Silane, dichlorodimethyl-, reaction products with silica

Hydrophobic colloidal silica occurs as a light, fine, white or almost white amorphous powder, not wettable by water.

Hydrophobic colloidal silica is prepared by the flame hydrolysis of chlorosilanes, such as silicon tetrachloride, at 1800°C using a hydrogen–oxygen flame. It is rapidly cooled to create an amorphous productand immediately treated with dichlorodimethyl silane in a fluid bed reactor. The resulting surface is covered with dimethylsilyl groups. Aerosil R972 is manufactured by modifying the surface of Aerosil 130 and thus has the same primary particle size and surface area, but the density of silanol groups is reduced from approximately 2.0 SiOH/nm2 to 0.75 SiOH/nm2.

large specific surface area, which provide desirable flow characteristics in dry powders used in tableting and capsule filling. The hydrophobic grades absorb less moistureand may offer an advantage in moisture-sensitive formulations.
Hydrophobic colloidal silica is also used to thixotropically control viscosity, to thicken and stabilize emulsions, or as a suspending agent in gels and semisolid preparations. Hydrophobic colloidal silica has a less pronounced effect on solution viscosity but can thicken and stabilize the oil phase of a water–oil emulsion. With other ingredients of similar refractive index, transparent gels may be formed. Generally, the uses and ranges of hydrophobic colloidal silica are similar to the concentrations used of the standard hydrophilic colloidal silicon dioxide.

The safety profile of hydrophobic colloidal silica is the same as for the hydrophilic silica types, as the modified silica surface does not significantly alter the toxicological properties.
LD50 (rat, IV): 0.015 g/kg

Hydrophobic colloidal silica should be stored in a well-closed container. It will not absorb moisture but may still absorb volatile substances owing to its high surface area.

Use of the hydrophobic colloidal silica Aerosil R972 reduces the strength of starch-based tablets.

Approved for use in pharmaceuticals in Europe. Approved by FDA and Europe for food contact articles. Included in nonparenteral medicines (oral tablets; rectal suppositories) licensed in the UK.