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

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 m 2 V -1 at pH 14), the number of Stern layer water molecules (1 × 10 15 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.