A simple guiding principle for the temperature dependence of the solubility of light gases in imidazolium-based ionic liquids derived from molecular simulations

Abstract

We have determined the temperature dependence of the solvation behavior of a large collection of important light gases in imidazolium-based ionic liquids with the help of extensive molecular dynamics simulations. The motivation of our study is to unravel common features of the temperature dependent solvation under well controlled conditions, and to provide a guidance for cases, where experimental data from different sources disagree significantly. The solubility of molecular hydrogen, oxygen, nitrogen, methane, krypton, argon, neon and carbon dioxide in the imidazolium based ionic liquids of type 1-n-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cmim][NTf]) with varying alkyl side chain lengths n = 2, 4, 6, 8 is computed for a temperature range between 300 K and 500 K at 1 bar. By applying Widom's particle insertion technique and Bennet's overlapping distribution method, we are able to determine the temperature dependent solvation free energies of those selected light gases in simulated imidazolium based ionic liquids with high statistical accuracy. Our simulations demonstrate that the magnitude of the solvation free energy of a gas molecule at a chosen reference temperature and that of its temperature-derivatives are intimately related to one another. We conclude that this "universal" behavior is rooted in a solvation entropy-enthalpy compensation effect, which seems to be a defining feature of the solvation of small molecules in ionic liquids. The observations lead to simple analytical relations, determining the temperature dependence of the solubility data based on the absolute solubility at a certain reference temperature. By comparing our results with available experimental data from many sources, we can show that our approach is particularly helpful for providing reliable estimates for the solvation behavior of very light gases, such as hydrogen, where conflicting experimental data exist.

A molecular dynamics study of the solvation of carbon dioxide and other compounds in the ionic liquids [emim][B(CN)4] and [emim][NTf2]

“…16, where they applied the Bennett Acceptance Ratio (BAR) method [73] to independent samples obtained with different values of λ vdW and λ Coul . Moreover, the ∆G sim value we computed for [emim][NTf 2 ] is very close to the one obtained by Kerl et al [37] (−0.534 kcal/mol, with no reported uncertainty) using the Free-Energy Perturbation (FEP) method [74].…”

“…The authors characterized this refinement as unusual, given that they modified the LJ parameters while keeping the partial charges unchanged. Kerl et al [37] recently employed this model to accurately predict Henry constants of CO 2 in [emim][NTf 2 ] at several temperatures.…”

Molecular Simulations of Thermodynamic Properties for the System α‐Cyclodextrin/Alcohol in Aqueous Solution

“…These relations were applied frequently to calculate the solubility of light gases in ionic liquids [41], in water, or aqueous solutions [42,43], or the solubility of n-alkanes in water [44], respectively. For the prediction of the solubility of solutes that are solid in their pure state see [45,46].…”

Solubility and Diffusivity of Oxygen in Ionic Liquids

“…In an early stage of investigation, researchers argued that the O 2 solubility increased with increasing temperature, which would mean no attraction between the O 2 and the IL (i.e., a positive enthalpy of dissolution). ,,− However, publications from recent years claim that the O 2 solubility in ionic liquids does decrease with increasing temperature. Among those works, some argued that the temperature dependence is strong, and the enthalpy of absorption ranged from −2 to −20 kJ/mol. ,,,− Others argued that O 2 absorption is only weakly exothermic or independent of temperature, ,− which means that the enthalpy of absorption is close to zero. To clarify the temperature dependence of O 2 dissolution and characterize the strength of interaction between O 2 and the ILs, the O 2 solubility was measured at 293, 313, and 333 K in selected ionic liquids, and the standard enthalpies and entropies of absorption were obtained.…”