Ionic conductivity in aqueous electrolyte solutions: Insights from computer simulations

Abstract

There exist a large amount of data on conductivity of various ions in aqueous and non-aqueous solvents as a function of ionic radius, pressure, temperature, ionic concentration, etc. Because of its importance in chemistry, biology, electrochemistry and a number of other areas, a consistent picture of how the various factors influence the conductivity is essential. We present detailed molecular dynamics simulations of ions in water in order to understand the influence of three factors: (i) ionic radius (ii) temperature and (iii) pressure. We propose an alternative explanation for the variation of ionic conductivity as a function of ionic radius for ions in water. The explanation is based on the Levitation Effect which arises from mutual cancellation of forces on the diffusant or ions when the ions are of the same radius as the bottleneck present in the solvent. This theoretical framework not only accounts for the variation of conductivity with ionic radius but also its variation with pressure. Experimental data on variation of ionic conductivity shows contrasting dependence on pressure for ions of different radius. We show that the proposed theoretical framework also accounts for the variation of ionic conductivity as a function of pressure for ions of different radius. Thus, we have been able to propose a theoretical framework which unifies our understanding of three different factors, namely, ionic radius, temperature and pressure.