Dispersion of Pyrogenic Silica

Dispersion of silica to a sol of separate, discrete, ultimate particles is difficult because the particles are coalesced to varying degrees. Also in many cases, the surface is partly anhydrous with only a few hydrophilic silanol groups. For these reasons the properties of sols of this type are generally different from those made In aqueous solution. They do not form strong gels and are of little use as an inorganic binder. 

The patent literature suggests that intense mechanical shearing forces, both on the dry voluminous powder and on the subsequent suspension in water, are required to attain reasonable dispersion in water or polar organic liquids. In water, wetting agents are used to promote wetting of the hydrophilic siloxane surface areas and alkali to promote surface hydration and dispersion (116, 117).  

A more stable dispersion is obtained by adding to the silicon tetrachloride a certain amount of titanium or aluminum chloride to produce silica containing a 

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Cab-O-Sperse [Cabot], TM for aqueous dispersions of pyrogenic silica for use in the paper and textile industries. 

As further evidence, Rubio and Goldfarb (288) reported that in aqueous dispersions of pyrogenic silica such as Aerosil, the amount of quaternary ammonium ions required for coagulation was much less than in the case of hydrated silicas also the sol was restabilized with much smaller excess of these organic cations. 

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