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

Abstract

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.

Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces

“…4,8−28 In particular, the Geiger group demonstrated that measurement of the χ total (2) phase in an off-resonance SHG experiment enables the surface potential and χ SL (2) to be determined. [10][11][12][13][14]29,30 Another major contribution to the field has been the development of second harmonic scattering (SHS) experiments and the accompanying theory by the Roke group to obtain the surface potential and χ SL (2) for colloidal nanoparticles in solution. 8,31−36 The Wei group developed a scheme based on off-resonance SHG where scanning the wavelength was an effective means of varying the coherence length if the nonlinear susceptibility remains constant, thereby facilitating the separation of χ SL (2) and χ DL (2) and extracting the surface potential.…”

“…Eisenthal’s group first demonstrated that second harmonic generation (SHG) offers the ability to tease apart the Stern layer (SL) and diffuse layer (DL) contributions to the electrical double layer (EDL) through a decomposition of the nonlinear susceptibility into χ total (2) = χ SL (2) +χ DL (2) and, in doing so, provides access to the interfacial potential . The theory behind this analysis has been further developed and refined by several groups in recent years. ,− In particular, the Geiger group demonstrated that measurement of the χ total (2) phase in an off-resonance SHG experiment enables the surface potential and χ SL (2) to be determined. − ,, Another major contribution to the field has been the development of second harmonic scattering (SHS) experiments and the accompanying theory by the Roke group to obtain the surface potential and χ SL (2) for colloidal nanoparticles in solution. ,− The Wen group developed a scheme based on off-resonance SHG where scanning the wavelength was an effective means of varying the coherence length if the nonlinear susceptibility remains constant, thereby facilitating the separation of χ SL (2) and χ DL (2) and extracting the surface potential . However, there is a selectivity advantage to probing the water response on resonance with a water-specific vibrational mode as in SFG experiments. , Hence, a two-angle SFG method has been proposed to separate the two EDL components and was recently experimentally realized .…”

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics

“…We recently advanced nonresonant second-order nonlinear spectroscopy to probe specific ion interactions at fused silica: aqueous interfaces via the SHG phase and amplitude. − This approach avoids the strong absorber problem of liquid water and provides the total interfacial potential as well as the second-order nonlinear susceptibility across interfaces in contact with ions of various chemical identities. We now employ this method to quantify the adsorbed ion density for Al 3+ , Mg 2+ , and Na + cations at fused silica surfaces maintained at pH 4, and during exposure to varying concentrations of the metal cation chlorides.…”