Features of ion hydration and association in aqueous rubidium fluoride solutions at ambient conditions

Aqueous RbF solutions have been investigated at ambient conditions by dielectric relaxation spectroscopy (DRS) over the concentration range of 0 ≤ c/M ≤ 5 and by statistical mechanics (RISM integral equation theory) in the concentration range of 2.15-9.18 M. The behavior of the dielectric spectra and their characteristic parameters (dispersion amplitudes and relaxation times) is discussed, as well as the pair correlation functions and partial coordination numbers derived from statistical mechanics. The paper focuses on the influence of salt concentration on the features of ion hydration and association. The observed changes in the dielectric properties indicate a decrease of the effective hydration number from ∼ 15-18 at infinite dilution to ∼ 3.5 at 5 M and possibly weak association involving stable ion pairs at c ≤ 0.5 M. A major finding is that both ions, Rb+ and F−, slow down surrounding water molecules, leading to the emergence of a separate “slow water” relaxation in the spectra. According to the analysis of RISM equilibrium properties, an average number of ∼ 6 water molecules coordinates both Rb+ and F− at c = 2.15 M. This number decreases by ∼ 19% for the cation and by ∼ 10% for the anion as c growths up to 9.18 M. The RISM-data indicate a significant increase in the number of contact and solvent-separated ion pairs with increasing c. The possibility of ion clustering is discussed for concentrated RbF(aq) solutions.