Synthesis of mesoporous silica spheres under quiescent aqueous acidic conditions

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 mu m 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.