In Situ Dielectric Spectroscopy Monitoring of Silica Nanoparticle Synthesis in Cationic Water-in-Oil Microemulsions

In situ monitoring of microemulsion-based nanoparticle synthesis is significant for understanding the particle formation mechanism and for advancing controlled nanoparticle synthesis by this means. In this study, the processes of silica nanoparticle synthesis in a CTAB/n-hexanol/cyclohexane/ammonia microemulsion were monitored via dielectric spectroscopy in situ and in real time, with the influences of the water content and precursor concentration being considered. Two dielectric relaxations in addition to a water-induced one were observed in the frequency range of 1 MHz to 3 GHz, which persist throughout the synthesis processes. It is suggested that the lower-frequency relaxation is ascribed to interfacial polarization and the higher-frequency one is caused by the orientational polarization of the ion pair consisting of a counterion and a surfactant polar group. The latter and water-induced relaxations were found to be barely changed during the synthesis processes, while the former changes obviously with synthesis time. The evolution of the lower-frequency relaxation and direct current conductivity with synthesis time are presented and discussed, on the basis of which the particle formation process is inspected from a dielectric spectroscopic point of view.