Potassium Hydroxide Concentration-Dependent Water Structure on the Quartz Surface Studied by Combining Sum-Frequency Generation (SFG) Spectroscopy and Molecular Simulations

Abstract

Vibrational sum-frequency generation (SFG) spectroscopy and molecular simulations were used to investigate the molecular structures at the quartz surface, and the influence of bulk potassium hydroxide concentration was systematically examined. It was found that when the potassium hydroxide concentration was less than 10 −2 M, the structure of water molecules at the quartz surface was dependent on the quartz surface potential as evidenced by the increase of SFG signal as a function of the alkaline concentration. However, when the alkaline concentration was more than 10 −2 M, a monotonic decrease of interfacial water SFG spectra intensity was observed, which has been proposed to be due to the decreased number of interfacial water molecules and proton disordering caused by the screening effect originated from the adsorption of cations. Furthermore, besides the typical hydrogen-bonded interfacial water peaks (3200 and 3400 cm −1 ), the quartz/H 2 O interface showed an additional red-shifted peak centered at ∼2930 cm −1 . The results of SFG spectra and chemistry calculations confirmed that the red-shifted vibrational peak was due to the O−H stretch vibration of water molecules strongly hydrogen bonded with the OH − adsorbed at the surface.

Structure of the Silica/Divalent Electrolyte Interface: Molecular Insight into Charge Inversion with Increasing pH

“…Given its relatively low frequency, we suggest it arises from the OH stretch of the first solvation shell water molecules strongly hydrogen bonded to the OHion in basic solutions, as previously proposed. [62][63][64][65] It is also consistent with a recent SF study at the silica/aqueous interface with varying KOH(aq) concentrations, where a distinct shoulder at 3000 cm -1 was attributed to the strong hydrogen bonding interaction between the surface water molecules and OH -; 66 however the contributions from intra-and intermolecular coupling of this mode were not determined. 26,[67][68][69][70] As mentioned in the introductory statements, the mechanism of charge reversal on silica and other mineral surfaces by divalent ions has been extensively discussed.…”

Investigation of the Atmospheric Moisture Effect on the Molecular Behavior of an Isocyanate-Based Primer Surface

“…Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a suitable tool to understand the change of molecular structures of surfaces and interfaces. − SFG enables precisely measuring the molecular structure/behavior of surfaces/interfaces by its surface/interface-sensitive nature. − SFG has been extensively applied to study the surface and interfacial molecular structures of polymers, including various polymer adhesives. − ,− In this study, the atmospheric moisture effect on the surfaces of isocyanate-based primers was probed by SFG, and the molecular behaviors of such surfaces were correlated to the macroscopic properties to investigate the relationship. Different conditions in the experiments were set up in a variation of humidity levels and exposure time to understand the influences of their effects on the primer surfaces.…”

Applications of sum-frequency generation vibrational spectroscopy in friction interface

“…SFG vibrational spectroscopy is an effective technique for investigating buried interfaces. This technique has been used in numerous studies pertaining to buried interfaces, including liquid-solid [39][40][41][42][43][44] and solid-solid interfaces [44][45][46][47][48][49][50]. Friction is a typical interface behavior.…”