Spontaneous Growth of Particles

Almost all the initial stages in the polymerization of monomer to oligomers and three-dimensional particles 1-2 nm in diameter occur in only a few minutes at above pH 7. Thereafter the polymerization involves only the increase in size and decrease in numbers of silica particles. At the same time, the concentration of monomer in equilibrium with these particles decreases as the average particle diameter increases. 

Under conditions where the relative rate of aggregation of silica particles is low the particles increase in size and decrease in numbers owing to Ostwald ripening, a term commonly applied to the coarsening of crystals in a precipitate such as BaSO As described in Chapter 1, small particles, especially smaller than about S 

Experience has shown that alkali-stabilized sols of silicic acid polymerize during a few months at 30 C until the surface area slowly drops to about 500 m g , corresponding to 5 nm particles, after which there is little change. At 90 C, the particles grow in a few days to around 7-8 nm, after which spontaneous growth is very slow. . .  

Growth of Silica Particles by Heating a 4% Sol of Silicic Acid at 

Mole Ratio SiOjiNajO -Time Temperature. - rc) Specific Sur-, face Area (m g- ). Estimated Particle Diameter (m ) 

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