Appearance colourless liquid Formula SiC8H20O4 Melting point -77 °C | Density 940 kg/m³ Boiling point 169 °C | |
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Tetraethyl orthosilicate
Tetraethyl orthosilicate, formally named tetraethoxysilane, is the chemical compound with the formula Si(OC2H5)4. Often abbreviated TEOS, it is a colorless liquid that degrades in water. TEOS is the ethyl ester of orthosilicic acid, Si(OH)4. It is the most prevalent alkoxide of silicon.
Contents
TEOS is a tetrahedral molecule. Like its many analogues, it is prepared by alcoholysis of silicon tetrachloride:
SiCl4 + 4 EtOH → Si(OEt)4 + 4 HClwhere Et is the ethyl radical, C2H5, and thus EtOH is ethanol.
Applications
TEOS is mainly used as a crosslinking agent in silicone polymers and as a precursor to silicon dioxide in the semiconductor industry. TEOS is also used as the silica source for synthesis of some zeolites. Other applications include coatings for carpets and other objects. TEOS is used in the production of aerogel. These applications exploit the reactivity of the Si-OR bonds.
Other reactions
TEOS easily converts to silicon dioxide upon the addition of water:
Si(OC2H5)4 + 2 H2O → SiO2 + 4 C2H5OH
An idealized equation is shown, in reality the silica produced is hydrated. This hydrolysis reaction is an example of a sol-gel process. The side product is ethanol. The reaction proceeds via a series of condensation reactions that convert the TEOS molecule into a mineral-like solid via the formation of Si-O-Si linkages. Rates of this conversion are sensitive to the presence of acids and bases, both of which serve as catalysts. The Stöber process allows the formation of monodisperse and mesoporous silica.
At elevated temperatures (>600 °C), TEOS converts to silicon dioxide:
Si(OC2H5)4 → SiO2 + 2 (C2H5)2OThe volatile coproduct is diethyl ether.
Safety
TEOS has low toxicity by ingestion. While tetramethoxysilane is highly damaging to eyes since it deposits silica, TEOS is much less so due to lower hydrolysis rate of the ethoxy groups.
