Systematic variations of Ion hydration in aqueous alkali metal fluoride solutions

Aqueous solutions of three alkali metal fluorides—NaF, KF, and CsF—have been studied by dielectric relaxation spectroscopy (DRS) over the frequency range 0.2 ≲ ν/GHz ≤ 89 at 25 °C and at concentrations 0.05 ≲ c/mol L–1 ≲ 1. The combination of these measurements with analogous literature data for RbF(aq) and M′Cl(aq) (M′ = Li, Na, K, and Cs) made possible a systematic analysis of the hydration of F– and the alkali metal cations. Unlike the other halide ions, F– was found to have a well-defined hydration shell which contains ∼7 water molecules, consistent with the appearance of a “slow”-water mode at ∼10 GHz in the spectra. Limiting total effective hydration numbers (Zt0) for M′F(aq), obtained from the solvent-related modes, did not follow a simple sequence, varying in the order CsF K+ > Rb+ > Cs+. It was also found that Zs0(M′+), the number of moderately bound (“slow”) water molecules, varied in the same order: K+ > Rb+ > Cs+. However, the presence of ib water molecules in the hydration shells of Li+ and Na+ attenuates their further interaction with surrounding water molecules such that Zs0(M′+) ≈ 0 for both ions