Silica Hollow Spheres by Templating of Catanionic Vesicles

A simple and effective means for obtaining hollow silica particles of controlled diameter from about 60 to 120 nm is presented. The synthesis utilizes equilibrium vesicles as templates for the directed growth of silica. Two different surfactant systems are used to form the vesicular templates: (a) mixtures of cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate (FC7) and (b) mixtures of cetyltrimethylammonium tosylate (CTAT) and sodium dodecylbenzenesulfonate (SDBS). These templates were chosen because these mixtures of surfactants in water form unilamellar vesicles spontaneously that appear stable in the chemical environment required for silica synthesis. Tetramethoxysilane (TMOS) is added to the vesicular templates as a precursor for silica formation via acid-catalyzed hydrolysis and polycondensation. The morphology of the silica products as observed with transmission electron microscopy (TEM), quasi-elastic light scattering (QLS), and small-angle neutron scattering (SANS) is consistent with silica deposition at the vesicle surface, creating hollow silica particles with a 1-2-nm-thick shell and with a core diameter identical to that of the template. TEM reveals under different conditions either discrete hollow particles or networks of linked or aggregated hollow silica shells.