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Colloidal silica is a liquid dispersion of silicon dioxide used in various industries. It can be monodisperse or polydisperse, with particle size affecting stability and appearance. Applications include abrasives, catalysts, coatings, dewatering agents, and refining beverages. Silica gels can also be formed for use as desiccants.
Colloidal silica is a liquid dispersion of silicon dioxide, the same material found naturally in sand. Like other colloids, it consists of microscopic particles dispersed evenly in a different substance, in this case a solid dispersed in a liquid. This mixture is known as sol and has numerous industrial and commercial applications.
Silica sols are created using a variety of chemical procedures. Colloidal silica is usually monodisperse, which means that all particles in the sol are the same size. The particles are chemically “grown” from tiny silica nuclei suspended in aqueous solution and can be large or small depending on the desired properties of the finite sol.
Large particles tend to make a silica mixture more stable or remain in a dispersed form. At high concentrations, smaller particles can clump together and make the mixture sticky and viscous. Particle size also affects the appearance of the sol, and mixtures with small particles are more transparent.
Colloidal silica is used in a multitude of environments. It can be used as an abrasive or polishing agent for semiconductors, as a catalyst for various chemical reactions, and as a coating product. Silica coatings are useful for increasing traction on surfaces such as floors and railroad tracks as well as textile fibers. This is due to both the abrasive action of the small particles and their ability to chemically react with the opposing surface.
Papermaking is another area where colloidal silica is used extensively. Here, the silica acts as a dewatering agent, allowing more starch to be contained in the paper when processing the aqueous pulp. The additional starch increases the toughness of the paper.
In wine and juice production, colloidal silica is used to refine a beverage by removing organic compounds from the suspension. These compounds, which would otherwise form an undesirable impurity in the beverage or adversely affect the flavor, are attracted to oppositely charged particles in the sol. A sediment of silica and impurities forms, which can then be mechanically removed, leaving a purified drink.
While in many industrial applications it is important to maintain the silica in its uniformly dispersed form, in other cases the formation of lumps or chains of particles is desired. Silica gels, which are often used as a desiccant in packaged products, can be formed in this way. When the pH of colloidal silica is reduced to less than 7 or salt is added, the particles in the sol begin to stick together, forming chains. Finally, a silica gel is created.
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