Diagrama de fases do sistema brometo de cetiltrimetilamonio - silicato de tetrametilamonio - agua

AUTOR(ES)

Angela Albuquerque

DATA DE PUBLICAÇÃO

2002

RESUMO

The molecular sieves of the M41S family have been studied since their discovery in 1992, however, the mechanism of their formation is not yet very clear. In this work, a phase diagram for the system cetyltrimethylammonium bromide (CTAB)/tetramethylammonium silicate/water has been constructed, in order to obtain basic information on the interactions that exist between these precursors of formation of the M41S materiaIs. With this purpose, the system has been divided in three concentration ranges: the diluted region, which comprehends solutions with CTAB concentrations below the CMC, that is, before the formation of micelles, until solutions where the surfactant micelles are approximately spherical; the semi-diluted region, which comprehends solutions which CTAB concentrations are above the ones in the dilute region, that is, solutions that involve cylindrical micelles; and the concentrated region, involving systems that present liquid-crystalline phases. In each of the regions characterizations of the phases found have been made. In the dilute region, the CMC of the surfactant was determined by conductivimetry and the region in which the formation of MCM-41 was observed has been delimited by turbidimetry. In the semi-diluted region, the sphere-cylinder morphological transition was determined as well as the region of MCM-41 formation. In the concentrated region, the liquid-crystalline phases have been detected by birrefringence in polarized light and characterized by polarized optical microscopy. Our results give experimental basis to the proposed, but not extensively tested, cooperative mechanism to the formation of mesoporous molecular sieves, which suggests that the presence of silicate anions leads to the formation of organic-inorganic mesophases at low concentrations of surfactant.

ASSUNTO(S)

quimica inorganica compostos halogenados silicatos

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