Relationship between mesostructures and pH conditions for the formation of silica–cationic surfactant complexes

As is well known, hexagonally ordered framework mesostructures have been constructed with negatively charged silica species and positively charged micelles under a basic condition. On the other hand, the hexagonal array was also prepared in an acidic solution. We investigated the formation of the mesostructures consisting of cationic surfactants and silica in the intermediate pH region. Under weakly basic conditions (pH 8–10), bimodal mesostructures originating from textural mesopores as an interparticle space were formed with the miniaturization of the silica grains. The ordered array of the framework in the particles was degraded with a decrease of pH in neutral and weakly acidic solutions (pH 3–7). These structural changes are ascribed to an increase in the influence of neutral micelles as a suppressant of grain growth with a decrease in the negative charge of silica species. Unimodal disordered mesostructures were produced in the vicinity of the iso-electrostatic point of silica (pH ∼ 2) with further miniaturization of the grains and merging of the framework pores and the textural pores. As the acidity of the precursor solution increased, the silica species were charged positively. Consequently, a hexagonal mesostructure was constructed through reversal arrangement of the positively charged silica species and the neutral micelles as a template under a strongly acidic condition (pH < 0).