Electrolyte effects on dissolution rate at pH 1–5 of polymerized tetraethoxysilane

The dissolution rate of polymerized tetraethoxysilane (TEOS) was investigated in the pH range 1–5. The dissolution rates of TEOS in aqueous HCl-solutions are independent of pH and the addition of electrolytes to the reactant solution did not affect the dissolution rate at pH < 2. In contrast, at higher pH-values, 3–5, the rates increased with increasing electrolyte concentration. The effect of electrolytes also increased with increasing pH and the increase in dissolution rate is an effect of the cation concentration. The cation-enhanced dissolution rates may be modeled as an adsorption of cations on reactive sites. The model resembles a Freundlich adsorption isotherm, R/R0=kCn where R is the actual rate, R0 is the rate in HCl, C is the electrolyte concentration, and n is the reaction order. SEM-microphotographs show that the material weathered in pure HCl-solutions were still coated with TEOS, while only a few flakes of TEOS were found after experiments performed at pH 5 in 0.1 M NaCl-solution. The polymerization of TEOS at the relative humidities (RH) of 50% and 95% was investigated using laser Raman spectroscopy. The polymerization of TEOS is found to be considerably faster at the higher humidity.