Kinetics of formation of micelle-templated silica mesophases monitored by electron paramagnetic resonance

The mechanism of formation of micelle-templated silica (MTS) was investigated by means of computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of three different probes inserted in the micelle. Information were obtained on (a) the kinetics of formation of MTS; (b) the interactions between the micelle surface and the solid silica; and (c) the modifications of the micelle structure during the MTS formation. As a reference, the formation of the micelles in the absence of silica, in neutral and alkaline media, and the interactions of the probes with already formed hexagonal mesoporous silica (MCM-41) in the absence of surfactants were investigated. The MTS synthesis at 323 K was shown by EPR analysis and by X-ray diffraction measurements to involve two steps, starting from a "zero-time" for addition of the silica solution to the micelle solution: (a) in the first few minutes, silicate species coated the micelles and "froze" them (decrease in the rotational mobility evaluated from EPR analysis), forming a disordered silica-micelle aggregate. An order parameter was needed to simulate spectra of probes in the CTAB micelles interacting with the hydrophobic region of mesophases. In the second step (b), an increasing fraction of cationic head groups of the surfactant strongly interacted with the solid walls of the structured silica. The destabilization of the first silicate precipitate and the formation of the more stable hexagonal mesophase corresponded to an Ostwald ripening mechanism, which was monitored by the change of the order parameter in the EPR spectra. The lengthening of micelles created an anisotropic cylindrical structure evidenced by a tilting of the surfactant chains (tilt of the rotational axis with respect to the axis of the elongated micelle in the EPR spectral. (C) 1998 Academic Press.