Schaefer Jan, Gonella Grazia, Bonn Mischa, Backus Ellen H G
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Phys Chem Chem Phys. 2017 Jun 28;19(25):16875-16880. doi: 10.1039/c7cp02251d.
Surface-specific vibrational sum-frequency generation spectroscopy (V-SFG) is frequently used to obtain information about the molecular structure at charged interfaces. Here, we provide experimental evidence that not only screening of surface charges but also interference limits the extent to which V-SFG probes interfacial water at sub-mM salt concentrations. As a consequence, V-SFG yields information about the ∼single monolayer interfacial region not only at very high ionic strength, where the surface charge is effectively screened, but also for pure water due to the particularly large screening length at this low ionic strength. At these low ionic strengths, the large screening lengths cause destructive interference between contributions in the surface region. A recently proposed theoretical framework near-quantitatively describes our experimental findings by considering only interference and screening. However, a comparison between NaCl and LiCl reveals ion specific effects in the screening efficiency of different electrolytes. Independent of electrolyte, the hydrogen bonding strength of water right at the interface is enhanced at high electrolyte concentrations.
表面特异性振动和频产生光谱（V-SFG）常用于获取有关带电界面分子结构的信息。在此，我们提供实验证据表明，不仅表面电荷的屏蔽，而且干涉也限制了V-SFG在亚毫摩尔盐浓度下探测界面水的程度。因此，V-SFG不仅在非常高的离子强度下（此时表面电荷被有效屏蔽），而且在纯水中（由于在此低离子强度下特别大的屏蔽长度）都能产生有关约单分子层界面区域的信息。在这些低离子强度下，大的屏蔽长度会导致表面区域贡献之间的相消干涉。最近提出的一个理论框架通过仅考虑干涉和屏蔽，近乎定量地描述了我们的实验结果。然而，NaCl和LiCl之间的比较揭示了不同电解质在屏蔽效率方面的离子特异性效应。与电解质无关，在高电解质浓度下，界面处水的氢键强度会增强。
