Entropy-driven helical mesostructure formation with achiral cationic surfactant templates

The development of a simple, synthetic route for helical mesoporous silica, using achiral cationic surfactants as template in a highly concentrated ammonia solution is demonstrated. The hydrated ammonia molecules provided a crowded environment for the rod-like surfactant micelles. The repulsive interaction between ammonium ions and the head groups of cationic surfactants made the surfactant micelles impenetrable for the ammonia molecules, which was similar to the spheres/tube interaction in the entropically driven model. The surfactant micelles took on a helical conformation to maximize the entropy of the system, and the resulting silica possessed well-defined spiral particle morphology and helical mesoporous channels. The particle morphology and the helical pitch of this material could also be systematically controlled by simply varying the ammonia concentration.