Nonlinear Optical Studies of Water at Gypsum–Air and Gypsum–Aqueous Interfaces

• Published in: Journal of physical chemistry. C Vol. 128; no. 5; pp. 2076 - 2085
• Main Authors: Yiyen, Galip, Sobolewski, Tess N., Walker, Robert A.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 08.02.2024
• Subjects: C: Physical Properties of Materials and Interfaces
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.3c04844

Vibrational sum frequency generation (VSFG), Fourier transform infrared (FTIR) absorbance, and Raman scattering were used to investigate the water structure at the gypsum (CaSO4·2H2O)–air and gypsum–water interfaces under ambient thermal and atmospheric conditions. Results show that water structurally embedded in the gypsum matrix consists of two different populations. One population has decoupled −OH bonds with one bond oriented in-plane, while the second water population has a more traditional, normal mode structure with the molecular C 2 symmetry axis aligned along the surface normal. Based on previously reported molecular dynamics simulations, we propose that surface water molecules having the decoupled −OH bonds (population 1) result from strong hydrogen bond donation to gypsum’s sulfate oxygens, whereas water molecules in the second population sample a more symmetric environment that shifts water’s vibrational frequencies to ∼3400 cm–1. Gypsum samples in contact with bulk water show significantly diminished VSFG signals with no observable new features. This result is attributed to loss of VSFG response due to the signal transmission into the bulk water and to an absence of surface-induced structure in the adjacent aqueous phase, consistent with a mineral surface near its point of zero charge. Removal of the bulk aqueous phase results in VSFG spectra that are very similar to those measured prior to water immersion, even when the aqueous phase consisted of D2O although relative intensities of individual bands change following exposure to D2O. These results imply that some fraction of gypsum’s surface structural water remains associated with the substrate and does not readily exchange with an adjacent phase. Furthermore, these findings provide direct molecular-level insight into previous force microscopy studies that proposed the existence of tightly bound surface waters at the gypsum/aqueous interface.