Fluorescence Probing Investigations of the Mechanism of Formation of Organized Mesoporous Silica

Spectrofluorometry (pyrene and dipyrenylpropane emission spectra) and time-resolved fluorescence quenching have been used to study the effect of addition of a large excess of sodium hydroxide and sodium silicate on the properties of micelles of cetyltrimethylammonium bromide and chloride (CTAB and CTAC). The systems investigated had overall compositions similar to those used when preparing organized mesoporous silica. Our purpose was to test whether the surfactant counterions are effectively exchanged by silicate ions and micelles grow in the presence of silica. The results show the CTAC or CTAB micelle micropolarity is not affected by the presence of the additives. The pyrene lifetime measurements with the CTAB systems indicate that only a small fraction of micelle-bound bromide ions, about 16%, is exchanged by hydroxyl and silicate ions even when they are largely in excess. The presence of the two additives was found to increase very little the micelle aggregation number in CTAB- and CTAC-containing systems, indicating that there is hardly any micelle growth under the experimental conditions used. Last the micelle microviscosity is also little modified by the effect of the additives. These results call for a reexamination of some of the assumptions underlying models used to describe the synthesis of these extremely interesting materials.