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We combine old, unpublished data on ionic liquids containing quaternary ammonium cations with new data on salts of aromatic cations containing a variety of anions, to demonstrate the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 attractive part of the pair potential. On the basis of the relation between the glass-transition temperature and the molar volume for salts with less-polarizable anions, we find evidence for a broad minimum in the ionic liquid cohesive energy at an internuclear separation of ca. 0.6 nm. This minimum lies between those of the BF4- and TFSI- anions for the small quaternary ammonium cations of this study. The minimum is expected to be narrower and less well-defined for salts with polarizable anions. The relation of fluidity to conductance is considered in terms of a Walden plot that is shown to provide a useful basis for organizing the observations on ionic liquids and solutions. Low vapor pressure and ideal Walden products are intimately related.