AGGREGATION PROFILES OF SILICA SOLS IN A SOL-GEL PROCESS

The kinetics of aggregation of sols in a sol-gel system was followed by measuring the particle size profile of sols from the start of the chemical reaction. The particle size was measured using dynamic light scattering. The particle size profiles show three different regions. The first one corresponds to an unstable regime characterized by reproducible oscillations in the size of the sols. This is due to a wide variety of different dissipative structures formed during the early stage of the reaction because of the autocatalytic character of the polymerization reaction. The second region is a steady-state regime which is characterized by the fact that the particle size remains practically constant for a long period of time. In this regime, the relative scattered intensity grows in an approximately linear way with the reaction time, i.e. the number of sols is increasing with time. In the last region, the particle size profile shows a rapid increase of the size of sols, and this corresponds to gelation. The number concentration of sols is so large that they begin to touch each other to form the gel. This is the mechanism by which the system reaches gelation.