SYNTHESIS OF ALUMINUM-BASED SURFACTANT MESOPHASES MORPHOLOGICALLY CONTROLLED THROUGH A LAYER TO HEXAGONAL TRANSITION

The morphologically controlled synthesis of aluminum-based surfactant mesophases by the homogeneous precipitation method using urea is demon strated, together with a model for the templating mechanism pathway. D odecyl sulfate surfactant initially forms a layered mesophase with an interlayer spacing of 3.5 nm in which the surfactant molecules are arr anged as a bilayer between the linked sheets of aluminum oxyhydroxide groups. The aluminate sheets consist mostly of octahedral Al but conta in some fraction of tetrahedral Al dependent on urea concentration. On further hydrolysis of urea, the layered mesophase is transformed into a hexagonal form through the interlayer condensation and cross-linkin g of the Al-OH groups in any adjacent aluminate sheets. The structural transition accompanies the rearrangement of the bilayered surfactant molecules into a rodlike assembly, followed by the additional growth i nto enlarged particles. The hexagonal mesophase resulting from the lam ellae containing comparatively large amounts of tetrahedral Al forms a ringed or curved wormlike morphology, while the phase grown from octa hedral Al-enriched forms appears in versatile patterns including spher ical, funneled, and tubular particles. Such morphologically versatile characteristics of the hexagonal mesophases reflect those of their pre cursors produced by the folding of aluminum-based flexible sheets of a luminum oxyhydroxide octahedral groups mixed with their tetrahedral on es at a fraction dependent on urea concentration.