Chemical Properties
Hydrophobic colloidal silica occurs as a light, fine, white or almost
white amorphous powder, not wettable by water.
Production Methods
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.
Pharmaceutical Applications
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.
Safety
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
storage
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.
Incompatibilities
Use of the hydrophobic colloidal silica Aerosil R972 reduces the
strength of starch-based tablets.
Regulatory Status
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.
