Control of mesostructured silica particle morphology

The formation of mesostructured silica was investigated using acidic conditions with tetraethoxysilane (TEOS) as the silica precursor and cetyltrimethylammonium bromide (CTAB) as the structure directing agent. A synthesis mixture of composition 2.2TEOS : 1.1CTAB : 75HCl ∶ 1000H(2)O gave a well ordered material exhibiting a range of particle shapes from extended rope-like structures of hexagonal cross-section to gyroid structures. Addition of monomethyltriethoxysilane (MTES) or tetrahydrofuran (THF) to the synthesis mixture gave rise to spherical particles. Studies of particle development showed initial growth of gel-like particles within which the mesostructure developed. A new model is proposed for the formation of mesostructured precipitates in which (1) growth of inorganic oligomers gives rise to a thermodynamically unstable mixture, (2) phase separation occurs giving droplets of a second liquid (or liquid crystal) phase rich in oligomer and surfactant, (3) the droplets are colloidally stabilized, (4) microphase separation occurs within the droplets, giving rise to a mesostructure and (5) further polymerization fixes the mesostructure.