SOME PROPERTIES OF SILICA DISPERSIONS

Abstract

When silicic acid sols are formed from solutions of sodium silicate by acid neutralization, a pronounced minimum in the time required for the sol–gel transformation is observed if the titration is stopped near pH 8. If the sol is prepared by electrodialysis, a similar minimum is obtained when the operation is stopped at about pH 5.5. Intermediate values for the minimum are found if the original silicate solution is partially dialyzed followed by neutralization with acid. When the water content of the gels is replaced by liquid hydrocarbons, maximum interaction between the silica particles occurs when the aqua gel has been formed or conditioned at pH 5.5 regardless of the method of preparation of the sol. A minimum in electrophoretic mobility and a maximum in sedimentation rate of the particles are observed at a similar pH value. These properties are modified considerably if the system is thoroughly dehydrated.