The importance of layer-dependent molecular twisting for the structural anisotropy of interfacial water

The unique structural properties of interfacial water are at the heart of many important processes in electrochemistry, climate science, and biophysics. At interfaces, water molecules exhibit preferential orientations and an altered intermolecular H-bond connectivity. Characterizing this layer-dependent anisotropic structure for such a thin molecular boundary, however, is a veritable challenge, with many important details remaining unknown. Here, we combine a novel depth-resolved second-order spectroscopy with molecular dynamics simulations to study the anisotropic structure at the air-water interface through the H─O─H bending vibration. We first uncover the elusive anisotropic interfacial response by removing the bulk-like (quadrupolar) term that is found to dominate the spectrum and has hampered previous experimental investigations of the interfacial structure. Thereafter, we reveal that the molecular structure at the interface shows a pronounced layering of alternating tilt-twist motifs. This highlights the often-disregarded anisotropy in the molecular twist angle and offers a revised picture of aqueous interfaces.