Reactivity and Aggregative Stability of Colloidal Silica

Abstract

The properties of colloidal silica taking into account the reactivity of silica, as well as the kinetics of polycondensation and depolymerization, are analyzed. The breaking of siloxane bonds during depolymerization leads to the appearance of reactive Si–OH and Si–O–Me+ groups, destruction of the silica structure, formation of gel-like layers on the surface and in the volume of particles (at a high level of alkalinity), growth of their hydrophilicity, and the dissolution of silica (limiting depolymerization). The formation of gel-like structures is accompanied by an increase in the hydrodynamic radii of the particles, an increase in the viscosity of aggregative stable sols, a decrease in their turbidity upon transition from acidic to alkaline media, sol dilution, and the introduction of electrolytes. Alkalis affect the solid phase depolymerization process by increasing the cation basicity in the series K+ > Na+ > Li+. The formation of gel-like structures causes an increase in the aggregative stability of the sol at pH> 5.5–6.0. Upon reaching pH 9.0–9.5, the quasi-equilibrium of the polycondensation rates and depolymerization reactions ensures the high aggregative stability of the sol. The further increase in alkalinity (increase in pH, silica modulus) in the aqueous phase increases the concentration of depolymerization products and silicate anions containing ionized and non-ionized silanol groups. Under these conditions, the oligo- and polymeric molecules of silicic acids are formed in the aqueous medium during the secondary polycondensation, and, based on them, linear supramolecular structures, the viscosity of the colloidal system grows, and gels are formed.