Home > Technical Information

Colloidal silica is produced by dispersing negatively charge amorphous silica particles in water. The stability of these particles depends on several variables: silica solids content, temperature, size or surface area of particles, pH, particle charge, salt concentration and the compatibility with various additives such as surfactants, stabilizers, and organic solvents.

Silica solids content
All grades of Silco colloidal silica are extremely stable in the absence of any freezing or evaporation of the solution, which can cause the particles to aggregate and gel. The stability of the solutions will increase with dilution (especially that of water) provided there is not a change in pH, temperature or other additives.

Typical colloidal silica picture below, with OH surface groups.

Temperature
A majority of the applications will be run at or near room temperature which does not cause any problems with the colloidal silica. Only the extremes cause problems, such as freezing which increases the concentration of silica in the unfrozen portion which accelerates gelation. The other extreme, caused by heating can both lead to evaporation which has the same effect or to movement of the colloidal particles in suspension and the dissociation of electrolytes present.

pH
As shown by the diagram below the stability of colloidal silica in relation to pH. The higher the concentration or the smaller the particle size, the greater the effect of change in pH. Above the pH of 10.5 the silica becomes increasingly solubilized and the alkali silicate acts like any other soluble salt destabilizing the remaining colloidal silica.

pH adjustment
The pH of a colloidal silica can be adjusted, for instance by the addition of an acid, however it differs by the type added. Typically, a weaker acid (like acetic) is much more effective than a strong one (like hydrochloric).

Particle Size
The relative stability of colloidal silicas increase with increasing particle size, if other variables remain constant. Typically, smaller particle sizes (less than 5-6nm) cannot be concentrated over 20%, while larger particles can be concentrated as high as 48.5%.

Particle Charge
Colloidal silica are spherical particles with a negative charge as can be seen from the diagram below. The SiOH groups and OH ions exist at the surface of the particles and form an electric double layer from the alkali ions. Stabilization is obtained by the repulsion between same negative charges of the particles. If this charge balance is disturbed in any way will cause the particles to aggregate, obtain a higher viscosity and eventual gelation.

Salt and Electrolytes
Salts cause the counter ions in the electric double layer to reduce in distance and move much closer to the particle which the repelling forces act. This causes a reduction in sol stability by increasing the probability of interparticle collision. Univalent slats react slowly, but polyvalent salts tend to react quickly, especially at higher pH’s. Electrolytes are typically added to induce gelation, however is not necessarily uniform in nature. This will vary depending on type, concentration, temperature, etc. of the electrolyte used. Ones typically used are NaCl, NH4Cl, acetates or nitrates.

Surfactants
These are typically added when trying to increase the wetting properties of your formulation. Silco’s colloidal silica are compatible with both anionic and nonionic surfactants, however may become incompatible if there are impurities in the surfactant.

Organic Solvents
Colloidal silicas are compatible with alcohol, acetone and other polarized solvents, however the HP grades are more stable. Compatibility increases with increasing dielectric constant, decreasing silica pH, decreasing silica concentration, and increasing particle size.