Mean distance of closest approach of ions: Lithium salts in aqueous solutions

Abstract

Numerical values for the mean distance of closest approach of ions, “a”, for lithium salts in aqueous solutions are presented and discussed. These values were obtained from both experimental activity and diffusion coefficients, and estimated by using different theoretical approaches.

Introduction

The importance of lithium salts has long been recognized, and stems from their unique properties that allow them to be used in a wide range of applications. They have been indicated as ideal electrolytes for many electrochemical systems (batteries, fuel cells, double-layer capacitors, actuators, and dye-sensitised solar cells) and are becoming increasingly important in areas such as renewable electricity generation [1], [2], [3], [4]. They are also relevant to other areas, such as pharmacology, where they are used primarily in the treatment of bipolar disorder, but also find applications in reinforcing the efficacy of other antidepressant medication (lithium carbonate or alternatives such as lithium citrate (Li₃C₆H₅O₇), lithium sulfate (Li₂SO₄) and lithium aspartate and orotate) [5], [6], [7], [8]. Other examples of their application can be found in fine chemical and polymer synthesis (LiCl) and also as air purifiers (LiOH) [9], [10], [11].

New demands in science and technology for many of these applications require precise data concerning the fundamental thermodynamic and transport properties of ionic solutions [12], [13], [14], [15], [16], [17]. For the explanation of this data, and perhaps more important, to estimate them when no experimental information is available, it is necessary to know parameters such as the “mean distance of closest approach of ions, a” (when this parameter is expressed in angstroms, it is represented by å). While there is a clear need for accurate values of such thermodynamic data on electrolyte solutions, frequently this data is not available from the literature and must be calculated by means of either empirical expressions or theoretical models in which the parameter “a” is involved. This parameter, “a”, depends on the nature and concentration of the electrolyte, and also on the nature and concentration of other species present in the solution which participate in the formation of an ionic atmosphere. While there is no direct method for the measurement of the parameter “a”, it can be estimated by using finite-ion-size equations from measurements of the activity and diffusion coefficients in solutions.

Turq et al. [18], [19] developed a theory which illustrates simultaneously the different transport (e.g., diffusion) and equilibrium properties from very dilute solutions to high concentrations (1–2 mol dm^− 3) by managing only the diameter of the ions as an adjustable parameter. It is possible to draw several qualitative conclusions from the systematic comparison of the magnitude of the “a” parameter, calculated from those equations with the results of studies of unrelated properties, such as ionic mobilities, and with the results obtained from theoretical approaches. A variety of different techniques ranging from diffraction methods (X-rays, neutrons or electrons) to computer simulations (molecular dynamics or Monte Carlo methods) have been applied to this goal [20], [21]. The available results of the ionic radius, particularly in solutions, up to the end of 1986 have been collected by Marcus in a review paper [21]. However, despite the intense work, the data available on this area are still scarce.

The objective of this paper is to present values for this “a” parameter for lithium salts, estimated from the concurrence of experimental data and theoretical approaches, which will be useful, in our case, for pharmaceutical studies, but will have more general applications for all researchers working with solutions of electrolytes containing this cation.