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

Abstract

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.

Physicochemical and structural properties of lidocaine-based ionic liquids with anti-inflammatory anions

“…17 The rst group lies quite close to the KCl line (0.1 < DW < 0.5) suggesting that the ionic liquid is made up of almost independently mobile ions and can be classied as good ionic liquids. [17][18][19] The second groups lies lower on the plot (0.5 < DW < 1.0) and is named "poor" ionic liquids. In this group existence of H-bonds and other specic interaction between ions in the pure state is more pronounced, leading to a signicant reduction of ions mobility.…”

“…The third group represents the liquids that are at least an order of magnitude below the ideal line, also described as liquid ion pairs or subionic liquids (DW > 1.0). [17][18][19] In these liquids, ion conductivity is substantially less than would be expected based on their viscosity. The simplest example is the ion pair, which is neutral and therefore, does not contribute at all to the measured conductivity.…”

Electrolytes for Lithium (Sodium) Batteries Based on Ionic Liquids: Highlighting the Key Role Played by the Anion

“…For all these examples, the Nernst‐Einstein parameter Δ is less than 0.5. On the other hand, in the few cases where association does occur, Δ lies between 0.5 and 1 …”

“…that is, the cation‐anion resistance coefficient is the geometric mean of the cation and anion resistance coefficients, then Δ is zero . Δ approaches 0.5 as r cc and r aa approach zero (from positive values) . The self‐diffusion coefficients are given by Equation : …”

“…This approach has been criticized on a number of grounds, primarily for its arbitrary nature, there being no clear reason why 1 mol/L aqueous KCl should be a model for molten salts . The method incorrectly predicts that a number of molten alkali halides are “superionic”, as their Walden plots lie above the KCl reference line, when they have normal Δ values lying between 0 and 0.5 . Clearly this method can lead to misleading conclusions and its use should be discontinued.…”