Ordered mesoporous silicas prepared from both micellar solutions and liquid crystal phases

In the preparation of mesoporous materials it has been shown that the liquid crystalline properties of the surfactant are an important factor in determining the morphology of the mesostructures, In this study the regularity and morphology of the mesopore structure was examined by X-ray diffraction, transmission electron microscopy and BET surface area analysis as a function of surfactant concentration for the TMOS/Brij 56/HCl (0.5 M) system. The preparation of ordered HI mesoporous silica was shown to take place from both a dilute solution of surfactant (1 Brij 56:1000HCl (0.5 M)) and a concentrated surfactant solution (1Brij 56:1HCl(0.5 M)). Ordered H-1 silica monoliths were prepared with 3.6 nm diameter pores and 1.7 nm thick walls from concentrated Brij 56 mixtures. Similarly ordered H-1 silica micron sized powders were prepared with 3.8 nm diameter pores and 2.1 nm thick walls from dilute Brij 56/mixtures, Comparison of the mesostructure of the silicas with the liquid crystal phase behaviour of the binary Brij 56/water system showed a correlation between the order of the mesoporous material and the phase behaviour of the surfactant at the high concentration of surfactant, At the low concentrations of surfactant the binary Brij 56/water phase diagram does not exhibit liquid crystalline behaviour, However, phase separation of the micellar solution, induced by the presence of silica species, resulted in micron sized domains of liquid crystalline material. These tiny domains acted as moulds for the formation of mesoporous silica. (C) 2001 Published by Elsevier Science B.V.