Perspectives on Colloidal Silica
A historical and technical perspective
It all began in the 1930s when I.G. Farben started selling and producing a 10% (by weight) silica sol. Since then, aqueous, amorphous colloidal silica dispersions have been commercially available. In the 1950s, broader uses were found when dispersions could be made at a higher solid concentrations. This improvement in colloidal stability and subsequent increases in concentration was due to the advance of ion exchange technology leading to lower levels of destabilizing ions and impurities.
Since then, the uses for colloidal silica as well as the variety of grades of colloidal silica dispersions has increased. Today, colloidal silica dispersions are available in a variety of grades determined by particle size, structure, concentration, pH, surface charge and surface modification.
Characteristics & Chemistry
Colloidal silica dispersions are fluid, low viscosity dispersions. There are many grades of colloidal silica, but all of them are composed of silica particles ranging in size from about 3 nm up to about 150 nm. These particles may be spherical or slightly irregular in shape, and they may be present as discrete particles or slightly structured aggregates. These particles may also be present in a narrow or wide particle size range - depending on the process in which they were created.
The maximum weight fraction of silica in the dispersion is limited based on the average particle size. Dispersions with a smaller average diameters have larger overall specific surface areas and are limited to low concentration dispersions. Conversely, dispersions with larger average diameters have lower overall specific surface areas and are available in more concentrated dispersions.
The appearance of colloidal silica dispersion depends greatly on the particle size. Dispersions with small silica particles (< 10 nm) are normally quite clear. Midsize dispersions (10-20 nm) start to take on an opalecent appearance as more light is scattered. Dispersions containing large colloidal silica particles (> 50 nm) are normally white.
Standard colloidal silica dispersions are stable against gelling and settling in pH range of 8 - 10.5. These colloidal silicas are charge stabilized with an alkali (normally alkalis of sodium, potassium, or lithium) or stabilized with ammonia. Under these conditions, the particles are negatively charged. The dispersion can be destabilized though the addition of excessive electrolytic species (sodium, calcium, chloride, lithium, potassium). These colloidal silica particles can achieve additional anionic charge stability when as aluminosilicate sites are formed by incorporation of aluminum into the surface layer of the silica particles.
Low pH versions of colloidal silica are also available by the adsorption of cationic aluminum oxide onto the surface of the particles. This results in a cationic particle that is stabilized with anionic species - commonly this is chloride. These dispersions are stable below a pH of 4.
Low pH grades can also be obtained by completely deionizing the dispersion. These grades do not require the presence of stabilizing ions and are also stable below a pH of 3.
Dispersion stability can also be enhanced with surface modification to incorporate silanes. The silanol groups can be isolated silanol groups or even geminal (silanediol groups) or vicinal types. Not only do these silanes provide reactive sites for the grafting of other chemicals, but they provide enhanced stability by physically preventing the formation of siloxane bridges that can result in the formation of aggrigates or gel structures.