Hydrophilic and Hydrophobic Hydration of Sodium Propanoate and Sodium Butanoate in Aqueous Solution

Aqueous solutions of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concentrations of c ≲ 3 M by broadband dielectric relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centered at (approximately) 1, 8, and 18 GHz, for both sets of solutions. The two faster modes were assigned to the cooperative relaxation of “slow” and bulk water molecules. Detailed analysis of the spectra indicated that both OPr^– and OBu^– were strongly hydrated, with ∼23 and ∼33 slow water molecules per anion, respectively, at infinite dilution. These effective hydration numbers include ∼6 water molecules hydrophilically bound to the carboxylate moiety, with the remainder arising from the hydrophobic hydration of the nonpolar alkyl chains. The latter shows a characteristic rapid decrease with increasing solute concentration, which facilitated the separation of the hydrophobic and hydrophilic contributions. The lowest frequency mode was a composite with contributions from ion-cloud, ion-pair, and anion relaxations. Although this low intensity mode provided specific evidence of weak ion pairing between Na⁺(aq) and the carboxylate anions, reliable estimates of the association constant could not be made because of its composite nature.