Twisting water reveals hidden order across four molecular layers at air-water interface

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Researchers from the Department of Physical Chemistry at the Fritz Haber Institute and Freie Universität Berlin have revealed the arrangement of water molecules at the interface between liquid water and air. Their findings help to better understand interfacial chemistry, which is largely determined by the specific arrangement of the water molecules. Published in Science Advances, the study shows that one parameter in particular—one that has been neglected until now—is of fundamental importance: the water twist.

Why study the air-water interface?

Water is arguably the most important molecule on Earth. Interfaces of water play a critical role in numerous processes within physiology, at the ocean surface, and in the atmosphere. In these processes, it is primarily the incredibly thin region of water directly at the boundary that governs their behavior.

Crucially, the sheer presence of the interface perturbs the molecular structure of water, generating preferential orientations and an altered hydrogen-bond network, which give rise to profoundly different properties of water in that region. While these unique structures are at the heart of many interfacial phenomena, characterizing them is monumentally difficult.

The challenge: Obtaining structural details from the extremely thin interfacial water region

The interfacial water region is incredibly thin (~8 Angstroms), involving only about four layers of water molecules. Below this interfacial region, water adopts its bulk properties. To elucidate details about the molecular water structure in the interfacial region, researchers need to probe only these first four water layers and retrieve information on molecular orientations in each of them. However, performing such experiments has so far not been feasible, with the result that the exact molecular structure of interfacial water remained unknown despite decades of intensive research.

Depth-resolved vibrational spectroscopy in combination with simulations

The research team at the Fritz Haber Institute overcame this challenge through their recently developed depth-resolved vibrational spectroscopy that uses a combination of infrared and visible lasers to irradiate the water surface and excite nonlinear vibrations in the water molecules. This process generates two new laser beams at different visible frequencies, namely the sum- and difference-frequency signals.

Exploiting small differences in the phase and amplitude of these signals, the team managed to extract precise depth information and isolate the vibrational response from the interfacial water region. The resulting spectra were then combined with high level computer simulations performed by the team from the FU Berlin to obtain a clear picture of the orientations of water molecules within the interfacial region.

Thanks to this combined approach and their novel experimental technique, the researchers revealed that the water molecules in the first four layers possess a very well defined orientational structure with layer-to-layer alternating molecular tilt and twist angles. While the tilt angle is defined as the angle between the water dipole and the surface normal, the molecular twist angle describes a rotation about the dipole axis.

Based on these findings, the research team was able to show that the common structural analysis of interfacial water in terms of molecules pointing "up or down" is largely insufficient by underlining the importance of the previously ignored depth-dependent molecular twist distribution at the interface to air. This leads to a revised structural picture of interfacial water with important implications on our understanding of processes at aqueous interfaces.

Publication details

Alexander P. Fellows et al, The importance of layer-dependent molecular twisting for the structural anisotropy of interfacial water, Science Advances (2026). DOI: 10.1126/sciadv.adz5505

Journal information: Science Advances

Key concepts

Biological fluid dynamicsSurface & interfacial phenomenaInterfaces

Provided by Fritz Haber Institute