Abstract
The water bending mode provides a powerful probe of the microscopic structure of bulk aqueous systems because its frequency and spectral line shape are responsive to the intermolecular interactions. Furthermore, interpreting the bending mode response is straightforward, as the intramolecular vibrational coupling is absent. Nevertheless, bending mode has not been used for probing the interfacial water structure, as it has been yet argued that the signal is dominated by bulk effects. Here, through the sum-frequency generation measurement of the water bending mode at the water/air and water/charged lipid interfaces, we demonstrate that the bending mode signal is dominated not by the bulk but by the interface. Subsequently, we disentangle the hydrogen-bonding of water at the water/air interface using the bending mode frequency distribution and find distinct interfacial hydrogen-bonded structures, which can be directly related to the interfacial organization of water. The bending mode thus provides an excellent probe of aqueous interfacial structure.
Highlights
Hydrogen-bonds in the topmost water layer affect various phenomena such as surface premelting of ice,[1,2] the slipperiness of ice surfaces,[3,4] evaporation of water[5−7] or/and sublimation of ice,[8] the formation of aerosols,[9] and the anomalously high surface tension of water.[10]
Bending mode has not been used for probing the interfacial water structure, as it has been yet argued that the signal is dominated by bulk effects
Molecular-level insights into the structure of interfacial water have been obtained by using sum-frequency generation (SFG) spectroscopy
Summary
Hydrogen-bonds in the topmost water layer affect various phenomena such as surface premelting of ice,[1,2] the slipperiness of ice surfaces,[3,4] evaporation of water[5−7] or/and sublimation of ice,[8] the formation of aerosols,[9] and the anomalously high surface tension of water.[10]. Through the sum-frequency generation measurement of the water bending mode at the water/air and water/charged lipid interfaces, we demonstrate that the bending mode signal is dominated not by the bulk but by the interface.
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