The complex dielectric constant and relaxation in aqueous solutions of alkali-metal nitrates

The concentration dependences of the microwave dielectric properties of aqueous solutions of alkali-metal nitrates are studied in the frequency range 7-25 GHz (in several cases, 3.4-25 GHz) and the temperature range 283-313 K. The dielectric parameters of the solutions are determined: static dielectric constant epsilon(s), dielectric relaxation time tau, and the activation energy of dielectric relaxation. A decrease in the static dielectric constant in solutions of different compositions correlates with changes in the hydration interaction of ions: Li+>K+>Na+>Cs+. In the range of high concentrations, for lithium and sodium nitrates a negative temperature coefficient of epsilon(s) (which is characteristic of water and dilute salt solutions) disappears and it seems that a positive coefficient of epsilon(s), appears instead. This is associated with the absence of the structure of bulk water in the solutions (bulk water resembles net water in its properties) and with the development of ion-ion association. In going from water to solutions, the dielectric relaxation time decreases for all of the systems studied. Concentrational variations in dielectric relaxation times are less pronounced at elevated temperatures, where the initial water structure is already perturbed by thermal motion. The changes in dielectric relaxation time caused by the perturbation of the water structure in solutions with different ions are in fact undifferentiated. This is due to the perturbing effect of nitrate ions on the molecular-kinetic parameters of cation hydration shells, this effect being in general similar to the temperature effect.