The unique molecular water structure at charged interfaces governs various important interfacial processes in electrochemistry, environmental chemistry, and biology. Despite its great relevance, very little is known about the interfacial water structures, particularly their evolution with depth, which is mainly due to the lack of appropriate experimental techniques.

In their latest work the Nonlinear Interfacial Spectroscopy group presents a novel experimental approach which combines structural sensitivity with depth resolution on the nanometer scale. Using this technique, which exploits the complementary information from phase resolved sum- and difference frequency generation signals, the authors are able to successfully isolate the spectroscopic signal from the first water layers in direct contact with the surface charges (bonded interfacial layer, BIL) and compare it to the signals from water in the layers below (diffuse layer, DL). The analysis shows a remarkable change in structural anisotropy at the transition from DL to BIL by 2 orders of magnitude while the spectral analysis reveals that the anisotropy in both regions is clearly dominated by an anisotropic orientational molecular distribution without any notable changes in the hydrogen-bond structure. These findings significantly refine our understanding of the anisotropic water structure at charged interfaces and showcase the large potential of the presented depth-resolved spectroscopic technique.