Ion Pairing and Solvent Relaxation Processes in Aqueous Solutions of Sodium Malonate and Sodium Succinate

Abstract

Dielectric relaxation spectra have been measured for aqueous solutions of sodium malonate and sodium succinate over a wide range of frequencies (0.2 ≤ v/GHz ≤ 89) and solute concentrations (0.025 ≤ c/M ≤ 1.0). In addition to the usual processes associated with bulk water, two further relaxation processes were detected. The faster of these, with a relaxation time of ∼16 ps, is unusual and was attributed to the presence of slow water in the hydration shells of the anions. The extent of this hydration shell is rather fragile (concentration-dependent) for malonate but not for succinate. The second, slower, process was attributed to the presence of ion pairs NaX-(aq). Unambiguous determination of the structure of the ion pair was not possible because of the internal flexibility of the anions, but the data were most consistent with a chelated solvent-shared ion pair. The kinetics of formation and dissociation of the ion pairs were evaluated from the effects of solute concentration on their relaxation times.