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Hydrated silica is a form of silicon dioxide, which has a variable amount of water in the formula. When dissolved in water, it is usually known as silicic acid . It is found in nature as opal [ 2 ] (which has been mined as a gemstone for centuries), and in the cell walls of diatoms .
Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica nuclei. The subunits of colloidal silica particles are typically in the range of 1 to 5 nm. Whether or not these subunits are joined depends on the conditions of polymerization.
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In 1990, the safety of synthetic magnesium silicate was reviewed by the Scientific Committee on Food (SCF) together with that of silica and the other metal alkali silicates. [10] The SCF noted that “the available data, including a number of short-term studies in two species, appear to substantiate the biological inertness of those compounds”.
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Silicic acids can be seen as hydrated forms of silica, namely 2 H 2x SiO x+2 = SiO 2 ·(H 2 O) x. Indeed, in concentrated solutions, silicic acids generally polymerize and condense, and ultimately degrade to silicon dioxide and water. The intermediate stages may be very thick liquids or gel-like solids.
An idealized equation is shown, in reality the silica produced is hydrated. This hydrolysis reaction is an example of a sol-gel process. The side product is ethanol. The reaction proceeds via a series of condensation reactions that convert the TEOS molecule into a mineral-like solid via the formation of Si-O-Si linkages.