Esterification, condensation, and deprotonation equilibria of trimethylsilanol

We investigate by ²⁹Si NMR the equilibrium behavior of trimethylsilanol in both acidic and alkaline ethanol/water solutions. This system is of interest not only for passivation (silylation) of organic and inorganic hydroxide-containing compounds, but also as a model of higher silane functionality systems yielding silica coatings, silicones, gels, and zeolites. Because silanol esterification, condensation, and deprotonation reactions are often coupled, one cannot easily monitor the individual reaction equilibria in multifunctional silane systems. Here, though, we measure the equilibrium species distribution in a monofunctional model system to estimate the equilibrium constants - including for the first time the esterification equilibrium constant of a silanol in alkaline conditions. Our main findings are: (1) the measured esterification equilibrium constants agree with previous values from dioxane-based solutions and with pseudoequilibrium data for multi-functional silanes in ethanol-water solutions (suggesting that substitution effects for silanol esterification equilibrium coefficients are negligible), (2) the measured equilibrium deprotonation constant agrees with silanol acidities reported in a dioxane-based system, and (3) the solvent environment affects the apparent silanol condensation equilibrium constants significantly. In alkaline systems, while silanol deprotonation affects solution pH even at low base concentrations, it affects the silicate speciation only at high base concentrations. Finally, we find a strong and nearly linear correlation between ²⁹Si NMR chemical shift and the degree of trimethylsilanol deprotonation.