Micellization of CTAB in the presence of silicate anions and the exchange between bromide and silicate at the micelle surface: A step to understand the formation of mesoporous molecular sieves at extremely low surfactant and silicate concentrations

The effect of silicate anions, from dilute aqueous tetramethylammonium silicate (TMASi) solutions (0-3.0 mmol L(-1) in silicon), on the formation of hexadecyltrimethylammonium bromide (CTAB) micelles was investigated by means of a series of simple conductivity experiments. These two compounds are used in the preparation of mesoporous silicate molecular sieves. An increase in the monovalent silicate anion concentration decreases the critical micelle concentration (cmc) of CTAB, as might be expected from the decreased repulsive forces between the polar heads of the surfactant molecules. However, the decrease in cmc values is less pronounced than that observed in the presence of bromide ions, suggesting that Br- binds more strongly than Si(OH)3O- at the micelle surface. Through the ion-exchange formalism, a selectivity coefficient for Si(OH)3O-/Br- exchange of 0.30+/-0.03 was estimated from the conductivity data. This value compares well with that of 0.4+/-0.1 also determined in this work by the pyrene fluorescence quenching method. The experimental results were used to rationalize the formation of a surfactant supramolecular-templated mesoporous molecular sieve at extremely low surfactant (0.63 mmol L(-1)) and silicate (4.00 mmol L(-1)) concentrations.