Colloidal Silica and Historical Development

The term colloidal silica here refers to stable dispersions or sols of discrete parti-cks of amorphous silica. By arbitrary definition, the term excludes solutions of poly-silicic acid in which the polymer molecules or particles are so small that they are not stable. Such solutions, which are usually obtained by acidifying sodium silicate solutions or by hydrolyzing silicon esters or halides at ordinary temperatures, have been discussed in Chapter 3 as precursors of colloidal particles. 

Stable concentrated silica sols that do not gel or settle out for at least several years became available-in the 1940s, after it was learned how to make uniform colloidal particles larger than about 5 nm in diameter, stabilized with an optimum amount of base. 

The history of the development and the state of the art in about 1954 was summarized by Her (8). Further refinements by Alexander (9) in controlling particle size, 

Sols containing discrete particles as-large as 300 nm or more in diameter, which settle out on standing, have been made by autoclaving wet silica gel with a base under superatmospheric pressure and then breaking the lightly aggregated particles apart in a colloid mill (12a). 

Thus in the past 30 years methods have been developed for making discrete silica particles covering the whole range of colloidal size and stabilizing these as concentrated commercial sols. 

---