Hydration of Formate and Acetate Ions by Dielectric Relaxation Spectroscopy

Dielectric relaxation (DR) spectra have been measured for aqueous solutions of sodium formate (NaOFm) and sodium acetate (NaOAc) over a wide range of frequencies (0.2 <= v/GHz <= 89) up to solute concentrations c less than or similar to 3.2 M and less than or similar to 3.7 M, respectively, at 25 degrees C. Measurements were also made on NaOAc(aq) at 15 <= T/degrees C <= 35. In addition to the usual dominant bulk-water relaxation process at similar to 20 GHz, one or two further relaxation modes were detected. One process, centered at similar to 8 GHz and observed for both NaOFm(aq) and NaOAc(aq), was attributed to the presence of slow water in the hydration shells of the anions. A lower-frequency process at similar to 0.6 GHz, observed only for NaOAc(aq) at c less than or similar to 1 M, was thought to be due to the presence of very small concentrations of ion pairs. Detailed analysis of the spectra indicated that very few (<2 per anion) water molecules were irrotationally bound (frozen) on the DR time scale. Nevertheless, both anions are strongly hydrated, as evidenced by the significant amounts of slow water detected. Such H(2)O molecules with reduced dynamics result from two distinct effects. The first is the relatively strong hydrophilic interaction of water with the -COO(-) moiety, which is similar for the two anions and little affected by increasing solute concentration. The second (for OAc(-) only) is the hydrophobic hydration of the -CH(3) group, which is fragile, decreasing markedly with increasing solute concentration and temperature. The activation parameters for bulk-water relaxation in NaOAc(aq) indicated a breakdown of the bulk water structure at high solute concentrations.