Water is arguably the most important molecule on Earth – playing a critical role in processes within physiology, at the ocean surface, and in the atmosphere. As these processes involve interfaces between water and other media, however, it is primarily the incredibly thin layer of water directly at the boundary that governs their behaviour. Crucially, the sheer presence of the interface perturbs the molecular structure of water, generating preferential orientations and an altered H-bond network, which give rise to profoundly different interfacial properties. While these unique structures are at the heart of many interfacial phenomena, characterising them is monumentally difficult.

In their latest work published in Science Advances, the Nonlinear Interfacial Spectroscopy group in collaboration with the group of Roland Netz from FU Berlin combine their newly developed depth-resolved interfacial spectroscopy technique with high-level simulations to investigate the fine details of the layer-dependent water structure at the interface to air. Using this experimental method, which combines phase-resolved sum- and difference-frequency generation spectroscopy, the authors could isolate the purely interfacial water spectrum. The analysis of the depth-dependent sum-frequency signals then reveals the pronounced layered structure of interfacial water with alternating tilt-twist orientations. These findings show that the common structural analysis in terms of “pointing up or down” of water molecules is largely insufficient by underlining the importance of the depth-dependent molecular twist distribution at the interface to air providing a revised structural picture of interfacial water.