Hydration of Formate and Acetate Ions by Dielectric Relaxation Spectroscopy
H. M. A. Rahman, G. Hefter and R. Buchner* ,
J. Phys. Chem. B 116(2012) 314-323. DOI: 10.1021/jp207504d
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/GH ≤ 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 cj 1M, 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 H2O 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 -CH3 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.