Micelle and mesoporous silica formation with different cethylammonium surfactants

Full text: MCM-41 hexagonal mesoporous silica is of high interest by the scientific community because its easy preparation, pore diameter adjustment (between 1.5 and 10 mm), high superficial area, porosity and short range of pore diameter distribution. These characteristics allow this material to be used as catalysts, supports, adsorbents and membrane in many processes. This material is formed by the combination between polyanionic silica species and the micelles of the cationic surfactant. The way that this combination performs has high influence on the mesoporous phase obtained. Therefore, the objective of this study is evaluate the influence of surfactants with different hydrophilic head sizes on micelles formation and then on the MCM-41 hybrid silica. The studies were performed by small angle x-ray scattering curves of aqueous solutions of cethylammonium surfactants. The results showed a decrease of the micellar organization and of the bromide anion layer thickness around the micelles when the size of surfactant head increases (C16TMABr < C16TEABr < C16TPABr). It is believed that the high size of the surfactant head provides a decrease on the condensation of bromide anions around the micelles [1] and larger steric hindrance between heads, interfering on the mesoporous silica formation. According to the SAXS results, the X-ray patterns of the mesoporous silicas prepared by C16TEABr and C16TPABr showed the formation of structures with less organization and with only the (100) diffraction peak, compared to the MCM-41 prepared by C16TMABr surfactant which has, additionally, the (110), (200) and (210) peaks. (author)