Quasi-Universal Solubility Behavior of Light Gases in Imidazolium-Based Ionic Liquids with Varying Anions: A Molecular Dynamics Simulation Study
Majid Namayandeh Jorabchi, Ralf Ludwig, Dietmar Paschek
Abstract
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 ([Cnmim][PF6]) and 1-n-alkyl-3-methylimidazolium tetrafluoroborate ([CnmimBF4]) 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.