Evaluation of ATR−FTIR Spectroscopy as an “in Situ” Tool for Following the Hydrolysis and Condensation of Alkoxysilanes under Rich H₂O Conditions

The fate of ∼0.5 M tetraethyl orthosilicate (TEOS) in HNO 3 solutions (pH 2−3) was followed by “in situ” ATR−FTIR spectroscopy. ATR spectra with excellent signal-to-noise ratios were obtained with collection times of 2 min. A qualitative examination of spectra measured at different reaction times showed that TEOS was first converted into ethanol and Si(OH) 4, and then amorphous SiO 2 was precipitated from the solution. The TEOS hydrolysis rate increased with decreasing pH of the nitric solution. In addition, under reaction conditions that make the system homogeneous (stable micelles, solution, or stable suspension), the use of standards enabled measurement of concentrations of the reactant products: EtOH, Si(OH) 4, and SiO 2 . Calibration curves for EtOH in water and in methanol, for TEOS in ethanol, and for SiO 2 in water fit equations of the form y = bx, with R 2 values between 0.999 and 0.99. The mass balance on the acidified TEOS/water systems was good: initial concentrations of 0.56 F of TEOS resulted in [EtOH] = 2.12 M and [SiO 2 ] = 0.53 M.