Hydrogen bonding and molecular orientations across thin water films on sapphire

Jean-François Boily; Li Fu; Aashish Tuladhar; Zhou Lu; Benjamin A Legg; Zheming M Wang; Hongfei Wang

doi:10.1016/j.jcis.2019.08.028

Hydrogen bonding and molecular orientations across thin water films on sapphire

J Colloid Interface Sci. 2019 Nov 1:555:810-817. doi: 10.1016/j.jcis.2019.08.028. Epub 2019 Aug 9.

Authors

Jean-François Boily¹, Li Fu², Aashish Tuladhar², Zhou Lu³, Benjamin A Legg², Zheming M Wang², Hongfei Wang⁴

Affiliations

¹ Department of Chemistry, Umeå University, SE 907 87, Umeå, Sweden. Electronic address: jean-francois.boily@umu.se.
² Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
³ Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
⁴ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200438, China.

PMID: 31425917
DOI: 10.1016/j.jcis.2019.08.028

Abstract

Hypothesis: Water vapor binding to metal oxide surfaces produces thin water films with properties controlled by interactions with surface hydroxo sites. Hydrogen bonding populations vary across films and induce different molecular orientations than at the surface of liquid water. Identifying these differences can open possibilities for tailoring film-mediated catalytic reactions by choice of the supporting metal oxide substrate.

Experiments: The (0001) face of a single sapphire (α-Al₂O₃) sample exposed to water vapor and the surface of liquid water were probed by polarization dependent Sum Frequency Generation-Vibration Spectroscopy (SFG-VS). Molecular dynamics (MD) provided insight into the hydrogen bond populations and molecular orientations across films and liquid water.

Findings: SFG-VS revealed a submonolayer film on sapphire exposed to 43% relative humidity (R.H.), and a multilayer film at 78% R.H. Polarization dependent SFG-VS spectra showed that median tilt angles of free OH bonds on the top of films are at ∼43° from the normal of the (0001) face but at 38° on neat liquid water. These values align with MD simulations, which also show that up to 36% of all OH bonds on films are free. This offers new means for understanding how interfacial reactions on sapphire-supported water films could contrast with those involving liquid water.

Keywords: Atmosphere; Film; Hydration; Molecular dynamics; Sapphire; Spectroscopy; Sum frequency generation; Water.