Colloidal silica facts: What is silica?

Of all the oxide minerals in the Earth’s crust, silicon dioxide, or silica, is the most abundant. It is present in not only in combination with other oxide minerals but also in its isolated forms such as sand. The semi-precious mineral opal is a form of amorphous silica that has been prized for centuries.

Besides being the most abundant mineral on the Earth, it is also very important to life on our planet. Diatoms, a type of phytoplankton forming the base of the ocean’s food chain, have skeletons composed of silica.  Many plants use silica to stiffen stems for holding fruit and to form external needles for protection. The role of silica is less obvious in animals, but each one of us contains about half a gram of silica – without which our bones, hair, and teeth could not be formed.

Not only does silica play an important role in biology, it had played an important role in civilization. Flint is a form of silica that was used in ancient tools.The sand used in pottery is also a form of silica. Two-thousand year-old Roman cement contains amorphous silica from volcanic ash which helps give it high strength and durability. Present technology would be very different without the silica used to create the catalysts of our oil refineries, bind the molds for casting super-alloys, form modern glass and ceramics, and polish electronic materials.

What's the difference between sodium silicate (water glass) and colloidal silica?

Colloidal silica consists of dense, amorphous particles of SiO₂.The building blocks of these particles are randomly-distributed [SiO₄]-tetrahedra. This random distribution is what makes amorphous silica different from crystalline silica - ordered on a molecular level.  Sodium silicates are alkaline solutions with pH ranges of 12-13, compared to 9-11 for colloidal silica. Sodium silicates are also composed of silicate monomers, as opposed to colloidal silica composed of polymeric silicates.The composition of sodium silicates have a SiO₂/Na₂O ratio of approximately 3.4, whereas colloidal silica generally has a SiO₂/Na₂O ratio greater than 50. Finally, the viscosity of sodium silicates is much higher - closer to that of a syrup, while colloidal silicas have viscosities close to that of water.

Where can colloidal silica be used?

Collodial silica can be used in numerous applications and it enhances functionality in an ever-growing number of products. To give a couple of examples our products enhances the performance of waterborne coatings by delivering anti-soling properties as well as provides increased durability and strength in cementing operations. Choosing the right colloidal silica can be a challenge. Subtle differences in particle morphology, particle size, and ionic species can make all the difference.

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 through 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 aggregates or gel structures.