Chemical structure:

SiO4 tetrahedron is the base unit of the structure of the macromolecular network nSiO₂

Surface chemistry:

SAS, as produced by wet or thermal route, has silanol (Si-OH) groups on the surface that render untreated SAS hydrophilic.

SAS may be rendered hydrophobic by surface treatment. The two main categories of surface treated SAS are:

• Alkylsilyl treated silica: SAS is treated with silanes and siloxanes resulting in unreactive alkyl groups at the surface to bring hydrophobic properties.
• Functionalised alkylsilyl treated silica: functional organic groups are grafted onto SAS surface to improve the cross-linking between the inorganic filler and the organic matrix in a polymer, rubber or a coating.