Development of Quantitative Structure-Property Relations for Ionic Liquids with Refrigerant Gases

Samantha E. McLeese, Aaron M. Scurto, and Kyle V. Camarda. Department of Chemical and Petroleum Engineering, The University of Kansas, 1530 W. 15th St., 4132 Learned Hall, Lawrence, KS 66045

Ionic liquids (ILs) are currently being considered for novel uses in hydrofluorocarbon (refrigerant) gas separations and as solvents in absorption refrigeration systems. Due to the low to immeasurable volatility of ILs, these compounds may be well-suited for such applications, and could potentially be considered as environmentally-benign solvents. Their molecularly-tunable nature yields an extraordinary number of possible cation and anion combinations, the majority of which have never been synthesized. This project has developed quantitative structure-property relations (QSPRs) to allow prediction of such important properties of ionic liquids as solubility in a given refrigerant, diffusivity, viscosity, toxicity, molar volume, melting and decomposition temperatures, as well as many excess thermodynamic properties directly from structure. The electronic structure of ionic liquids is quantified using modified connectivity indices, which describe bonding environments, charge distribution, orbital hybridization and other interactions within and between ions, and between the ions and the refrigerant molecules present. These topological descriptors provide a unique set of values which identify a specific ionic liquid structurally. QSPR relations have been developed using linear regression between these descriptors and experimentally measured physical and chemical properties. Both the quality of fit and the predictive capability of these expressions will be presented. These relations provide a methodology for quick estimation of the most relevant properties of ionic liquids, and can be used within a computational molecular design framework to provide candidate structures for use as environmentally benign solvents with refrigerant gases.