| Abstract
| - When cationic surfactants are added to the Stöber process, spherical particles with radially oriented mesoporescan be prepared by precipitation of silica from a solution of ethanol, water, and ammonia. Van Tendeloo andco-workers proposed that these particles form by epitaxial growth of cylindrical assemblies from the facetsof Ia3d cubic (MCM-48) seeds. [J. Phys. Condens. Matter2003, 15, S3037.] Here, we reexamine this hypothesisby detailed characterization of intermediate and final mesoporous silica particles formed from ethanol/water/ammonia solutions. We find that the presence of a cubic core is not required to explain the synthesis ofspherical particles with radially oriented pores. Instead, we hypothesize that the radial orientation originatesat the particle surface because of the preferred alignment of CTAB micelles normal to that interface. Consistentwith previous studies of the Stöber process, we initially observe small, irregular silica/surfactant clusters.After an induction time, these clusters suddenly form spherical particles larger than 100 nm in diameterbecause of aggregation or collapse of weakly bound clusters. No ordered micellar structure forms initially,but shortly after the appearance of large spherical particles, cylindrical surfactant micelles appear and alignperpendicular to the particle/solution interface. The micelles appear to maintain their alignment normal to theinterface even during particle coalescence, supporting the idea that radial orientation originates at the surfacerather than the interior of the particles. A study of particles formed with varying amounts of ethanol suggeststhat ethanol acts as a cosolvent and as a low-dielectric constant solvent to induce cooperative effects onmicelle organization and particle morphology, leading to particles with radially oriented pores at a largeethanol concentration.
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