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 (Z_t⁰) for M'F(aq), obtained from the solvent-related modes, did not follow a simple sequence, varying in the order CsF < NaF < RbF < KF. However, it is shown that this anomalous sequence results from subtle variations in the strength of the M'⁺-OH₂ bonding. Thus, it was established that Z_ib⁰(M'⁺) values, corresponding to the numbers of strongly ("irrotationally") bound (ib) water molecules around the cations, do vary with charge density in the order Li⁺ ≫ Na⁺ > K⁺ > Rb⁺ > Cs⁺. It was also found that Z_s⁰(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 Z_s⁰(M'⁺) ≈ 0 for both ions.