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

Abstract

Aq. RbF solns. have been investigated at ambient conditions by dielec. relaxation spectroscopy (DRS) over the concn. range of 0 ≤ c/M ≤ 5 and by statistical mechanics (RISM integral equation theory) in the concn. range of 2.15-9.18 M. The behavior of the dielec. spectra and their characteristic parameters (dispersion amplitudes and relaxation times) is discussed, as well as the pair correlation functions and partial coordination nos. derived from statistical mechanics. The paper focuses on the influence of salt concn. on the features of ion hydration and assocn. The obsd. changes in the dielec. properties indicate a decrease of the effective hydration no. from ∼15-18 at infinite diln. to ∼3.5 at 5 M and possibly weak assocn. involving stable ion pairs at c ≤ 0.5 M. A major finding is that both ions, Rb+ and F-, slow down surrounding water mols., leading to the emergence of a sep. "slow water" relaxation in the spectra. According to the anal. of RISM equil. properties, an av. no. of ∼6 water mols. coordinates both Rb+ and F- at c = 2.15 M. This no. 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 no. of contact and solvent-sepd. ion pairs with increasing c. The possibility of ion clustering is discussed for concd. RbF(aq.) solns.