Investigation of the Formation of MCM-41 by Electron Spin−Echo Envelope Modulation Spectroscopy

Abstract

The electron spin−echo envelope modulation (ESEEM) technique was used to investigate the formation mechanism of the mesoporous material MCM-41. The spin-probes 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6,-tetramethyl piperidine-oxyl iodide (CAT16) and 5-doxyl stearic acid (5DSA) were introduced into the surfactant (cetyltrimethylammonium bromide, CTAB) solution in minute amounts followed by the addition of a base and a silica source to initiate the reaction. The reaction was then quenched at different times by rapid insertion into liquid nitrogen. The preservation of the micellar structure upon freezing was proved by a series of ESEEM measurements carried out on 5DSA in CTAB solutions of various concentrations, which showed that the 14N modulation depth was sensitive to the transition from spherical to cylindrical micelles. Variations in the immediate environment of the spin-probes occurring during the room temperature formation of MCM-41 were followed by tracing the 2H modulation depth k(2H) induced by ...