Large- and small-nanopore silica prepared with a short-chain cationic fluorinated surfactant

A cationic partially fluorinated surfactant with four carbons in the chain 1-(3,3,4,4,4-pentafluorobutyl)pyridinium chloride is employed as a structure-directing agent to synthesize nanoporous silica. Samples are prepared in dilute ammonia solutions at room temperature with a range of surfactant:Si ratios. The sample with the largest surfactant:Si ratio forms particles with wormhole-like micropores with an average diameter of 1.6 nm, which corresponds to the anticipated small size of the surfactant aggregates. On the other hand, the sample with the smallest surfactant:Si ratio forms a gel that, upon drying, has uniform 11.1 nm pores. The formation and stabilization of the latter large-mesopore structure is unusual for a sample prepared and dried under ambient conditions, and may reflect favourable roles of the surfactant both in inducing gelation and in stabilizing the pore structure during drying.