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 ha
s been segregated and separated from the sol during the progress of the rea
ction. FTIR spectral analysis reveals that this intermediate liquid predomi
nantly consists of four-membered siloxane rings (Si4O12) as identified by t
heir absorption frequencies in the ranges 550-600 and 670-710 cm(-1). The s
egregated liquid phase has been transformed into glass-like non-porous dens
e microspheres during stirring. FTIR analyses of these silica spheres are s
imilar to that of fused silica. Density, surface area and size of the dried
(150 degrees C) microspheres vary in the ranges 2.10-2.16 g cm(-3), 0.5-0.
9 m(2)/g and 10-60 mu m, respectively. Their high densities and FTIR spectr
al data indicate a similar structure to that of silica glass. (C) 2000 Else
vier Science B.V. All rights reserved.