Quasi-Universal Solubility Behavior of Light Gases in Imidazolium-Based Ionic Liquids with Varying Anions: A...

In this work, the temperature-dependent solvation behavior of a number of important light gases, such as carbon dioxide, xenon, krypton, argon, oxygen, methane, nitrogen, neon, and hydrogen, in two important imidazolium-based ionic liquids (ILs) of the type 1-n-alkyl-3-methylimidazolium hexafluorophosphate ([Cmim][PF₆]) and 1-n-alkyl-3-methylimidazolium tetrafluoroborate ([CmimBF₄]) with varying chain lengths (n = 2, 4, 6, and 8) are investigated using molecular dynamics simulations for a temperature range between 300 and 500 K at a pressure of 1 bar. The aim of this work is first to propose a reliable estimate for the temperature-dependent solubility behavior of (very) light gases, e.g., hydrogen and nitrogen, where reported experimental data are inconsistent. Moreover, we would like to rationalize the common features of the temperature-dependent solvation of light gases for various imidazolium-based ionic liquids. For the selected solute gases in our simulated imidazolium-based ILs, we applied the potential distribution theorem using both Bennet's overlapping distribution method (ODM) and Widom's particle insertion technique to determine the temperature-dependent solvation free energies with good statistical accuracy. We observed from the simulations that the quantity of the solvation free energy of a gas molecule and its temperature derivatives are connected in regard to each other at a chosen reference temperature. This trend was observed for all the studied light gases. Moreover, the computed solvation enthalpies of all gases obey an enthalpy-entropy compensation behavior, which is almost identical for all the studied ILs. Based on this observation, we report a correlation between the temperature-dependent solubility behavior of light gases in various ILs at their reference state so that we are now able to semiquantitively predict the temperature-dependent solubility behavior of a certain gas in various imidazolium-based ionic liquids based on a single solubility value of that gas in one of the ILs at a certain temperature.