Ionic Liquids:  Ion Mobilities, Glass Temperatures, and Fragilities

We combine old, unpublished data on ionic liquids containing quaternary ammonium cations with new data on salts of aromatic cations containing a variety of anions, to demonstrate the existence for ionic liquids of an unexpectedly wide range of liquid fragilities. The pattern is one now familiar for other liquids. Here, the pattern is important in determining the relative fluid properties at ambient temperatures. We find that the optimization of ionic liquids for ambient temperature applications requiring low-vapor-pressure fluid phases involves the proper interplay of both cohesive energy and fragility factors. The cohesive energy is discussed in terms of the coulomb and van der Waals contributions to the…