Synthesis of Aluminum-Based Surfactant Mesophases Morphologically Controlled through a Layer to Hexagonal Transition

Abstract

The morphologically controlled synthesis of aluminum-based surfactant mesophases by the homogeneous precipitation method using urea is demonstrated, together with a model for the templating mechanism pathway. Dodecyl sulfate surfactant initially forms a layered mesophase with an interlayer spacing of 3.5 nm in which the surfactant molecules are arranged as a bilayer between the linked sheets of aluminum oxyhydroxide groups. The aluminate sheets consist mostly of octahedral Al but contain 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-linking 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 into enlarged particles. The hexagonal mesophase resulting from the lamellae containing comparatively large amounts of tetrahedral Al forms a ringed or curved wormlike morphology, while the phase grown from octahedral Al-enriched forms appears in versatile patterns including spherical, funneled, and tubular particles. Such morphologically versatile characteristics of the hexagonal mesophases reflect those of their precursors produced by the folding of aluminum-based flexible sheets of aluminum oxyhydroxide octahedral groups mixed with their tetrahedral ones at a fraction dependent on urea concentration.

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