As silicon dioxide, or silica, is one of the most abundant chemical compounds on earth, it is quite easy to undervalue its utility in today’s chemical formulations. Silica that is refined and manufactured to impart specific functionality is referred to as synthetic amorphous silica (SAS). There are four types of SAS: colloidal silica, fumed silica, precipitated silica, and silica gel. Each type of silica product provides unique functionalities that make them ideal for specific applications.
What Is Colloidal Silica?
While most silica products are solid, colloidal silica is unique in that it is a liquid. Colloidal silica is an electrostatically stabilized dispersion of amorphous silica particles in water. The silica is stabilized by the presence of a counterion, typically sodium. This type of dispersion is referred to as a colloid, hence the name colloidal silica.
While the silica particles in colloidal silica are highly reactive, the colloid does not react or separate while stored under normal conditions. Since it is a liquid, colloidal silica is one of the easiest types of silica to incorporate into liquid chemical formulations. It also does not present inhalation hazards like some solid silica products.
How Is Colloidal Silica Used?
The silica particles in colloidal silica are extremely small, on the order of a few nanometers to tens of nanometers. Accordingly, colloidal silica particles have very high surface areas that make them highly reactive via silanol groups on the particle surface. These silanol groups form ionic and hydrogen bonds with polymer functional groups, metal ions, various surfaces, and even other silica particles. This makes colloidal silica an excellent surface modifier, cross-linker, binder, and reinforcing agent in various industrial applications.
Colloidal Silica Applications
Increasing Concrete Densification: Colloidal silica’s small particles penetrate deep into concrete to react with calcium hydroxide to form calcium silicate hydrate, increasing concrete strength.
Creating and Enhancing Catalysts: In addition to being a highly pure raw material for catalysts, colloidal silica also improves attrition resistance, durability, and adhesion to substrates.
Enhancing Paint and Coating Resiliency: Colloidal silica forms strong ionic and hydrogen bonds with coating polymers and substrates to enhance adhesion and resistance to abrasion and blocking.
Strengthening Refractories: Ceramic fibers used in refractories are bonded and treated with colloidal silica to increase resistance to thermal shock and chemical exposure.
Frictionizing Paper Surfaces: Colloidal silica increases the coefficient of friction on paper and paperboard surfaces to reduce slipping and skidding.
What Is Fumed Silica?
Fumed silica is a light, fluffy powder made of amorphous silica particles produced by agglomerating chain-like silica aggregates and primary particles in a flame hydrolysis process. The fumed silica particles are extremely small, usually on the order of tens of nanometers. However, they feature a large surface area and high reactivity.
Fumed silica particles may be surface modified to impart hydrophobicity or introduce functional groups. When incorporated into formulations, these particles must be properly mixed, and care must be taken to prevent the risk of inhalation.
How Is Fumed Silica Used?
Fumed silica particles have a very high surface area with abundant silanol groups that promote hydrogen bonding and Van der Waals forces between various other constituents in a chemical formulation. This action restricts the movement of the fumed silica and other components in the formulation, increasing viscosity. Thus, one of the main uses of fumed silica is as a rheology modifier in liquid formulations. As fumed silica also has a high binding capacity when dry, it is also useful to prevent dry formulations from caking or clumping together by binding and providing an irregular surface roughness to surrounding particles.
Fumed Silica Applications
Improving Food and Beverage Consistency: The high binding capacity of fumed silica helps prevent caking in food products and thickens beverage products.
Paint and Coating Rheology Modification: Fumed silica controls rheology and imparts anti-settling and anti-sagging properties when added to paints and coatings.
Enhancing Flow in Tablet Production: Fumed silica improves the flow properties of powders used in pharmaceutical tablet production, preventing inconsistency and improving quality.
Controlling Cosmetic Appearance: Adding fumed silica to cosmetics can enhance their look by creating a smoother and matte appearance.
What Is Precipitated Silica?
Precipitated silica is a solid product consisting of amorphous silica particles produced via a precipitation reaction of silicate salts. Precipitated silica's particle size is much larger than fumed silica, where the former tends to be on the order of tens of micrometers, whereas the latter is on the order of tens of nanometers.
The larger size of these particles opens the possibility of leveraging the porosity of amorphous silica to create adsorptive properties. Manufacturers control both the particle size and porosity of precipitated silica to produce different grades that are more suited for specific applications. Precipitated silica must be mixed well into liquid formulations to create a suspension.
How Is Precipitated Silica Used?
The relatively large size and high total surface area of precipitated silica particles lend themselves to providing good liquid adsorptive capacity, made possible by a large network of interconnected pores within each particle. This ability to adsorb water can help prevent the clumping of solid particles in powdered products if exposed to water. Precipitated silica’s relatively large particle size is also useful for creating an abrasive effect within formulations to provide polishing or cleaning of various surfaces. Also, precipitated silica particles also have silanol groups available to interact with other particles, albeit not to the same degree as fumed silica. Thus, precipitated silica can be used to thicken liquid formulations.
Precipitated Silica Applications
Boosting Oral Care Cleaning: Precipitated silica provides polishing and cleaning and induces thickening in oral care products, such as toothpaste.
Enhancing Food and Pharmaceutical Consistency: Adding precipitated silica prevents caking and clumping in powdered food products and pharmaceutical formulations.
Providing Material Reinforcement: Incorporation of precipitated silica into materials such as rubber can provide reinforcement and strengthening.
Modifying Film Appearance: Precipitated silica reduces gloss and provides a matte effect to coatings, paints, and inks.
What Is Silica Gel?
Silica gel is a solid synthetic amorphous silica product characterized as coarse granules or beads. It is produced by acidifying a solution of sodium silicate, causing the precipitation of a gel. Silica gel has by far the largest particle size of the silica products described in this article; each individual bead is typically a few millimeters in diameter.
Like precipitated silica, silica gel is also highly porous, with pore diameters ranging from a few nanometers to a few micrometers, depending on the type of silica gel. Compared to the other types of silica described in this article, silica gel is not typically added to chemical formulations but is usually packaged as a finished product.
How Is Silica Gel Used?
As mentioned above, silica gel has a relatively large particle size and is extremely porous. Accordingly, silica gel has a very high specific surface area of around 750–800 m2/g. Since silica gel is naturally hydrophilic, it provides utility as an adsorbent of water and moisture. When silica gel is in contact with water or moisture, the water or moisture readily adsorbs into the vast silica pore network within the silica particle. This property is widely leveraged in creating silica gel packets, where silica gel is used as a desiccant in product packaging, ranging from electronics to food items, to help prevent damage and spoilage from moisture. As such, silica gel is not commonly used outright as an ingredient in formulations but is nonetheless a useful product in packaging. Silica gel is also used in column chromatography applications as a stationary phase.
About ChemPoint
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