Counterion and alcohol effect in the formation of mesoporous silica

The adsorption of silicate anions onto the cationic micellar surface and th e slow condensation of silicate species control the early phase of the alka line synthesis of mesoporous materials. We found that there exists a well-d efined induction time (tp) of the transition from clear solution to white p recipitation gel. The induction time tp increases with the concentration of added salt NaX. At a fixed concentration of the salts, the tp decreases in the order: ClO3- > NO3- > Br- > SO32-, SO32- > Cl- > F-, with F- being the most effective ion in forming the hexagonal phase gel. The order in induct ion time reflects the strength in counter-ion binding of X- to micelles. Th e stronger adsorbing X- would block the adsorption of silicate ions on mice lles and delay the formation of the silica-surfactant mesophases. The order of the ion series agrees with the Hofmeister series in many lyotropic syst ems such as in ion-protein and ion-surfactant interactions, Surfactant chai n length and the addition of I-alkanols affects the counterion adsorption b ehavior and they also have strong influences on the rate of formation of th e silicate-surfactant mesophase. (C) 2001 Elsevier Science B.V. All rights reserved.