Just how anisotropic is the air-water interface? Using depth- resolved SFG/DFG spectroscopy to determine its structure and thickness Alexander P. Fellows 1 , Alvaro Diaz-Duque 1 , Louis Lehmann 2 , Roland Netz 2 , Martin Wolf 1 and Martin Thämer 1 1 Fritz Haber Institut der Max Planck Gesellschaft, Deutschland, 2 Freie Universität Berlin, Deutschland Liquid water possesses a well-studied three-dimensional bulk structure that is governed by its highly dynamic hydrogen bond network. Close to an interface this structure is greatly perturbed leading to the formation of an anisotropic interfacial layer with modified chemical and physical properties. These specialized properties of interfacial water govern various important processes in nature. Examples are many cellular functions in biology which are governed by the water properties at the interface to cell membranes as well as atmospheric aerosol chemistry where the properties of the air-water interface are considered crucial for the uptake mechanisms of gases as well as the kinetics of gas-phase reactions. In all of these examples not only is the specific characteristics of the first interfacial layer an essential parameter, but also the length-scale to which the structural perturbation extends towards the bulk. Despite the importance and an enormous amount of research our current understanding of interfacial water on a molecular level is still rather limited and often solely relies on results from molecular dynamics simulations. This is true even for the “simple” case of the pure air—water interface where important questions such as the thickness of the perturbed water layer and the depth dependent structural evolution remain largely unanswered. Recent simulations have suggested a surprisingly small thickness of the interfacial water layer on the sub-nanometer level; however, direct experimental evidence remains elusive. The work presented here shows a molecular level investigation of the air-water interface combining molecular dynamics simulations with experimental observations. The study focusses on revealing the depth dependent evolution of the water structure using the recently developed technique of depth resolved nonlinear vibrational spectroscopy (SFG/DFG method 1 ). By performing isotopic exchange experiments the obtained second-order spectra are decomposed into their resonant and non-resonant contributions which allows a direct comparison between experiment and results from simulations yielding a comprehensive molecular picture of the interfacial region. Based on these experiments the length-scale of the interfacial anisotropy decay is also determined providing the first direct experimental evidence of the thickness of the interfacial water layer at the boundary to air. References 1. J. Phys. Chem. C2022, 126, 26, 10818–10832
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