Counterion and alcohol effect in the formation of mesoporous silica

The adsorption of silicate anions onto the cationic micellar surface and the slow condensation of silicate species control the early phase of the alkaline synthesis of mesoporous materials. We found that there exists a well-defined induction time (t_p) of the transition from clear solution to white precipitation gel. The induction time t_p increases with the concentration of added salt NaX. At a fixed concentration of the salts, the t_p decreases in the order: ClO₃^- > NO₃^- > Br^- > SO^2-₄, SO^2-₃ > Cl^- > F^-, with F^- being the most effective ion in forming the hexagonal phase gel. The order in induction time reflects the strength in counter-ion binding of X^- to micelles. The stronger adsorbing X^- would block the adsorption of silicate ions on micelles and delay the formation of the silica-surfactant mesophases. The order of the ion series agrees with the Hofmeister series in many lyotropic systems such as in ion-protein and ion-surfactant interactions. Surfactant chain length and the addition of 1 -alkanols affects the counterion adsorption behavior and they also have strong influences on the rate of formation of the silicate-surfactant mesophase.