Probing the molecular structure and bonding of the surface of aqueous salt solutions

The surfaces of aqueous solutions of NaF, NaCl, NaBr, and NaI have been examined using vibrational sum-frequency spectroscopy. Spectra of these salts in mixtures of HOD/H2O/D2O have been used to provide insight into how simple salts alter the hydrogen bonding structure of water in the surface region. As the anion is changed, the observed interfacial hydrogen bonding also changes, indicating the presence of anions in the interfacial region. The isotopic dilution experiments performed on each solution enable separation of the contributions arising from interfacial water with differing degrees of hydrogen bonding. Frequency shifts in the peaks attributed to tetrahedrally coordinated water molecules within the interfacial region display the structure-making characteristics of F- and the structure-breaking characteristics of Cl-, Br-, and I-. However, water molecules residing in the topmost surface layer show minimal perturbation by the presence of these anions as probed by the characteristics of the donor OH mode of water molecules that straddle the air/water interface. These results indicate a significantly diminished population of the anions at the uppermost layer of the surface region.