Findings by researchers at the RIKEN Advanced Science Institute and their colleagues at Tohoku University and in the Netherlands have resolved a long-standing debate over the structure of water molecules at the water surface. Published in the Journal of The American Chemical Society, the research combines theoretical and experimental techniques to pinpoint, for the first time, the origin of water’s unique surface properties in the interaction of water pairs at the air-water interface.
The most abundant compound on Earth’s surface, water is essential to life and has shaped the course of human civilization. As perhaps the most common liquid interface, the air-water interface offers insights into the surface properties of water in everything from atmospheric and environmental chemistry, to cellular biology, to regenerative medicine. Yet despite its ubiquity, the structure of this interface has remained shrouded in mystery.
At the heart of this mystery are two broad bands in the vibrational spectrum for surface water resembling those of bulk ice and liquid water. Whether these bands are the result of hydrogen bonds themselves, of intra-molecular coupling between hydrogen bonds within a single water molecule, or of inter-molecular coupling between adjacent water molecules, is a source of heated debate. One popular but controversial hypothesis suggests one of the spectral bands corresponds to water forming an actual tetrahedral “ice-like” structure at the surface, but this interpretation raises issues of its own.
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