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 addit ion of a large excess of sodium hydroxide and sodium silicate on the proper ties of micelles of cetyltrimethylammonium bromide and chloride (CTAB and C TAC). The systems investigated had overall compositions similar to those us ed when preparing organized mesoporous silica. Our purpose was to test whet her the surfactant counterions are effectively exchanged by silicate ions a nd micelles grow In the presence of silica. The results show the CTAC or CT AB 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 h ydroxyl and silicate ions even when they are largely in excess. The presenc e of the two additives was found to increase very little the micelle aggreg ation 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 additi ves. These results call for a reexamination of some of the assumptions unde rlying models used to describe the synthesis of these extremely interesting materials.