Corresponding States Theory for the Freezing of Ionic Liquids

Abstract

Corresponding states analyses for understanding the freezing temperatures of existing ionic liquids, and predicting freezing properties of ionic liquids yet to be discovered are described. Room ionic liquids exist as such because they broadly obey a scaling relationship that describes all 1:1 electrolytes, including the alkali halides. In a zeroth-order treatment, the reduced freezing temperature (Tf*) is simply expressed in units of a characteristic ion-ion pair potential energy containing a single size or length parameter (r0): Tf* = kBTf r0/e2, where kB is Boltzmann's constant. All ionic liquids in the same conformal group have the same reduced freezing point (Tf*). Organic ionic liquids have r0 values roughly 3-times greater than those of alkali halides, which melt around 1000 K, hence their room temperature freezing points. Corresponding states analyses are reported for conformal groupings obtained for both the DME (distance of minimum energy) definition of r0 and also the scaling obtained from the sum of the isolated cation and anion polarizability trace radii (PTR). We discuss the inclusion of first-order effects of nonconformable perturbations, such as the polarizability anisotropy and ion-size assymmetry. Scaling concepts promise to be a valuable tool for predicting freezing points of ionic liquids.