Silica, also known as hydrated silica or colloidal silica, has the molecular formula SiO2-x·nH2O (x=0.4-0.8). It is a white, non-toxic, amorphous porous substance. The primary particle size ranges from approximately 16-40 nm, with a specific surface area of about 40-170 m²·g⁻¹. Its specific gravity is approximately 2.319-2.653, and its melting temperature is 1750°C. It is insoluble in water and solvents but dissolves in strong alkalis and hydrofluoric acid. Silica possesses excellent properties such as being lightweight, heat-resistant, chemically stable, non-flammable, and having good electrical insulation.
Silica particles bear some resemblance to carbon black particles; they are spherical in shape, with particles contacting each other and interlocking into chain-like structures. These chain structures interact through hydrogen bonding forces to form clustered network-like three-dimensional structures, commonly referred to as secondary structures. These secondary structures can be pulled apart or disrupted by external forces but can subsequently re-aggregate. The degree of structure of silica is often indicated by its oil absorption value: the higher the structure, the greater the oil absorption value. A higher oil absorption value corresponds to smaller particles and a larger specific surface area. Microscopic observation shows that silica exhibits grape-like clusters of aggregates with a high specific surface area. X-ray diffraction patterns confirm that the overall structure of silica is amorphous, although the amorphous structure may vary depending on the production method.
Silica particles have a very small diameter, with primary particle sizes typically ranging from 0.01 to 1 mm. The surface of these fine particles contains various types of hydroxyl groups, making them hydrophilic. Infrared spectroscopy studies have confirmed the presence of three types of hydroxyl groups on the surface of silica particles: isolated hydroxyls, adjacent hydroxyls, and siloxane groups.
