pH-Dependent Behavior of Quartz Surfaces: Insights from XPS, DRIFT, and Potentiometric Titrations

Quartz is among the most abundant minerals on Earth, but its surface chemistry under varying pH conditions remains not fully understood. In particular, the interplay between pH, amphoteric behavior, and water adsorption properties has been the subject of a long-standing debate. This study presents a comprehensive, multitechnique investigation into the pH-dependent interfacial chemistry of quartz. The findings redefine the acid-base properties of quartz surfaces. Potentiometric titrations revealed two distinct reactive silanol groups with pK_a values of 5.7 and 7.6. Diffuse reflectance infrared Fourier transform spectroscopy reveals a dynamic hydrogen-bonding network at the quartz-water interface, where the speciation of water molecules shifts with pH, thereby modulating adsorption properties. X-ray photoelectron spectroscopy analysis identifies three distinct oxygen species:neutral SiOH⁰, negatively charged SiO^-, and adsorbed water; whose relative surface densities evolve systematically with pH, correlating with changes in surface electronegativity and charge distribution. This study clarifies long-standing ambiguities about quartz surface ionization by identifying the pH-dependent distribution of surface species and revealing how their speciation controls water adsorption environments and interfacial charge. These findings have broad implications across multiple disciplines, as this molecular level understanding enables the optimization of surface interactions in aqueous environments, from improving mineral processing and catalysis to functional surface engineering.