In Situ Investigations on Cetyltrimethylammonium Surfactant/Silicate Systems, Precursors of Organized Mesoporous MCM-41-Type Siliceous Materials

Organized mesoporous silica (OMS) constitutes new materials with many potential applications. These materials with very well organized pores can be obtained by lowering the pH of precursor systems, i.e., highly alkaline silicate solutions containing cationic surfactants such as cetyltrimethylammonium bromide (CTAB) or chloride (CTAC). The precursor systems of OMS have been investigated prior to precipitation by in situ techniques. ²⁹Si liquid-state NMR spectroscopy has been used to determine the type of silicate species in the investigated systems and how it is affected by the presence of the surfactant. Fluorescence probing techniques with the fluorescent probes pyrene and dipyrenylpropane have been used to study the variation of properties of the CTAB and CTAC micelles upon successive additions of a large excess of base (sodium hydroxide or tetramethylammonium hydroxide) and silicate (water glass or octameric species Si<INF>8</INF>O<INF>20</INF>^8-) as to generate the precursor systems that have the compositions that are used when synthesizing OMS. The effect of the surfactant concentration, the pH, and the nature of the silicate species used as a source of silica has been investigated. The results show that there is little binding of the hydroxyl and silicate ions on the CTAB micelles and little micelle growth in the precursor systems, under the experimental conditions used. Thus, the interactions between the silicate species and the surfactant micelles are weak in the precursor systems used. The absence of any organization in the system prior to precipitation suggests that the first and most important step in the synthesis of organized mesoporous silica is the formation of siliceous prepolymers. As these prepolymers grow, they bind an increasing amount of surfactant ions. At some point the precipitation of a mesomorphous surfactant ion/polymerized silica complex takes place, as is commonly observed in many systems made up of a surfactant and an oppositely charged polyelectrolyte. A mechanism of formation of OMS based on these ideas and supported by experimental results on polyelectrolyte/oppositely charged surfactant systems is presented.