Feng Ran-Ran, Guo Yuan, Wang Hong-Fei
International Center for Quantum Materials, Peking University, Beijing 100871, People's Republic of China.
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China.
J Chem Phys. 2014 Nov 14;141(18):18C507. doi: 10.1063/1.4895561.
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.
许多实验和理论研究已经证实了特定阴离子以及阳离子对电解质溶液气/水界面处氢键结构的影响。然而，离子对气/水界面最顶层的影响，即以非氢键结合的所谓“游离OH”基团为标志的影响，尚未得到明确的讨论或研究。在本报告中，我们使用界面选择性和频振动光谱（SFG-VS）在ssp和ppp偏振模式下，测量了不同浓度氟化钠（NaF）溶液气/水界面处“游离OH”基团取向角的变化。偏振相关的SFG-VS结果表明，随着NaF浓度从0增加到0.94 M（接近饱和），“游离OH”的平均倾斜角从约36.6°±0.5°变为44.1°±0.6°。这种倾斜角变化是围绕气/水界面最顶层同一水分子的另一个O-H基团的轴进行的，该O-H基团与最顶层下方的水分子形成氢键。这些结果提供了NaF盐的离子效应在气/水界面最顶层水分子结构方面的定量分子细节，尽管Na(+)阳离子和F(-)阴离子都被认为是最容易从气/水界面被排斥的离子之一。
