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

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, n(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 n(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 n(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.