Can the Transport Properties of Molten Salts and Ionic Liquids Be Used To Determine Ion Association?

Abstract

There have long been arguments supporting the concept of ion association in molten salts and ionic liquids, largely based on differences between the conductivity and that predicted from self-diffusion coefficients by the Nernst-Einstein equation for noninteracting ions. It is known from molecular dynamics simulations that even simple models based on charged hard spheres show such a difference due to the (anti)-correlation of ion motions. Formally this is expressed as a difference between the velocity cross-correlation coefficient of the oppositely charged ions and the mean of those for the two like-charged ions. This article examines molten salt and ionic liquid transport property data, comparing simple and model associated salts (ZnCl, PbCl, and TlCl) including weakly dissociated molecular liquids (HO, HCOOH, HSO). Analysis employing Laity resistance coefficients (r) shows that the common ion-association rationalization is flawed, consistent with recent direct measurements of the degree of ionicity in ionic liquid chlorides and with theoretical studies. However, the protic ionic liquids [PyrOMe][BF] and [DBUH][CHSO] have larger than usual NE deviation parameters (>0.5), and large negative like-ion r, analogous to those of ZnCl. Structural, spectroscopic, and theoretical studies are suggested to determine whether these are indeed genuine examples of association.

On the temperature dependence of the double layer capacitance of ionic liquids

“…25 and 26 suggested to interpret RTILs as dilute electrolytes [49,50], assuming that most of the ions are ion paired. Theoretical [51][52][53][54], computational [52,[55][56][57][58][59] and experimental [26,27,60] papers have found that the extent of ion pairing in RTILs is not to the extent that was suggested in Refs. 25 and 26 (a detailed review of this work can be found in Refs.…”

“…61 and 62). It is almost widely accepted now that the extent of ion pairing suggested in that highly intriguing work [25] cannot be literally taken for RTILs, owing to a number of other physical properties not agreeing with that conclusion [3,53,56,63]. In fact, as summarised in Ref.…”

Dynamic Ion Correlations in Solid and Liquid Electrolytes: How Do They Affect Charge and Mass Transport?

“…In this context, it is worth mentioning that Harris and coworkers calculated correlations functions of the ion dynamics in ionic liquid and molten salts from experimental diffusion and conductivity data. The underlying equations were based on the assumption of strict ion momentum conservation …”

Free and Bound States of Ions in Ionic Liquids, Conductivity, and Underscreening Paradox

“…The deviations from it lead to smaller conductivities attributed to correlations and ion pairing [50]. This interpretation has been debated, with some authors suggesting that the concept of ion pairing and clustering in RTILs is not required [51,52] but some supporting it [53]. There have been investigations into the properties of ion pairs in RTILs [54], but we are not aware of any reports estimating the fraction of ions in a state that contribute to conductivity.…”