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

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• 作者：Kenneth R. Harris
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：December 1, 2016
• 年：2016
• 卷：120
• 期：47
• 页码：12135-12147
• 全文大小：692K
• ISSN：1520-5207

文摘

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 (ZnCl₂, PbCl₂, and TlCl) including weakly dissociated molecular liquids (H₂O, HCOOH, H₂SO₄). Analysis employing Laity resistance coefficients (r_ij) 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][BF₄] and [DBUH][CH₃SO₃] have larger than usual NE deviation parameters (>0.5), and large negative like-ion r_ii, analogous to those of ZnCl₂. Structural, spectroscopic, and theoretical studies are suggested to determine whether these are indeed genuine examples of association.