A Study of the Assembly Mechanism of the Mesoporous MSU-X Silica Two-Step Synthesis

Several synthetic pathways of mesostructured porous silica using nonionic surfactants or copolymers have been reported since the discovery of this new family of materials. We developed a process, which we call a two-step synthesis, that allows us to synthesize MSU-X mesoporous silica, with very good reproducibility and high reaction yields. This reaction implies a first assembly step between silica oligomers and nonionic micelles, followed by a condensation step induced by the addition of sodium fluoride as catalyst. We characterized nanoscopic hybrid objects made of silica and nonionic poly(ethylene oxide) (PEO) surfactants obtained once the assembly between silica and surfactants had occurred, as intermediates in the two-step synthesis of mesoporous silica of the MSU-X family. NMR, small-angle X-ray scattering, and dynamic light scattering experiments allowed us to propose a structural model involving nucleophilic participation of external OH groups, where a low-density framework of silica grows out of the micellar hydrophilic outer shell made of PEO chains, and builds a third shell around the initial spherical micelle. Stable and isolated micellar hybrid objects with this three-layered structure (alkyl core, PEO first shell, silica second shell) were observed, forming monodisperse objects with diameters adjustable between 7 and 12 nm, depending on the amount of silica.