Surface chemistry of SiO2 and TiO2–SiO2 glasses as determined by titration of soot particles

Surface charge is a critical issue for various types of glass processing including surface finishing (polishing) and cleaning. Unlike monomeric silicic acid with a pKa of 9.8, glass surfaces can have variable pKa values as determined by extent of Si–O–Si bonding, composition, and structure. The purpose of this study was to determine the surface charge, point of zero charge, and pKas for high purity fused SiO2 and TiO2–SiO2 (∼7 wt% TiO2) glasses. To achieve necessary surface areas for titration, colloidal soot particles were used in this study rather than glass monolith. Soot particles have the same inherent physical and chemical properties as the product glasses that are made by the same chemical vapor flame hydrolysis deposition process. Titration experiments performed in 10−1–10−5 M NaCl solutions revealed dissociation constants (i.e., intrinsic pKa1 and pKa2 values) of 0.0±0.2 and 7.0±0.1, respectively for the fused SiO2 particles, and 0.0±0.1 and 5.0±0.2, respectively for the TiO2–SiO2 particles. Points of zero charge for each material were calculated as 3.5±0.1 and 2.5±0.1 for the fused SiO2 and TiO2–SiO2 particles, respectively. The role of TiO2 in lowering point of zero charge and pKa2 values is hypothesized to be the result of four-fold coordination as compared to the six-fold coordinated TiO2 used in surface chemistry studies reviewed by this paper.