Microemulsion Liquid Crystal Templates for Highly Ordered Three-Dimensional Mesoporous Silica Monoliths with Controllable Mesopore Structures
journal contributionposted on 10.02.2004, 00:00 by S. A. El-Safty, T. Hanaoka
Nanostructure composite silica monoliths (HOM-n) with three-dimensional (3-D) structures and controllable pore size were fabricated by using microemulsion lyotropic liquid crystal mesophase as templates under acidic conditions. By addition of alkanes with different molecular size (C6-C19 alkyl chains) into the primary lyotropic liquid crystal mesophase of Brij 56 (C16EO10), quaternary microemulsion liquid crystal phases formed and the mesophase topology of the surfactant was significantly enhanced with 3-D structures. Hydrocarbon molecular size and the degree of solubilization significantly influenced the amphiphilic phase behavior and the mesopore morphological structure. The phase transitions between the different cubic symmetries could be controlled by using a different solubilizing agent in the microemulsion phases of Brij 56 amphiphile. Thereby, the phase transition of primary liquid crystal mesophases in the microemulsion systems were used to fabricate well-defined highly ordered mesoporous silica monoliths HOM-n, including hexagonal P6mm (HOM-2), 3-D hexagonal P63/mmc (HOM-3), primitive-centered cubic Pn3m (HOM-7), Pm3n (HOM-9), and Pm3m (HOM-4) symmetries and body-centered cubic Im3m (HOM-1), bicontinuous cubic Ia3d (HOM-5), and face-centered cubic Fm3m (HOM-10) materials. In these systematic trends, the transitions were primarily governed by the interfacial curvature surface of the amphiphile aggregates. The interactions between alkanes and aggregates substantially influenced the interfacial surface curvatures of micelles, thereby affecting the preferable mesophase structures. However, the primitive cubic (Pm3n) and face-centered cubic (Fm3m) monoliths were the most stable phases among all mesophases, indicating that surfaces with high interfacial curvature were obtained by the addition of long hydrocarbon chain lengths (C10-C19 range). The addition of short lengths, hexane (C6) and heptane (C7), to the aggregate did not significantly affect the shape geometry of the phases, with the exception of the microemulsion lamellar phases formed in the 2-D hexagonal silica monoliths (HOM-2). Our results show that domains with highly curved interfaces were favored in these microemulsion systems. A tailored pore size was obtained within the transition between the 3-D phases with all mesophases. The extent of transition between the phases with enlarged pore dimensions was further studied by varying the amount of solubilizing agent in the microemulsion composition. Large 3-D monoliths of millimeter-sized particles had a wide range of uniform pore size between 30 and 80 Å, high surface area up to 900 m2/g, and thick walls of about 100 Å. Such properties will allow a wide range of mesoscopic applications.
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hydrocarbon chain lengthscrystal mesophaseFm 3 mmicroemulsion lamellar phasessurfacemicroemulsion systemsEOÅ.amphiphilic phase behaviortransitionPm 3 mPm 3 nC 19 rangepore sizeuniform pore sizeBrij 56 amphiphileMicroemulsion Liquid Crystal TemplatesC 6Im 3 mP 6 mmsolubilizing agentControllable Mesopore Structures NanostructurePn 3 mC 19 alkyl chainsmesoporous silica monoliths HOMsilica monoliths