STRUCTURAL CHEMISTRY OF ORGANIC-INORGANIC MESOPHASES. A. Monnier, C.C. Landry and G.D. Stucky. Dept. of Physical Chemistry, University of Geneva, 30, Q.E. Ansermet, 1211 Geneve 4, Switzerland.

Since the discovery of highly ordered silicates mesoporous materials of the MCM41 family in 1992 (pore size in the range 20-100 A), much effort has been devoted to the study of organic-inorganic supramolecular self-assemblies and their subsequent polymerization, leading to highly structured mesoporous materials. The research in this field is largely fueled by the search of new materials to be used as heterogeneous catalysts, ultraselective molecular sieves, mesoscopically structured hosts, for optically or biologically active guests, or more marginally, as substrates to enter into the realm of mesoscale electronic devices.

Several morphologies, most of which correspond to various phases already known in the water-surfactant lyotropic liquid crystal systems, were synthetised as silicate mesoporous materials. However, a thorough understanding of the various molecular forces acting cooperatively for the formation of organic-inorganic mesophases is crucial in order to improve the quality and increase the variety of the targeted mesoporous materials. This quest represent a research topic on its own, and it is the goal of this talk to present the current status of this topic at both the experimental and theoretical levels.

The experimental data accumulated to date on the synthesis and characterization of silicates mesoporous materials strongly support the view that a good theoretical model will be most suitably grounded on the aggregation colloids chemistry and on the lyotropic liquid crystal physico-chemistry. Some aspects, specific to the unique ability of silicates to polymerize must also be included to get a coherent picture. A short review of the backgrounds in these fields, with a special emphasis on X-ray diffraction techniques, will be made.

Then, experimental data illustrating the differences and similarities existing between the standard lyotropics liquid crystals and the silicatropic liquid crystals will be presented. This comparison helps to identify the leading forces governing the formation of the silicatropic mesophases and their subsequent polymerization. In turn this allows a model to be presented which is consistent with the behavior of the silicate-surfactant supramolecular assemblies observed so far. The proposed model provides insight into how the various synthesis conditions favor a particular morphology and also suggests possible synthesis modifications to enhance the quality of the final material.

Finally, recent diffraction experiments revealing various phase transitions occurring during the synthesis of the mesoporous material will be presented. The occurrence of these phases transitions will be discussed within the frame of the above proposed model.

As a conclusion, several directions for future investigations will be outlined with their potential consequences for the development of this rapidly growing research field.