Probing the Molecular Structure and Bonding of the Surface of Aqueous Salt Solutions

Abstract

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.

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

“…24,64,74,99,100,101,102,103,104 We suggest that detailed polarization and experimental configuration analysis of the SFG vibrational spectra be applied to these interfaces.…”

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…107 Vibrational sum frequency spectroscopy of aqueous salt solutions show changes in the interfacial water by the addition of alkali halide salts, although the interpretation in terms of enhancement of interfacial ion concentrations is not as direct. [108][109][110] In light of the MD simulations, surface reactions of Br À and I À would be expected to be even more important than in the case of Cl À . A significant complication in probing these reactions is that the increased reactivity of bromide and iodide ions in bulk solution in many cases overshadows the interface chemistry.…”

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…1 However, a combination of computational [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] and experimental studies [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] has shown that ions can reside and even be enhanced at the interface.…”