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 ≤ ν/GHz ≤ 89) up to solute concentrations c ≲ 3.2 M and ≲ 3.7 M, respectively, at 25 °C. Measurements were also made on NaOAc(aq) at 15 ≤ T/°C ≤ 35. In addition to the usual dominant bulk-water relaxation process at ∼20 GHz, one or two further relaxation modes were detected. One process, centered at ∼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 ∼0.6 GHz, observed only for NaOAc(aq) at c ≲ 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 (