onstrated their exciting properties in acidic (8, 9) and oxida-The mechanism of formation of micelle-templated silica (MTS) tive (10, 11) catalysis of large organic molecules, as hosts was investigated by means of computer-aided analysis of the elecfor polymeric structures (12, 13), in sorption studies (14, tron paramagnetic resonance (EPR) spectra of three different 15), and as minireactors for electron transfer reactions (16).
probes inserted in the micelle. Information were obtained on (a)
The discovery of these new materials led to the extension the kinetics of formation of MTS; (b) the interactions between the of the concept of ''templating'' from microporous to mesomicelle surface and the solid silica; and (c) the modifications of the porous molecular sieves. The micropore structure of zeolites micelle structure during the MTS formation. As a reference, the is templated by hydrated cations or individual organic moleformation of the micelles in the absence of silica, in neutral and alkaline media, and the interactions of the probes with already cules. The mesopore structure of M41s is templated by selfformed hexagonal mesoporous silica (MCM-41) in the absence of assembled micelle aggregates of surfactants. Many efforts surfactants were investigated. The MTS synthesis at 323 K was were devoted to determining the mechanism of mesophase shown by EPR analysis and by X-ray diffraction measurements to formation (2, 5, 17-20). Considering these results and their involve two steps, starting from a ''zero-time'' for addition of the likely interpretation, the most plausible mechanistic pathway silica solution to the micelle solution: (a) in the first few minutes, for mesophase formation implies a cooperative organization silicate species coated the micelles and ''froze'' them (decrease in of organic species and dissolved inorganic species. In this the rotational mobility evaluated from EPR analysis), forming a mechanism, silicate anions in solution neutralize the charge disordered silica-micelle aggregate. An order parameter was needed of the cationic surfactant and play an intimate role in directing to simulate spectra of probes in the CTAB micelles interacting with formation of the supramolecular micelle arrays, precursors the hydrophobic region of mesophases. In the second step (b), an increasing fraction of cationic head groups of the surfactant strongly of the mesostructures (5). Structures of the mesophases are interacted with the solid walls of the structured silica. The destabilisimilar to well-known lyotropic phases (18). Few studies zation of the first silicate precipitate and the formation of the more have examined the kinetics of formation of these mesophases. stable hexagonal mesophase corresponded to an Ostwald ripening Nevertheless, great variations of the rate of formation as a mechanism, which was monitored by the change of the order paramfunction of synthesis conditions clearly appear from the literaeter in the EPR spectra. The lengthening of micelles created an ture. The source of silica is of paramount importance, the anisotropic cylindrical structure evidenced by a tilting of the surfacduration of the synthesis being measured on a scale of minutes tant chains (tilt of the rotational axis with respect to the axis of the when tetraethoxysilane (TEOS) is used (4, 21-23) and on elongated micelle in the EPR spectra).