Controlling nanopore size and shape by fluorosurfactant templating of silica

Fluorinated surfactants are a special class of surfactants that assemble into aggregates and form novel intermediate mesophases more easily than hydrocarbon surfactants. Despite their unique properties, researchers have only recently begun to explore the use of these surfactants as templates for porous inorganic materials. Here, we report a comprehensive investigation of the use of cationic fluorinated surfactants as templates for ordered nanoporous silica. A homologous series of perfluoroalkylpyridinium chloride surfactants with tail lengths between 6 and 12 carbons is synthesized and characterized. Using these surfactants in aqueous solution, materials are synthesized with pore structures that, as the tail length of the surfactant increases, include uniform wormhole-like pores, ordered 2-D hexagonal pores, and mesh phase pores. The smallest pore diameter observed in this series (2.19 nm by the KJS method) is among the smallest observed for a porous ceramic made with a single chain cationic surfactant. To avoid initial immiscibility of the silica precursor, a series of materials is also prepared from aqueous ethanol using the same series of surfactants. The products in this series include spherical particles with wormhole-like pores, spherical particles with radially oriented close-packed cylindrical pores, flower-like particles with radially oriented slit pores, and holey sheets of silica. For materials prepared in both water and in aqueous ethanol we find, as has been observed for hydrocarbon templates, that the pore sizes increase as the tail length of the surfactant increases, as long as the pore architecture remains the same. Unlike materials prepared with hydrocarbon surfactants, the pore architecture rapidly evolves toward mesh-phase and bilayer structures as the chain length increases. These investigations show that cationic fluorinated surfactants have significant potential not only as templates for controlling the size of familiar pore structures such as close-packed cylinders but also for developing ceramic materials with novel pore architecture.