Ljerka Ukrainczyk

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

Abstract 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.

Acidic dispersion of fused SiO2 particles

The acidic dispersion behavior of fused SiO 2 particles was investigated and compared to two fumed SiO 2 particles. In contrast, the fused SiO 2 particles have a larger particle size, broader size distribution, and lower surface area. Fluoride adsorption was used to study surface activity, and acid-base titration was used to study surface charge in 10 - 1 to 10 - 3 M NaCI solutions over the pH range of 2-7.5. Each of the three SiO 2 particles exhibited similar titration behavior, with the fused SiO 2 particles displaying a higher intrinsic pK a 2 value of 7.0 as compared to 6.8 and 6.1 for the two fumed SiO 2 particles. Rheological experiments designed to test for dispersion and agglomeration/ gellation at 3 and 6 wt% solids loading in 10 - 3 M NaCl solutions adjusted to pH 2, 4, and 6 showed the fused SiO 2 particles to be more stable in suspension, exhibiting lower viscosity results for all test conditions. Results show that the fused SiO 2 particles tested in this report display superior dispersion properties as compared to conventional fumed SiO 2 particles for slurry applications under acidic conditions.