Synthesis of mesoporous silica spheres under quiescent aqueous acidic conditions

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Abstract
A gyroid-to-sphere shape transition has been unveiled in the growth of mesoporous silica morphologies that are synthesized under quiescent acidic aqueous conditions. It can be induced by a decrease of the acidity for a surfactant-based gyroid preparation. As the acidity is gradually lowered from the gyroid domain, the growth process changes from one involving a smooth continuous deposition of silicate–surfactant micellar solute species onto specific regions of an evolving silicate liquid crystal seed, to one in which deposition instead occurs on non-specific regions of the seed. This creates multigranular gyroid morphologies which at lower acidity emerge as sphere shapes. The gyroid-to-sphere metamorphosis appears to correlate with an acidity and/or temperature dependent switch in the mode of formation, from the gyroid involving fast and local polymerization of a growing silicate liquid crystal seed, to the sphere based upon a slower and global polymerization of a silicate liquid crystal droplet. Surface tension will cause such a droplet to adopt a spherical shape, ultimately to be rigidified in the form of a mesoporous silica sphere. Comparative gyroid and sphere information is presented on synthesis-size-shape-channel plan relations, degree of orientational order of the channels, extent of polymerization of the silica, thermal stability and nitrogen adsorption properties. The ability to synthesize 1–10 µm diameter mesoporous silica spheres with a narrow sphere size and pore size distribution portends a myriad of applications in large molecule catalysis, chromatographic separations and nanocomposites.

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