Synthesis of mesoporous silica particles controlling the CTABr-pluronic assembly

Abstract

In the synthesis of mesoporous silica materials, self-assembly of a charged surfactant (cetyltrimethylammoniumbromide, CTABr) and a pluronic (PEOx-PPOy-PEOx where PEO is CH2CH2O and PPO is CH(CH3)CH2O) into micelles have key. By controlling the hydrophilic-hydrophobic character of the CTABr-Pluronic micelles, mesoporous silica particles can be synthesized with different morphologies (sphere, wormlike, crystal-like etc.). The particles generally have 2D hexagonal mesostructure with a high surface area (as high as 800 m2 /g). Shape of the micelles as well as the morphology of the particles depend on the hydrophobic nature of the pluronic surfactant and the CTABr amount. The CTABr amount is carefully adjusted to control the morphology and structural order of the particles. The self-assembly of the CTABr-Pluronic micelles and silica species has been achieved by adjusting pH of the synthesis medium to 1.0 in order to produce mesoporous particles with a distinct morphology and mesostructure. Nature of the CTABr-Pluronic micelles can be influenced by adding organic and inorganic additives to the reaction media. The effect of the lyotropic (F- , SO4 2- and Cletc.) and hydrotropic (NO3 , SCNetc.) anions on the micellization of P85 has been first investigated in the aquoues media using UV-Vis Spectroscopy and ethyl orange dye. Then these inorganics and organic (Benzene) additives, in the synthesis of mesoporous silica, have been used to control the micellization of the CTABr-P123 couples as well as the morphology and the pore structure of the silica particles. Highly ordered particles with larger pores and various pore structures have been synthesized using lyotropic anions in the CTABr-P123 system. Furthermore, the hydrotropic anions control the CTABr content of the CTABr-P123 micelles. Increasing CTABr amount in the CTABr-P123 micelles decreases the wall thickness of the silica particles. The hydrophobic character of the micelles can also be enhanced by adding water insoluble organic additives (benzene). The silica particles, synthesized using CTABr-P123-Benzene system, are well structured, where the higher order X-ray diffraction lines can also be observed. Finally, the catalytic role of Fions on the polymerization of the silica has been studied in the CTABr-Pluronic system. Addition of Fion to the reaction medium speeds up the formation process and producing spherical and uniform mesoporous particles less than 20 minutes. The effect of each of the reaction component, Fion, CTABr and P123 molecules, to the assembly rate has also been investigated by determining the turbidity point (due to the formation of silica particles) of the solutions. A correlation between the particle size and reaction rate has also been brought out. The mesoporous silica particles synthesized in this thesis have been characterized using PXRD, FT-IR and Raman Spectroscopy, SEM, TEM and N2 sorption measurements.