Colloidal Silica

Colloidal silicas are suspensions of fine sized amorphous, nonporous, and typically spherical silica particles in a liquid phase.

Usually they are suspended in an aqueous phase that is stabilized electrostatically. Colloidal silicas exhibit particle densities in the range of 2.1 to 2.3 g/cc.

Most colloidal silicas are prepared as monodisperse suspensions with particle sizes ranging from approximately 5 to 100 nanometers in diameter. Polydisperse suspensions can also be synthesized and have roughly the same limits in particle size. Smaller particles are difficult to stabilize while particles much greater than 150 nanometers are subject to sedimentation.

Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica nuclei. The resulting suspension is then concentrated and stabilized. The initial neutralization step can be accomplished by acidification or electro-dialysis, but it most often achieved by contacting the silicate with a hydrogen form ion exchange resin. The maximum concentration at which this neutralization can be carried out is in the range of 10 to 15%. Higher concentrations result in gelation and plugging of the resin pores.

The colloidal suspension is stabilized by pH adjustment and then concentrated, usually by evaporation. The maximum concentration obtainable depends on the on particle size. For example, 50 nanometer particles can be concentrated to greater than 50 wt% solids while 10 nanometer particles can only be concentrated to approximately 30 wt% solids before the suspension becomes too unstable.