The formation of the mesoporous material MCM-41 as studied by EPR line shape analysis of spin probes

The formation mechanism of the hexagonal mesoporous material MCM-41, prepar ed with tetraethyl-orthosilicon (TEOS) and cetyl-trimethylammonium chloride (bromide) (CTAC/B), was investigated through the motional characteristics of the spin probe 5-doxyl stearic acid (SDSA). Electron spin echo envelope modulation (ESEEM) experiments, carried out on the final product, showed th at the spin probe is incorporated into the organic part and the nitroxide r adical is located near the organic-inorganic interface. The EPR spectra of SDSA were measured in situ during the formation of MCM-41 at 298 K. The spe ctra were analyzed by computer simulations that provide the time evolution of the rotational diffusion rates, R-perpendicular to and R-parallel to, an d of the ordering potential. As the reaction progresses, the spin probe, wh ich reflects the behavior of the surfactant molecules, experiences an incre asing order parameter, S, while its rotational diffusion rates decrease. Fr om the time evolution of these parameters two stages were distinguished. Du ring the first, which lasts about 12 min, S, R-perpendicular to and R-paral lel to change rapidly whereas during the second, which lasts about 1 h, R-p erpendicular to and R-parallel to remain essentially constant while S exhib its a mild increase. The fast stage is assigned to the onset of orientation al ordering and silicate condensation, which occur simultaneously, while th e slow process reflects the "hardening" of the silica wall.