Quantification of Stern Layer Water Molecules, Total Potentials, and Energy Densities at Fused Silica:Water Interfaces for Adsorbed Alkali Chlorides, CTAB, PFOA, and PFAS

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 127; no. 40; pp. 8404 - 8414
• Main Authors: Chang, HanByul, Lozier, Emilie H., Ma, Emily, Geiger, Franz M.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 12.10.2023
• Subjects: A: Aerosols; Environmental and Atmospheric Chemistry; Astrochemistry; Chemistry; Physics
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.3c04434

We have employed amplitude- and phase-resolved second-harmonic generation spectroscopy to investigate ion-specific effects of monovalent cations at the fused silica:water interface maintained under acidic, neutral, and alkaline conditions. We find a negligible dependence of the total potential (as negative as −400 mV at pH 14), the second-order nonlinear susceptibility (as large as 1.5 × 10–21 m2 V–1 at pH 14), the number of Stern layer water molecules (1 × 1015 cm–2 at pH 5.8), and the energy associated with water alignment upon going from neutral to high pH (ca. −24 kJ mol–1 to −48 kJ mol–1 at pH 13 and 14, close to the cohesive energy of liquid water but smaller than that of ice) on chlorides of the alkali series (M+ = Li+, Na+, K+, Rb+, and Cs+). Attempts are presented to provide estimates for the molecular hyperpolarizability of the cations and anions in the Stern layer at high pH, which arrive at ca. 20-fold larger values for α total ions (2) = α M + (2) + α OH – (2) + α Cl – (2) when compared to water’s molecular hyperpolarizability estimate from theory and point to a sizable contribution of deprotonated silanol groups at high pH. In contrast to the alkali series, a pronounced dependence of the total potential and the second-order nonlinear susceptibility on monovalent cationic (cetrimonium bromide, CTAB) and anionic (perfluorooctanoic and perfluoro­octanesulfonic acid, PFOA and PFOS) surfactants was quantifiable. Our findings are consistent with a low surface coverage of the alkali cations and a high surface coverage of the surfactants. Moreover, they underscore the important contribution of Stern layer water molecules to the total potential and second-order nonlinear susceptibility. Finally, they demonstrate the applicability of heterodyne-detected second-harmonic generation spectroscopy for identifying perfluorinated acids at mineral:water interfaces.