Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases

Self-assembled lamellar silica-surfactant mesophase composites have been pr epared with crystallike ordering in the silica frameworks using a variety o f cationic surfactant species under hydrothermal conditions. These material s represent the first mesoscopically ordered composites that have been dire ctly synthesized with structure-directing surfactants yielding highly order ed inorganic frameworks. One-dimensional solid-state Si-29 NMR spectra, X-r ay diffraction patterns, and infrared spectra show the progression of molec ular organization in the self-assembled mesophases from structures with ini tially amorphous silica networks into sheets with very high degrees of mole cular order. The silicate sheets appear to be two-dimensional crystals, who se structures and rates of formation depend strongly on the charge density of the cationic surfactant headgroups. Two-dimensional solid-state heteronu clear and homonuclear NMR measurements show the molecular proximities of th e silica framework sites to the structure-directing surfactant molecules an d establish local Si-O-Si bonding connectivities in these materials.