Colloidal Silica - Chemistry and Properties

Colloidal Silica - Chemistry and Properties

Colloidal silica emulsion is a liquid, low viscosity emulsion. Colloidal silica is available in many grades, but all colloidal silica particles consist of colloidal silica particles ranging in size from about 2 nm to about 150 nm

The particles can be spherical or slightly irregular in shape and can exist as discrete particles or slightly structured aggregates. They may also exist in a narrower or wider range of particle sizes, depending on the process by which they were produced.

The maximum weight proportion of silica in the dispersion is limited based on the average particle size. Dispersions with smaller mean diameters have larger specific surface areas and are limited to low concentration dispersions. Conversely, an emulsion with a larger average diameter has a smaller overall specific surface area, and a higher concentration emulsion can be used.

The appearance of colloidal silica emulsions depends largely on particle size. Dispersions of small silicon particles (< 10 nm) are usually clear. Medium size dispersions (10-20 nm) start to take on an opaque appearance as more light is scattered. Dispersions containing larger colloidal silica particles (>50 nm) are usually white in color.

Standard silica gel dispersions are stable against gelation and precipitation over a pH range of 8 - 10.5. These silica gels are stabilized with a base (usually a sodium, potassium or lithium base) or stabilized with ammonia. Under these conditions, the particles are negatively charged. Dispersions can be destabilized by adding excess electrolyte species (sodium, calcium, chloride, lithium, potassium). These colloidal silica particles achieve additional anionic charge stabilization by incorporating aluminum into the surface layer of the silica particles to form aluminosilicate sites.

A low pH version of colloidal silica is also available by adsorption of cationic alumina onto the particle surface. This produces cationic particles (usually chloride) that are stabilized with an anionic species. The pH of these emulsions was stable below 4. Low pH series can also be obtained by fully deionized dispersion. These series are stable below pH 3 without the presence of stabilizing ions.

In addition, the stability of the emulsion can also be improved by surface modification of the silane. The silanol groups may be isolated silanol groups, or even silanediol groups (silanediol groups) or phthalic acid groups. These silanes not only provide reactive sites for grafting of other chemicals, but also physically prevent the formation of siloxane bridges, leading to the formation of aggregates or gel structures, thereby increasing stability.