Hydration of Formate and Acetate Ions by Dielectric Relaxation Spectroscopy

Abstract

Dielec. relaxation (DR) spectra have been measured for aq. solns. of sodium formate (NaOFm) and sodium acetate (NaOAc) over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) up to solute concns. c .ltorsim. 3.2 M and .ltorsim. 3.7 M, resp., at 25°. Measurements were also made on NaOAc(aq) at 15 ≤ T/°C ≤ 35. In addn. 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 obsd. 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, obsd. only for NaOAc(aq) at c .ltorsim. 1 M, was thought to be due to the presence of very small concns. of ion pairs. Detailed anal. of the spectra indicated that very few (<2 per anion) water mols. were irrotationally bound (frozen) on the DR time scale. Nevertheless, both anions are strongly hydrated, as evidenced by the significant amts. of slow water detected. Such H2O mols. 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 concn. The second (for OAc- only) is the hydrophobic hydration of the -CH3 group, which is fragile, decreasing markedly with increasing solute concn. and temp. The activation parameters for bulk-water relaxation in NaOAc(aq) indicated a breakdown of the bulk water structure at high solute concns.