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

Limiting ionic conductance (&#923;<SUB>0</SUB>) of rigid symmetrical unipositive ions in aqueous solution shows a strong temperature dependence. For example, &#923;<SUB>0</SUB> 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 (H<SUB>2</SUB>O) to heavy water (D<SUB>2</SUB>O). In addition, &#923;<SUB>0</SUB> shows a nonmonotonic size dependence with a skewed maximum near Cs<SUP>+</SUP>. 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 &#923;<SUB>0</SUB> 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 &#923;<SUB>0</SUB> of unipositive ions in molecular terms. The marked change in &#923;<SUB>0</SUB> as the solvent is changed from H<SUB>2</SUB>O to D<SUB>2</SUB>O 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