Dielectric Spectroscopy Study On Synthesis Of SiO₂ Nanoparticles In Ionic Surfactant Microemulsion

Nanoparticles are widely used in various fields,and there are various preparation methods,among which microemulsion method is easy to operate and the product morphology is uniform,but there are still many problems in this method.Therefore,in-situ and real-time monitoring of nanoparticles synthesized by microemulsion method is helpful to understand the formation mechanism of nanoparticles.In this thesis,based on the in-situ and real-time monitoring of the synthesis process of silica nanoparticles by microemulsion using DS and the study of the growth mechanism of nanoparticles,the influence of anionic/cationic surfactants on the growth process of silica nanoparticles was compared.Firstly,the static dielectric relaxation behaviors of CTAB/n-hexanol/cyclohexane/ammonia microemulsions(CTAB system)and AOT/n-butanol/n-heptane/ammonia microemulsions(AOT system)with different water phase contents were studied by dielectric spectroscopy.Then,by adding precursors,the synthesis process was monitored in real-time by DS under different phase compositions,and the DS results were analyzed by logarithmic derivative(LD)method.The variation of relaxation strength and relaxation time of different dielectric relaxations in the measured frequency range are accurately obtained.The underlying mechanisms of the three superimposed relaxation of CTAB systems at frequencies of 20 MHz,120 MHz and 1 GHz were determined by analyzing and comparing different models.Relaxation around GHz is caused by bound water,low frequency relaxation comes from interfacial polarization,and middle frequency relaxation is ascribed to polarization of ion pair composed of Br~-and head group of CTAB.In AOT system,two overlapping relaxations are observed,which are located at 240 MHz and 1 GHz respectively.Among them,the relaxation at 1 GHz frequency is caused by bound water,while the relaxation near 240 MHz comes from the directional polarization of the ion pair composed of Na~+and the polar head group of AOT.By analyzing the characteristic dielectric parameters of the related relaxation in the above two systems,the influence of changing the precursor content and water content on the dielectric relaxation behavior in the synthesis process was explored.It was found that these two synthesis processes can be divided into three similar stages,and can be effectively identified by the relaxation characteristics and electrical properties of the synthesis system.In addition,the influence of anionic/cationic surfactants on the nanoparticle synthesis was compared and analyzed,and it was found that the dissociation of anionic/cationic surfactants would hinder/promote the hydrolysis of TEOS,which led to different evolution trends of dielectric relaxation behavior in the synthesis process.