Dielectric Spectroscopy of Aqueous Solutions of KCl and CsCl

Results of a dielectric relaxation study of the aqueous solutions of KCl and CsCl at 25 °C are reported using
measurements with a vector network analyzer (0.2 ≤ ν/GHz ≤ 20) and with waveguide interferometers (27
≤ ν/GHz ≤ 89). Similar to previously reported data for NaCl, the spectra of both salts are well fitted by a
Cole−Cole equation, although a double-Debye model is competitive at intermediate CsCl concentrations.
The impact of the solute on the water structure, reflected in a decrease in the bulk-water relaxation time with
rising electrolyte concentration, decreases in the sequence NaCl > KCl > CsCl and appears to be proportional
to the surface-charge density of the cation. The interactions of K+ and Cs+ with their hydration shells are too
weak to cause irrotational bonding of H2O molecules. It is argued that CsCl exhibits weak ion pairing, although
the data are not sufficiently accurate to determine the extent of association and the nature of the ion pair
formed.