Toward Understanding the Hofmeister Series. 3. Effects of Sodium Halides on the Molecular Organization of H₂O As Probed by 1-Propanol

We investigated the effects of NaF, NaCl, NaBr, and NaI on the molecular organization of H2O by a calorimetric methodology developed by us earlier. We use the third derivative quantities of G pertaining to 1-propanol (1P) in ternary 1P−a salt−H2O as a probe to elucidate the effects of a salt on H2O. We found that NaF and NaCl worked as hydration centers. The hydration numbers were 19 ± 2 for NaF and 7.5 ± 0.6 for NaCl. Furthermore, the bulk H2O away from the hydration shell was found unaffected by the presence of Na+, F-, and Cl-. For NaBr and NaI, in addition to the hydration to Na+, Br- and I- acted like a hydrophilic moiety such as urea. Namely, they formed a hydrogen bond to the existing H2O network and retarded the fluctuation nature of H2O. These findings were discussed with respect to the Hofmeister ranking. We suggested that more chaotropic anions Br- and I- are characterized as hydrophiles, whereas kosmotropes, F- and Cl-, are hydration centers.