Name: Hydrophobic Colloidal Silica
CAS No: Silane, dichloro-dimethyl-, reaction products with silica [68611-44-9]
BP: Hydrophobic Colloidal Anhydrous Silica PhEur: Silica, Hydrophobic Colloidal USP-NF: Hydrophobic Colloidal Silica
Aerosil R972; silica dimethyl silylate; silica hydrophobica colloidalis; silicic acid, silylated; silicon dioxide, silanated.
Silane, dichloro-dimethyl-, reaction products with silica [68611-44-9]
SiO2 (partly alkylated for hydrophobation) 60.08
Hydrophobic colloidal silica has nano-sized primary particles and a large specific surface area,(1) which provide desirable flow characteristics in dry powders used in tableting(2–4) and capsule filling.(3) The hydrophobic grades absorb less moisture(5) and 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.(3,5) With other ingredients of similar refractive index, transparent gels may be formed.(5) Generally, the uses and ranges of hydrophobic colloidal silica are similar to the concentrations used of the standard hydrophilic colloidal silicon dioxide.
Hydrophobic colloidal silica occurs as a light, fine, white or almost white amorphous powder, not wettable by water.
See Table I.
Density (bulk) 0.094 g/cm3(4) Density (tapped) 0.115 g/cm3 for Aerosil R972(4) Moisture content Less than 0.5% w/w at room temperature between 0 and 100% relative humidity.(5) Particle size distribution Primary particle size is 16nm for Aerosil R972. Forms loose agglomerates of 10–200 mm.(5) Refractive index 1.46(5) Solubility Solubility is 1 in 6.7 parts of water (pH 7, 258C).(5) Practically insoluble in organic solvents(5) and acids, except hydrofluoric acid; soluble in hot solutions of alkali hydroxide.(6) Specific gravity 2.0–2.2(5) Specific surface area 110 20m2/g for Aerosil R972
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.(4)
Hydrophobic colloidal silica is prepared by the flame hydrolysis of chlorosilanes, such as silicon tetrachloride, at 18008C using a hydrogen–oxygen flame. It is rapidly cooled to create an amorphous product(7) and immediately treated with dichlorodimethyl silane in a fluid bed reactor.(5) The resulting surface is covered with dimethylsilyl groups.(1) Aerosil R972 is manufactured by modifying the surface of Aerosil 130(5) 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.(4)
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.(8) LD50 (rat, IV): 0.015 g/kg(9)
Observe normal precautions appropriate to the circumstances and quantity of the material handled. Eye protection and gloves arerecommended. Considered to be a nuisance dust.(7) Inhalation of hydrophobic colloidal silica dust may cause irritation to the respiratory tract but it is not associated with fibrosis of the lungs (silicosis), which can occur upon exposure to crystalline silica.(7,9) Precautions should be taken to avoid inhalation of colloidal silicon dioxide. In the absence of suitable containment facilities, a dust mask should be worn when handling small quantities of material. For larger quantities, a dust respirator is recommended.
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.
Colloidal silicon dioxide.
The prefix R in Aerosil R972 stands for ‘repellent’. Hydrophobic grades have not been considered for use in food intended for human consumption but are permitted in food contact articles in Europe (listed as silicon dioxide, silanated) and as an anticaking agent in animal feed mineral premixes in Europe and the USA. The EINECS number for hydrophobic colloidal silica is 271-893-4.