F- MEDIATED SYNTHESIS OF MESOPOROUS SILICA WITH IONIC AND NONIONIC SURFACTANTS - A NEW TEMPLATING PATHWAY

Mesoporous silica precursors were synthesized at room temperature from solutions in the presence of fluoride anions at a pH between 6 and 10 using the cetyltrimethylammonium cation as the surfactant. The solids were obtained from the hydrolysis of SiF62- or mixtures of TMOS and N H4F. When the pH decreases from 10 to 6, the Q(4)/Q(3) ratio increases . This increase is due to the polycondensation of the =SiOH groups whi ch is catalyzed by the fluoride anions. Under these conditions, less s urfactant S+ is incorporated. The decrease of the number of =SiO- grou ps is partially compensated by the incorporation of F- anions into the neutral framework I-0 (S+[(IF-)-F-0] mechanism). Too high an ionic st rength gives rise to low-organized materials. Nevertheless, the increa se of the surfactant concentration improves the organization of the ma terials. By using non-ionic surfactants under similar conditions, the presence of F- enables the formation of materials from reaction mixtur es with low silica/surfactant ratios exhibiting characteristics close to those of the material obtained with conventional cationic surfactan ts.