Synthesis of micelle-templated silicas from cetyltrimethylammonium bromide/1,3,5-trimethylbenzene micelles

Micelle-templated silicas (MTS) were synthesized by self-assembly of inorganic silica and micelles of cetyltrimethylammonium bromide (CTAB) containing different amounts of 1,3,5-trimethylbenzene (TMB) at 343 K. Nitrogen sorption isotherms and transmission electron microscopy (TEM) showed that MTS pore size increased with the increase of TMB amount. However, at TMB/CTAB = 5 the material was characterized by double porosity, due to destabilization of the surfactant emulsion (demixed emulsion), whereas at TMB/CTAB = 13 a stable emulsion originated a solid at uniform pore size. The kinetics of formation of these MTS was followed by inserting a radical-surfactant probe in the surfactant mixture and monitoring, by computer-aided EPR analysis, the structural variations of the surfactant aggregates and of the solid over time. In the absence of TMB, the probes monitored the formation of a polar solid at the surface of the micelles. Only at t > 150 min a slow component started contributing to the EPR signal, indicating the formation of the final solid structure. As observed by EPR, at TMB/CTAB = 13, a fast organization of the micelles and a fast condensation of silica accelerated the synthesis process, and in the solid, two probe environments at different polarities were already formed and almost stabilized in the first hour of synthesis. The low polar environment, in which the nitroxide group experienced a relatively high mobility, was attributed to the localization of a portion of TMB in the vicinity of the headgroups of the surfactant aggregates. The proximity of TMB to the silica surface favored and accelerated the formation of siloxane groups. The formation of hydrophobic sites at the surface was also confirmed by H-1 NMR, which indicated a lower proton coordination of the surfactants in the presence of TMB, and was further proved on calcined MTS by the decrease in the C-BET parameter, which is an indicator of surface polarity. At TMB/CTAB = 5, the kinetics, in the first 100 min of the synthesis monitored by EPR, was equivalent to that found for TMB/CTAB = 13, corresponding to the fast formation of the large porosities. Then, at longer synthesis times, the same slow component as for TMB/CTAB = 0 progressively appeared and contributed to the EPR spectra, corresponding to the final formation of small pores. Therefore, in agreement with the nitrogen sorption isotherms and TEM, the solid formed at TMB/CTAB = 5 is originated by the demixing of the surfactant emulsion into two fractions: the first is TMB-rich and provides a kinetics similar to that of TMB/CTAB = 13; the second is deprived of TMB, providing a kinetics similar to that of TMB/CTAB = 0. Definitely, when TMB is used in the NITS synthesis, the synthesis rate increases and the polarity of MTS surface decreases.