Controlled formation of silica particles from tetraethyl orthosilicate in nonionic water-in-oil microemulsions

The formation of silica particles by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in nonionic water-in-oil microemulsions was investigated as a function of the molecular structure of the surfactant, the type of oil, and the concentrations of surfactant and water. Three types of nonionic surfactants and two oils, heptane and cyclohexane, were used. Microemulsion solutions were characterized using photon correlation spectroscopy, electrical conductivity measurement, and titration of phase separation points. The hydrolysis of TEOS and the size of synthesized silica particles were measured by FTIR spectroscopy and transmission electron microscopy measurements, respectively. It was found that the size distribution of silica particles correlates well with the size, connectivity, and stability of microemulsion droplets. This correlation indicates that the final size distribution of silica particles is predetermined in the silica particle's nucleation stage. In this stage, hydrolyzed (monomeric and polymeric) silica reacting species undergo interdroplet dynamic exchanges to contact one another and grow into nuclei. The results of this study showed that the nucleation of silica particles is enhanced (as to result in smaller final particles) by increasing the number of microemulsion droplets that compartmentalize reacting species in solutions or by increasing the strength of surfactant films in sterically hindering the dynamic exchange of reacting species between droplets. The effect of each variable described above on the formation of silica particles in microemulsions is detailed in the paper.