(643g) The Effect of Cation Polarity on the Melting Points of Ionic Liquids: An Experimental and Computational Study

A number of molten salts cannot be classified as ionic liquids (ILs) because they are not liquids at or below 100 C. These materials may have interesting or desirable properties however because they remain solids at standard temperatures this is likely to severely restrict their usage in a given process. Lowering the melting points of these materials without significantly affecting their other properties, effectively extending their liquidus range, could facilitate their adoption in industrial processes.

Recently, it was shown that the addition of different substituents to a series of thermally-robust IL cations could have a very substantial effect on the melting point. It was concluded that electron withdrawing effects in these groups create polarity within the cations, which could align, lowering the repulsion in these systems and hence the liquid phase enthalpy, leading to a depression in the melting point. From these results, it remained unclear if this principle was system specific or if it could be reasonably extended to other IL classes.

In this talk we present a systematic study of single fluorine substitutions on the structure and thermodynamics of different homologous series of IL cations. We systematically vary the position of the fluorine substitution in perarylsulfonium, perarylphosphonium, imidazolium, and pyridinium cations paired with bistriflimide, effectively altering the electronic charge and dipole moment of these cations. These ILs were synthesized and their melting points measured along with their enthalpies and entropies of fusion. Furthermore, the crystal structures of the compounds were determined using X-ray crystallography and used to simulate these structures using molecular dynamics. Using two-state simulations, we are able to predict the melting points of these ionic liquids and connect the effect that induced polarity in the IL cation has on the overall melting points.