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, prepared 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 (5DSA). Electron spin echo envelope modulation (ESEEM) experiments, carried out on the final product, showed that the spin probe is incorporated into the organic part and the nitroxide radical is located near the organic-inorganic interface. The EPR spectra of 5DSA were measured in situ during the formation of MCM-41 at 298 K. The spectra were analyzed by computer simulations that provide the time evolution of the rotational diffusion rates, R_⊥ and R_∥, and of the ordering potential. As the reaction progresses, the spin probe, which reflects the behavior of the surfactant molecules, experiences an increasing order parameter, S, while its rotational diffusion rates decrease. From the time evolution of these parameters two stages were distinguished. During the first, which lasts about 12 min, S, R_⊥ and R_∥ change rapidly whereas during the second, which lasts about 1 h, R_⊥ and R_∥ remain essentially constant while S exhibits a mild increase. The fast stage is assigned to the onset of orientational ordering and silicate condensation, which occur simultaneously, while the slow process reflects the "hardening" of the silica wall.