Silica sol preparation

Nanosized FezOs-SiOz composites are considered to be promising optical and magnetic materials, catalysts, and magneto-resistive sensors. To obtain the above composites most of authors [1,2] used the combined hydrolysis of Fe salt and tetraethylorthosilicate (TEOS) or introduction of Fe salt into silica sol prepared by TEOS hydrolysis. 

Figure 16. 29Si NMR spectra of spinnable silica sol prepared from TEOS (HsO/Si =1.5). Spectrum a, experimental, 1.0-Hz exponential line broadening spectrum b, experimental, 30-Hz exponential line broadening spectrum c, computer simulation and spectrum d, resonance components of computer simulation. (Reproduced with permission from reference 112. Copyright...

FIGURE 47.16 NMR spectra of spinnable silica sol prepared from TEOS (H20/Si = 1.5). Spectrum a,... 

Such a mechanism must have been involved in the formation of 200 nm spheres in a solution of pure silica sol prepared by hydrolyzing SiCU and removing HCl by electrodialysis, as reported by Radezewski and Richter (128b). The purified clear sol contained about 0.5% SiOj and the pH was 6.8. Similarly, uniform porous spherical silica particles up to 1 micron in diameter are formed by the aggregation of primary particles less than 5 nm in size formed by the hydrolysis of ethyl silicate in a water-alcohol-ammonia system as developed by Stdber and Funk (128c). 

Formation of silica gel has been observed in electron microscope studies by Radezewski and Richter (227). Silica sols prepared by the hydrolysis of silicon tetrachloride and purified by electrodialysis initially contained particles less than 10 nm in diameter. On standing at pH 6.8, spongelike aggregates were observed to be formed from the individual particles. The polymeric silicas which were so obtained... 

Two different sols were used to bind the USY particles together. One was a Super-D silica sol prepared by reaeting a sodium siheate solution with an aluminum sulfate/sulfuric acid solution imder high shear to a pH 3.0 and the other was an aluminum chlorohydrol sol stable at pH 4.3. 

This study provides several examples of how, using multi-step hydrolysis procedures, the sequence (acid-acid, acid-base, acid-acid-base) and timing of the successive steps inHuences the molecular and intermediate-scale structure of silica sols prepared from alkoxide precursors. These examples imply that the silica polymerization pathway is largely Jdnetically controlled at least for moderate values of r. 

---