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

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.