Abstract

The effect of an amphiphilic surfactant on the co-assembly of gibbsite and low-dimensional-order, liquid-crystalline mesophases using a hydrothermal-gelation-devitrification route was examined. Crystal growth by this method occurred either in three dimensions or was limited to only two dimensions. Gibbsite and mesophases were synthesized using a cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), as template and a commercial mineral sample as inorganic precursor. The commercial mineral sample contained pyrophyllite, α-quartz, and minor kaolinite. The syntheses were made at pH 10 under hydrothermal conditions followed by equilibration at room temperature. The hydrothermally soluble portion settled at room temperature to form a translucent hydrous gel. This translucent gel turned white after drying on a glass substrate because of the following events based on chemical analysis, X-ray diffraction, and optical/electron microscopy: (1) gibbsite preferentially nucleated at the gel/air and gel/glass interfaces to form spherulites of tabular gibbsite crystals with entrapped droplets; (2) a ~26 å basal-spacing, aluminate-encased, lamellar mesophase formed by 2D growth near the edge of the drying gel; and (3) residual solution in entrapped droplets within the gibbsite phase later devitrified abruptly into an optically isotropic material (an aluminosilicate gel possibly with minor mesophases) with a dendritic morphology. Formation of gibbsite and the lamellar mesophase was initially interface controlled, but later became 2D diffusion-controlled as CTAB concentrations and micelle lengths were increased with crystallization time. A relatively high surfactant/water ratio of the drying gel might account for predominant crystallization of an aluminate-encased lamellar mesophase rather than the hexagonal mesophase known as the Mobil Composition Material MCM-41.

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