Dense silica microspheres from organic and inorganic acid hydrolysis of TEOS

It has been observed that any acid–water mixture (irrespective of the type of acid organic and inorganic) in the pH range 1.35–2.25 can produce silica microspheres from tetraethyl orthosilicate (TEOS) with only a restriction of the water/TEOS molar ratio which varies in the ranges 1.0–1.5 for strong acids and 1.5–4.0 for weak acids. Microspheres have been found to form in situ from an intermediate heavier liquid phase of polysilicic acid which has been segregated and separated from the sol during the progress of the reaction. FTIR spectral analysis reveals that this intermediate liquid predominantly consists of four-membered siloxane rings (Si4O12) as identified by their absorption frequencies in the ranges 550–600 and 670–710 cm−1. The segregated liquid phase has been transformed into glass-like non-porous dense microspheres during stirring. FTIR analyses of these silica spheres are similar to that of fused silica. Density, surface area and size of the dried (150°C) microspheres vary in the ranges 2.10–2.16 g cm−3, 0.5–0.9 m2/g and 10–60 μm, respectively. Their high densities and FTIR spectral data indicate a similar structure to that of silica glass.