Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
Nature. 2011 Jun 8;474(7350):192-5. doi: 10.1038/nature10173.
The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 ångströms (1 Å = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers. However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the ångström scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength. Here we report a combined experimental and theoretical study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the 'free OD' transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase.
气-液界面可能是最常见的液体界面。它覆盖了地球表面的 70%以上，强烈影响着大气、气溶胶和环境化学。气-液界面也因其作为一个模型系统而引起了广泛关注，该系统允许对理论进行严格的测试，其中一个基本问题是它有多薄。理论研究表明，惊人的短“愈合长度”约为 3 埃（1 Å = 0.1 nm），在顶层几个单分子层内恢复了水的体相性质。然而，由于难以在 Ångström 尺度上对液体表面进行深度剖析，直接的实验证据一直难以获得。水的大多数物理、化学和生物性质，如粘度、溶剂化、润湿性和疏水效应，都由其氢键网络决定。通过观察 OH 伸缩模式的线形状可以探测到氢键网络，其频率位移与氢键强度有关。在这里，我们使用表面选择性异频探测振动和频光谱学对气-液界面进行了组合实验和理论研究，该技术专注于仅在最顶层水层中发现的“自由 OD”跃迁。通过使用氘化水和同位素稀释来揭示振动耦合机制，我们发现自由 OD 伸缩仅受分子内耦合到同一分子上另一个 OD 基团伸缩的影响。另一个 OD 伸缩频率表明在表面上遇到的第一个氢键的强度；这是跨越界面的水分子的供氢键，我们发现其强度仅略弱于体相水氢键。从这种观察结果中，我们推断出从空气进入水相时，体相行为的出现非常迅速。
