Limiting ionic conductance of symmetrical, rigid ions in aqueous solutions: temperature dependence and solvent isotope effects

Abstract

Limiting ionic conductance (Λ₀) of rigid symmetrical unipositive ions in aqueous solution shows a strong temperature dependence. For example, Λ₀ more than doubles when the temperature is increased from 283 to 318 K. A marked variation also occurs when the solvent is changed from ordinary water (H₂O) to heavy water (D₂O). In addition, Λ₀ shows a nonmonotonic size dependence with a skewed maximum near Cs⁺. Although these important results have been known for a long time, no satisfactory theoretical explanation exists for these results. In this article we present a simple molecular theory which provides a nearly quantitative explanation in terms of microscopic structure and dynamics of the solvent. A notable feature of this theory is that it does not invoke any nonquantifiable models involving solvent-berg or clatherates. We find the strong temperature dependence of Λ₀ to arise from a rather large number of microscopic factors, each providing a small but nontrivial contribution, but all acting surprisingly in the same direction. This work, we believe, provides, for the first time, a satisfactory explanation of both the anomalous size and temperature dependencies of Λ₀ of unipositive ions in molecular terms. The marked change in Λ₀ as the solvent is changed from H₂O to D₂O is found to arise partly from a change in the dielectric relaxation and partly from a change in the effective interaction of the ion with the solvent.