A hydrogen-bonding structure in self-formed nanodroplets of water adsorbed on amorphous silica revealed via surface-selective vibrational spectroscopy

• Published in: Physical chemistry chemical physics : PCCP 2020-01, Vol.22 (46), p.27031-27036
• Main Authors: Urashima, Shu-hei, Uchida, Taku, Yui, Hiroharu
• Format: Article
• Language: English
• Subjects: Adsorbed water; Bonding strength; Coefficient of friction; Contact angle; Humidity; Hydrogen; Hydrogen bonding; Room temperature; Silicon dioxide; Spectrum analysis; Surface properties; Vibrational spectra; Water; Water chemistry; Wettability
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d0cp03207g

Water adsorption onto a material surface is known to change macroscopic surface properties such as wettability and friction coefficient. While the role of the adsorbed water has been discussed for a long time, the interfacial structure of the adsorbed water has not been fully recognized in many cases. In this study, the hydration structure of water adsorbed on a vapor/silica interface at room temperature was studied via heterodyne-detected vibrational sum-frequency generation spectroscopy. The vibrational spectra of the interfacial molecules obtained here were different from those estimated via conventional sum-frequency generation spectroscopy. Interestingly, our results suggest that, at low humidity, the adsorbed water on silica forms nanodroplets instead of a uniform film. Because no silanol group was found to be hydrogen-bonding free, it was concluded that water molecules gather around the silanol group to form strongly hydrogen-bonded droplets. At high humidity, while the adsorbed water partially behaves like a bulk liquid, deprotonation of the silanol was not observed, unlike the case of silica surfaces in contact with bulk liquid water.