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/H2O 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.