On the Use of the Angell–Walden Equation To Determine the “Ionicity” of Molten Salts and Ionic Liquids

In this work, the Angell analysis of Walden plots of the conductivity of ionic liquids and other electrolytes against viscosity is used to examine simple molten salts at high temperatures, a test that does not appear to have been made previously. It is found that many simple salts such as alkali metal fluorides and chlorides are predicted to be "superionic" as their Walden plots fall above the arbitrary reference line introduced by Angell, which passes through the datum point for 1 M aqueous KCl at 25 °C. This contradicts long-standing molecular dynamics evidence in the literature showing that these salts conduct simply by ion migration in an electric field. Zinc chloride is also predicted to be "ideal", whereas one would expect it to be "subionic" in Angell's terminology given that it is an associated salt. Results for certain protic ionic liquids are also contradictory. Therefore, Angell-Walden analyses of this type do not convey any useful information other than a qualitative ranking of the conductivity of similar ionic liquids at a given viscosity and their use for estimating "ionicity" is best discontinued. It cannot and should not be used for classifying the interactions in ionic liquids. Instead, it is argued that an examination of Laity resistance coefficients is more useful in any discussion of true association in molten salts and ionic liquids where known examples show negative like-ion resistance coefficients with NE deviation parameters close to unity. Such an approach could be more fruitful in understanding the transport properties of molten salts and ionic liquids rather than simple comparisons of viscosity and conductivity.