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 2 nm up to about 150 nm. The particles may be spherical or slightly irregular in shape, and may be present as discrete particles or slightly structured aggregates. They 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 opalescent 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 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.

Can you explain surface charge and surface modification?

The majority of colloidal silica grades are anionic colloidal silicas. Their surface is composed mostly of hydroxyl groups with the formula of Si-O-H. However, other groups have also been identified including: silandiol, -Si-(OH)2; silanetriol, -Si(OH)3; surface siloxanes, -Si-O-Si-O-; and surface-bound water.This yields an anionic surface charge at alkaline pH and the particles are stabilized by cationic species such as sodium or ammonium.

Anionic colloidal silicas can be further stabilized by the incorporation of aluminum into the surface of the particle leading to the formation of -Al-OH- -groups. This results in very highly negatively charged surfaces even at a pH of 3. This increases the stability of the dispersion greatly!

In the case of cationic colloidal silica, the surface has been coated with Al2O3.  This reverses the charge of the surface of the particle to be positive, and the counter-ion is normally chloride. These sols are stable only below a pH of 4.

Surface modification with silanes reduces the surface charge of the particles, but the steric stabilization phenomenon allows these sols to be stable from pH 2-11.

What are the differences between the grades of colloidal silica?

Particle size and pH are what differ most between the grades of colloidal silica. Particle size can also be expressed in terms of specific surface area, i.e. the higher the specific surface area, the smaller the average particle size. The average particle size also affects the maximum possible SiO2 content (i.e. small particles are only only stable in dilute sols while larger particles are stable at higher concentrations). The pure silica sols are anionic and are typically sodium- or ammonium-stabilized to a pH of 9-11. Through modification using sodium aluminate, however, the sols are stable down to a pH of 3-4.  Cationic silica sols are stable at pH 4-5, and deionized sols are stable at a low pH, typically 2-3.

Can colloidal silica be modified?

Of course! Colloidal silicas can be modified to several configurations including but not limited to: adjustments to pH, stabilization ions, surface charge and surface modification.  Contact your local sales office for more information.

How do I measure specific surface area?

Particle surface area can be determined through titration.

How do I know that I'm using the particle size I ordered?

All of our products are shipped with a certificate of analysis if requested detailing the specific surface area.Particle size can be estimated through back-calculation from surface area.

Is colloidal silica dangerous to the environment?

Since colloidal silica products consist of amorphous silica and water, they rank as one of the most environmentally-friendly, industrial chemical products.

Does it pose any particular health hazards?

Colloidal silica products are aqueous dispersions of amorphous silica. Colloidal silica is not classified as harmful, but as mildly irritating. Because the products can have a drying effect on the skin, protective gloves should always be used. In case of skin contact, wash the area of contact with plenty of water. The use of safety glasses is always recommended. In case of eye contact, rinse with large amounts of water and seek professional medical advice. For further information, please reference the Safety Data Sheets for each product.

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