Abstract
Aq. solns. of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concns. of c ⪅ 3 M by broadband dielec. relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centered at (approx.) 1, 8, and 18 GHz, for both sets of solns. The two faster modes were assigned to the cooperative relaxation of "slow" and bulk water ...
Abstract
Aq. solns. of sodium propanoate (NaOPr) and n-butanoate (NaOBu) have been studied at concns. of c ⪅ 3 M by broadband dielec. relaxation spectroscopy over the frequency range 0.2 ≤ ν/GHz ≤ 89 at 25 °C. Three relaxation modes were resolved, centered at (approx.) 1, 8, and 18 GHz, for both sets of solns. The two faster modes were assigned to the cooperative relaxation of "slow" and bulk water mols. Detailed anal. of the spectra indicated that both OPr- and OBu- were strongly hydrated, with ∼23 and ∼33 slow water mols. per anion, resp., at infinite diln. These effective hydration nos. include ∼6 water mols. 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 concn., which facilitated the sepn. 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 ests. of the assocn. const. could not be made because of its composite nature.
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