The above reaction is typically performed in ethanol, with the final density of the aerogel dependent on the concentration of silicon alkoxide monomers in the solution. In practice, this amount of water leads to incomplete reaction, and weak, cloudy aerogels. Catalysts The kinetics of the above reaction are impractically slow at room temperature, often requiring several days to reach completion. For this reason, acid or base catalysts are added to the formulation. The amount and type of catalyst used play key roles in the microstructural, physical and optical properties of the final aerogel product.
Acid catalysts can be any protic acid, such as HCl.
Basic catalysis usually uses ammonia, or ammonia buffered with ammonium fluoride. Aerogels prepared with acid catalysts often show more shrinkage during supercritical drying and may be less transparent than base catalyzed aerogels. The microstructural effects of various catalysts are harder to describe accurately, as the substructure of the primary particles of aerogels can be difficult to image with electron microscopy. As condensation reactions progress the sol will set into a rigid gel. At this point, the gel is usually removed from its mold.