Mechanism of Condensation and Hydrolysis

As discussed in connection with the isoelectric point of silica the rate of polymerization of monomer and formation of silica gel is at a minimum at around pH 2. It has been assumed that since this is the isoelectric point of silica, the catalyst below pH 2 is the H ion which forms an active cationic comple.. Also, above pH 2 the OH ion is the catalyst in that active anionic silica is generated. 

However, Her (87) discovered that the pH at which the polymerization rate is at a minimum is strongly Influenced by traces of fluoride ion, as little as 10 M concentration (1.9 ppm F) having a marked effect. Since traces of fluorine are difficult to avoid it was suspected that the catalytic effect of H ion might depend entirely on fluoride impurity. However, in view of the observations by various workers on highly purified silica systems, it is now apparent that the effect of fluorine is merely superimposed on the effect of H ion alone. The self-condensation of monomer catalyzed by OH ion is commonly written  

In order to explain the mechanism of the polymerization of silicic acid, we would have to start with the fact that the tetravalent silicon is still unsaturated coordinaiively. Both of the strongly defined secondary valences which are active in the fluoride complexes of silicon must also play a role in the hydrated oxfde. 

In any case it is supposed that when silicon becomes coordinated with five or six oxygen atoms all the silicon-oxygen bonds are weakened so that rearrangements can occur. This has been considered in considerable detail by Strelko (93), who proposed the following rate equation for the pH range from 2 to 10  

This equation corresponds to a maximum in the rate of polymerization somewhere in the mid-pH range..  

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