The in situ phase transitions occurring during bicontinuous cubic phase formation

A novel family of mesoporous molecular sieves (M41-S) has recently been rep orted by scientists of Mobil Oil Research and Development. During its synth esis a base- or acid-catalyzed polymerization of inorganic compounds (silic a in most cases) takes place around surfactant micelles that serve as a tem plating agent. At the end of the synthesis, a silica matrix imbedded by mic elles, namely, a mesoporous material, is obtained. Its mechanism of formati on is not yet completely understood. We have studied the mechanism of forma tion and the phase transition sequence occurring during the synthesis of bi continuous cubic mesoporous phase. It was found that the changes in the sur factant packing parameter, dictated by the time- and temperature-dependent reaction parameters, result in a hexagonal --> lamellar --> hexagonal --> c ubic phase transition sequence. In this article, we present a small angle X -ray scattering (SAXS) in situ study of a new preparation procedure of the bicontinuous cubic (Ia3d) phase in which an organic base is used as a react ion catalyst. The coexisting hexagonal and lamellar phases detected at an e arly reaction stage could indicate their importance in the formation of a c ubic (Ia3d) mesophase. We found that the final cubic phase is formed only w hen heat is applied. The cubic phase is formed by the collapse of a precedi ng hexagonal phase, where a given hexagonal plane (1 0 0) evolves to a cubi c plane (2 1 1) having the same interplanar distance. (C) 2000 Elsevier Sci ence B.V. All rights reserved.