Making Particles Under 10 nm in Size

The earliest salt-free sols of 5-8 nm particles were made by Alexander, who defined the silica concentration limits and alkali content for stabilization in terms of particle size (9). The particle size was related to average specific surface area of silica within the following limits  

To increase silica concentration without particle growth beyond 5-10 nm size, a silicic acid solution is alkalinized with sodium silicate and then more silicic acid is added while water is removed by vacuum distillation. Under these circumstances the low temperature avoids excessive particle growth in spite of the addition of active silica to the system (33). An earlier approach was to form the particles to the desired size in dilute solution and then concentrate by evaporating water and adding more dilute sol. Since the added particles are the same size as those in the evaporator, no buildup occurs (34). 

The use of ammonia as a stabilizer apparently minimizes particle growth. Weldes and Derolf (35) claim that sols of particles size 2-3 nm are obtained in concentrations up to 12% when stabilized with both NaOH and NH OH, the (NH )jO Na,0 ratio being 25-150 and the Si02 (NH4)j0 2-8. Possibly the Adsorption of NH3 on the silica surface reduces the rate of dissolution of silica so that growth is retarded. Birkheimer (36) similarly claims stabilizing a sol with excess ammonia and then boiling out the excess. 

Vossos claimed sols of particles less than 5 nm in size stabilized at pH 9-11 containing up to 25% SiOj. The. alkalinized dilute sol was vacuum concentrated at less than 150 F to minimize growth (38).  

Marotta (40) made 5-10 nm particles by adding silicic acid solution to a dilute solution of sodium silicate over a period of hours at 25-50 C, then more at 50-90 C, and still more at 70-100 C while evaporating water. This apparently involved a combination of additional nucleation and particle buildup. 

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