Anionic surfactant induced mesophase transformation to synthesize highly ordered large-pore mesoporous silica structures

Successive mesophase transformation induced by an anionic surfactant such as sodium dioctyl sulfosuccinate (AOT) has been demonstrated to fabricate four kinds of large pore mesoporous silica materials in a triblock copolymer F127 surfactant assembly system. The transformation of the highly ordered mesostructures from face-centered cubic (space group Fm3m) to body-centered Im3m then towards two-dimensional (2-D) hexagonal p6m and eventually to cubic bicontinuous Ia3d symmetries has been achieved by tuning the amount of AOT and 1,3,5-trimethylbenzene (TMB). Characterization by small-angle X-ray scattering (SAXS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ sorption isotherms reveals that all mesoporous silica structures have highly ordered regularity in large domains and possess high surface areas, large pore volumes and uniform pore sizes. The expansion of hydrophobic volume in the amphiphilic Pluronic F127 surfactant associated with AOT and TMB molecules in an acidic media is attributed to the observed mesophase transformation. A further swelling of the surfactant micelles can be achieved by adding TMB molecules into the mixed AOT and F127 surfactants system due to their synergistic solubility enhancement, which gives rise to a long-range ordered 2-D hexagonal mesoporous silica structure with very large cell parameter (a = 16.5 nm) and pore size (∼12 nm). The understanding of the blend-surfactant assembly mechanism will lead to a more rational approach for economical and large-scale production of mesoporous materials with controllable structures.