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

摘要

We have determined the temperature dependence of the solvation behavior of alarge collection of light gases in imidazolium-based Ionic Liquids (ILs) withthe help of extensive molecular dynamics simulations. The solubility ofmolecular hydrogen, oxygen, nitrogen, methane, krypton, argon, neon and carbondioxide in the imidazolium based ILs of type 1-n-alkyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide ([C$_n$mim][NTf$_2$]) with varying chainlengths $n!=!2,4,6,8$ are computed for a temperature range between$300,mbox{K}$ and $500,mbox{K}$ at $1,mbox{bar}$. By applying Widom'sparticle insertion technique and Bennet's overlapping distribution method, weare able to determine the temperature dependent solvation free energies forthose selected light gases in simulated imidazolium based ILs with highstatistical accuracy. Our simulations show that the magnitude of the solvationfree energy of a gas molecule at a chosen reference temperature and itstemperature-derivatives are intimately related with respect to oneanother. Weconclude that this "universal" behavior is rooted in a solvationentropy-enthalpy compensation effect, which seems to be a defining feature ofthe solvation of small molecules in Ionic Liquids. We argue that this featureis based on a hypothesized funnel-like shape of the free energy landscape of asolvated gas molecule. The observations lead to simple analytical relations,determining the temperature dependence of the solubility data based on theabsolute solubility at a certain reference temperature, which we call"solvation funnel" model. By comparing our results with available experimentaldata from many sources, we can show that the "solvation funnel" model isparticularly helpful for providing reliable estimates for the solvationbehavior of very light gases, such as hydrogen, where conflicting experimentaldata exist.