Particle Size by Adding Active Silica

Increasing Particle Size By Adding Active Silica 

It was because of the small particle size of the colloidal silica made at ordinary temperature that stable concentrated sols could not be obtained. Because higher temperature was known to accelerate gelling, it seemed logical that sols should be made and kept at ordinary temperature. Hence the ultimate particle size seldom exceeded 2-3 nm. When a sol of this type was adjusted to pH 8-10 and vacuum concentrated at 20-30 C it gelled when the concentration reached about 10%. It was not realized that it could be heated and evaporated at 100 C to a stable concentration of 10-20% because at the higher temperature the particle size increased to 4-6 nm. 

However, to obtain still more concentrated sols, even larger particle sizes were needed. This was first achieved by the further deposition of soluble silica on the particles by adding silicic acid in the form of particles smaller than 5 nm, generally less than 2 nm in diameter or even smaller polymer species. These are active in the sense that they are more soluble and dissolve in the presence of larger particles or nuclei on which silica is deposited. 

In a modification of the Bechtold and Snyder process. Rule (7) started with a heel of alkalinized sot but added a solution of polysilidc acid made by ion exchange to which no alkali has been added. The silica particles were thus grown in a medium of constant alkali content, so that stable concentrated sols were produced containing a minimum of stabilizing alkali. Albrecht (15) patented the optimum rate of addition of polysilidc acid in the Rule process for producing silica particles 45-100 nm in diameter. By operating a similar process under superatmospheric pressure, particles up to 150 nm in diameter have been produced (16). 

This method of increasing the size of silica particles is sometimes referred to as the buildup process. Colloidal particles in which only the surface of the particles ts of silica, whereas the interior is another insoluble material, can be made by rith suitable colloidal nuclei other than silica, using the buildup process. Thus it is possible to produce sols having the dispersion and surface characteristics 

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