Most colloidal silicas are prepared as monodisperse suspensions with particle sizes ranging from approximately 30 to 100 nm in diameter.
Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4.
If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow.
Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge.
Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point.