Mechanism of Interparticle Bonding

The conversion of a sol of spherical particles to a uniform gel coctaiuing all the liquid in the sol is not easily understood. When particles collide it is assumed that adhesion can occur but in the case of silica particles there is reason to believe that the attachment is through the formation of Si-O-Si bonds. One reason for thinking so is that the same factors that promote polymerization of moncrr.er and low molecular weight silicic acids also promote the conversion of a sol of coUoidal silica particles to a gel. Thus sols consisting of well-defined spherical particles form gel least rapidly at about pH 2 and the process is accelerated by fluoride ior.s at low pH. 

One can only conclude that when particles collide, there are neutral sSiOH groups as well as sSiO ionized groups on the surface of the particles which condense to form Si-O-Si linkages by the same mechanism involved in the polymerization of low molecular weight species. However, the presence of soluble silica or monomer then plays a role of further cementing the particles together. 

Indeed it is even possible that the presence of Si(OH) at the point of contact between colliding particles may play a role in promoting the formation of the initial bond. 

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