Hydrothermal Phase Transformation Of Bicontinuous Cubic Mesoporous Material Ams-6

By Author: jayson elliot
Total Articles: 162
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The controlled synthesis of ordered anionic surfactant templated mesoporous silica with cubic structure (AMS-6) is reported via prolonged periods of hydrothermal treatment. The cubic mesophase transforms to hexagonal after 8 days of HT at 100 °C. Surprisingly, the hexagonal phase is stable only for a limited period after which a reversal to the cubic mesostructure is observed. Characterization methods such as powder X-ray diffraction (XRD), electron microscopy (SEM, TEM), N2-isotherms, magic-angle spinning (MAS)29Si NMR spectroscopy, and thermogravimetric analysis (TGA) have been employed to follow structural and textural changes of the materials prepared. Data show that the resultant mesostructure and its textural properties are highly dependent on the period of HT with less unit-cell shrinkage on calcination after extensive HT. Furthermore, evidence of two different solid−solid phase mechanisms during HT is presented. The initial transition is consistent with a restructuring of the surfactant packing and a depletion of the organic moieties from the organo-silica wall as evident from 29Si NMR spectroscopy. The return ...
... to the bicontinuous cubic phase is driven by changes in charge matching at the organic−inorganic interface as a result of increases in the polymerization of the silica wall. The textural properties, and in particular the presence or absence of surface porosity, has been controlled through variations in hydrothermal treatment. These are associated with specific growth directions of cubic AMS-6 crystals. The synthetic method described allows us to easily prepare phase pure and intermediate mesostructured functionalized nanoparticles.

A new mesoporous silica structure with cubic Ia mmetry has been synthesized. The structure has very large unit cell dimensions, up to 250 Å in the as-synthesized form and 222 Å after calcination, and the surface area is around 700 m2/g. The syntheses were done according to well-established synthesis conditions with the triblock copolymers Pluronics P103 or P123, except for the addition of NaI to the synthesis mixture. Small-angle X-ray scattering revealed that the sample has Iad symmetry. According to electron micrographs, the structure is similar to that of MCM-48, and we thus propose that the structure lies on the gyroid minimal surface.

An in-depth study of the synthesis and growth of bicontinuous cubic mesoporous AMS-6 material in the presence of polymeric dispersant P123 is presented, resulting in controlled particle shape attainable over a small range of synthesis conditions. Unlike previous syntheses a remarkable effect on facet formation of the Iad silica particles with cubic morphologies is observed. Trapezoidal icositetrahedra, spherical and elongated morphologies have been prepared with increased faceting on addition of higher concentration of dispersant. Particles are characterised extensively by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy and nitrogen sorption isotherms. Mesoporous Materials produced in this study show textural features suggesting a secondary porous network formed on the external surface of the particle and the origin of this secondary porosity is discussed. Based on the reduced dynamics generated by the presence of the P123 dispersant, we suggest possible growth models for the formation of mainly two different morphologies—the thermodynamically favoured trapezoidal icositetrahedron and elongated particles, the latter not following the expected 10 mm point group symmetry.

Jayson Elliot, For more information on Drug Delivery, Controlled Release, Targeted Delivery, Mesoporous Materials, transdermal delivery system.
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