Effects of Carboxylate Anions on the Molecular Organization of H₂O as Probed by 1-Propanol

Abstract

We characterized the effects of carboxylate anions, formate (OFm^–), acetate (OAc^–), and propionate (OPr^–), on the molecular organization of liquid H₂O by the 1-propanol (1P) probing methodology. The latter thermodynamic methodology provides two indices: one pertaining to the hydration number, <i>n</i>_H, and the other being related to the net increase/decrease of the entropy–volume cross fluctuation of the system. The results indicated that OFm^– is a hydration center with <i>n</i>_H = 1.2 ± 0.5 and leaves the bulk H₂O away from the hydration shell unperturbed. We suggest that this single H₂O hydrates preferentially one of the O’s in the COO^– group, showing the hydration center character. The values of <i>n</i>_H for OAc^– and OPr^– were found to be 3.7 ± 0.8 and 9 ± 2, respectively, out of which one H₂O molecule is used for hydrating the COO^– side and the remaining 2.7 and 8 H₂O molecules hydrate the respective alkyl group. Hence, OPr^– is more hydrophobic than OAc^– in terms of the hydration number. However, both alkyl moieties seem to equally retard the hydrogen bond probability of bulk H₂O away from hydration shells around nonpolar sites, as much as the probing 1P does