Formation mechanism of anionic surfactant-templated mesoporous silica

The synthesis mechanism of anionic surfactant-templated mesoporous silica (AMS) is described. A family of highly ordered mesoporous silica structures have been synthesized via an approach based on the self-assembly of anionic surfactants and inorganic precursors by using aminopropylsiloxane or quaternized aminopropylsiloxane as the co-structure-directing agent (CSDA), which is a different route from previous pathways. Mesophases with differing surface curvatures, varying from cage type (tetragonal P4₂/mnm; cubic Pm3̄n with modulations; cubic Fd3m) to cylindrical (two-dimensional hexagonal p6mm), bicontinuous (cubic Ia3̄d and Pn3̄m), and lamellar have been obtained by controlling the charge density of the micelle surfaces by varying the degree of ionization of the carboxylate surfactants. Changing the degree of ionization of the surfactant results in changes of the surfactant packing parameter g, which leads to different mesostructures. Furthermore, variation of the charge density of positively charged amino groups of the CSDA also gives rise to different values of g. Mesoporous silicas, functionalized with amino and quaternary ammonium groups and with the various structures given above, have been obtained by extraction of the surfactant. This report leads to a deeper understanding of the interactions between the surfactant anions and the CSDA and provides a feasible and facile approach to the mesophase design of AMS materials.