Ion size effects on the dynamic and static dielectric properties of aqueous alkali solutions

Dielectric spectroscopy studies of aqueous ionic solutions ACl/H2O (A=Li, Rb, Cs) were carried out at frequencies between 45 MHz and 20 GHz, using recently developed coaxial measurement techniques. The behavior of the static dielectric constant εs0 and the dielectric relaxation time τD of the solutions were studied as functions of ion size and concentration. For moderate concentrations both εs0 and τD decrease linearly with solution conductivity. While the behavior of εs0 can be understood in terms of either static or Hubbard–Onsager kinetic polarization models, the experimental results for τD are at present not understood quantitatively in terms of these models. However we point out the good correlation of the τD data with empirical viscosity results, which suggests an alternative explanation based upon the solution viscosity, modified by ion size effects, which play an important role in the dielectric response. We also discuss the various length scales relevant to dielectric and conductivity processes in the solutions.