Reorientation of the "free OH" group in the top-most layer of air/water interface of sodium fluoride aqueous solution probed with sum-frequency generation vibrational spectroscopy.

Many experimental and theoretical studies have established the specific anion, as well as cation, effects on the hydrogen-bond structures at the air/water interface of electrolyte solutions. However, the ion effects on the top-most layer of the air/water interface, which is signified by the non-hydrogen-bonded so-called "free OH" group, have not been explicitly discussed or studied. In this report, we present the measurement of changes of the orientational angle of the "free OH" group at the air/water interface of the sodium fluoride (NaF) solutions at different concentrations using the interface selective sum-frequency generation vibrational spectroscopy (SFG-VS) in the ssp and ppp polarizations. The polarization dependent SFG-VS results show that the average tilt angle of the "free OH" changes from about 36.6° ± 0.5° to 44.1° ± 0.6° as the NaF concentration increases from 0 to 0.94 M (nearly saturated). Such tilt angle change is around the axis of the other O-H group of the same water molecule at the top-most layer at the air/water interface that is hydrogen-bonded to the water molecules below the top-most layer. These results provide quantitative molecular details of the ion effects of the NaF salt on the structure of the water molecules at the top-most layer of the air/water interface, even though both the Na(+) cation and the F(-) anion are believed to be among the most excluded ions from the air/water interface.