Guanidinium vs. ammonium surfactants in soft-templating approaches: Nanostructured silica and zwitterionic i-silica from complementary precursor-surfactant ion-pairs
Résumé
We investigated the behaviour of guanidinium and ammonium surfactants as structure directing agents in template directed hydrolysis-polycondensation processes. For this purpose, a series of guanidinium surfactants was synthesized from long chain substituted primary amines. In a first series of experiments, we compared nanostructured silicas obtained via template directed hydrolysis-polycondensation reactions of TEOS in the presence of either guanidinium or ammonium based surfactants. We observed that typical MCM-41-type silicas displaying 2D hexagonal architecture were obtained in the presence of ammonium surfactants such as cetyl-trimethylammonium bromide (CTAB). In contrast, the use of guanidinium surfactants led to the formation of hollow silica spheres or spherical silica nanoparticles. In a second time, we investigated the use of ammonium and guanidinium surfactants in the formation of periodic mesoporous organosilicas containing ionic groups (i-silica). We studied in particular template directed hydrolysis-polycondensation reactions of zwitterionic ammonium sulfonate and ammonium carboxylate precursors. We observed that both reactions involving the ammonium-sulfonate precursor yielded structured i-silicas. In contrast, hydrolysis-polycondensation reactions of the ammonium carboxylate precursor afforded structured materials only in the presence of the guanidinium surfactant, whereas amorphous materials were obtained in the presence of CTAB. This result reflects specific surfactant-precursor interactions and suggests that the nature of both surfactant and precursor are determinant for the formation of i-silica materials displaying regular architectures on a mesoscopic length scale.