Connection between Phase Diagram, Structure and Ion Transport in Liquid, Aqueous Electrolyte Solutions of Lithium Chloride

Abstract

The nature of structure and ion transport in liquid electrolyte solutions are still not fully understood over the whole concentration range. In this work, we have studied aqueous solutions of lithium chloride as a model salt due to its very high solubility in water and ample knowledge of its structure and physiochemical properties. We have analyzed the ionic conductivity (κ) vs. concentration (C) plots based on free volume approach and our recently developed equation: κ = AC exp[−BC] and conductivity vs. temperature plots based on Arrhenius equation. We find that the solutions show little variation in free volume with concentration, or even temperature, but a rapid increase in activation energy and pre-exponential factor. We relate the significant changes in conductivity to changes in structure and transport in the solutions and connect them to the binary LiCl/H2O phase diagram. We hypothesize that the changes are caused by a breakdown of the bulk water structure near the eutectic composition that causes a change in transport mechanism. We believe that this connection between solution structure, ion transport and phase diagram is common in most aqueous and non-aqueous electrolyte solutions and explains the origin of maximum in conductivity in isothermal conductivity vs. concentration plots.