Experimental and in silico Study of the Thermophysical Properties of Ionic Liquids with Less Common Imidazoium Cations

Although there are at least 1 million ionic liquids possible, a great portion of the ILs that have been experimentally synthesized and studied are based on relatively simple 1-R-3-methylimidazolium cations.  Here, we present results for ILs that have additional functional group(s) at the 2, 4 and/or 5 positions of the imidazolium ring.  Research in such ILs requires characterization of each species in terms of thermophysical properties associated with mathematical models.  Given the vast number of possible structures and potential compounds, computational methods are required to predict these properties and benchmark against experimental data.  Our work focused on modeling a number of new cation structures in COSMOTherm software and the actual synthesize of selected species as a means of validating predicted IL density.  Excellent agreement was observed between calculated and predicted densities, which were then correlated to the IL fractional free volume (FFV) as calculated by COSMOTherm.  Increased substitution of the imidazolium ring was observed to increase FFV more than isomeric 1-R-3-methylimidazolium cations.  As FFV has been previously shown to influence the solubility of CO₂ in ILs, these results suggest there are still very large computational and experimental spaces to be studied in the search to identify “optimal” ILs for gas separation applications.