In situ investigations of the formation mechanism of mesoporous materials via the dynamics and ordering of spin-probes - pH and Si/surfactant effect

In this work we examine the effect of the pH and the Si/surfactant ratio on the rate of formation of MCM-41 at room temperature. The methodology applied is in situ EPR where minute amounts of the spin probe 5-doxyl stearic acid are added to the reaction gel and the EPR spectrum is followed during the course of the reaction. Within the basic pH range where MCM-41 can be obtained, the pH increase leads to a faster rate of formation while in the acid pH range, a decrease of the pH leads to faster reaction. This is consistent with the hydrolysis of the tetraethyl-orthosilicate being a rate determining step. However, when the pH values are slightly above or below the basic pH range which generates MCM-41 (keeping all other components constant) the reaction is considerably slower. Comparison of reactions with different Si/surfactant ratios showed that the polymerization of the silicate oligomers occurs at the interface of the surfactant aggregate and that its rate is determined by the density of oligomers at the interface. Finally, in situ EPR was also used to determine the factors that control the incorporation of Mn(11) into the silica matrix under acidic conditions.