility of the gases CO2, ethylene, propylene, 1-butene, and 1,3-butadiene at low pressure and constant temperature are correlated, with the estimated RTIL Hildebrand solubility parameters, into two categories: bis(trifluoromethyl(sulfonyl)imide ([Tf2N]) and non-bis(trifluoromethyl(sulfonyl)imide (non-[Tf2N]) RTILs. The RTILs used in this work are based on 1-alkyl-3-methylimidazolium, quaternary phosphonium, and quaternary ammonium cations. The non-[Tf2N] anions used in the trial set are chloride [Cl], hexafluorophosphate [PF6], trifluoromethanesulfonate [TfO], bis((perflurorethyl)sulfonyl)imide [BETI], dicyanamide [DCA], diethylphosphate [DEP], and dodecylbenzenesulfonate [DBS]. The analysis of the correlation parameters against theory indicates that solute/solvent interactions may be negligible, compared to solvent/solvent and solute/solute interactions, when correlating the relative trend in gas solubilities between RTILs. This even occurs for the non-[Tf2N] anion classification for CO2 solubility that contained anions over a range of electron donor potentials from [Cl] to [BETI], which is in contrast to the widely published statement that the relative CO2 solubility between RTILs is related to anion interactions. Within an anion class, the RTIL solubility parameters decrease as the length of the cation-alkyl chain increases. Overall, the viscosity correlations presented here give better universal correlations for gas solubility than those previously presented in the literature (surface tension, lattice-energy densities, and melting points).