Tuning the mesopore structure in the shells of hollow silica and organosilica particles formed by fluorosurfactant templating

Hollow ceramic and organic-inorganic hybrid particles with macropore cavities (> 50 nm in diameter) and mesoporous shells (by definition, pores 2-50 nm wide) have tremendous potential utility as designed nanosystems for controlled adsorption, storage, and release of solutes. The hollow cavity provides a large volume for storage of solutes, while the shell can be designed for controlled dissolution (e.g. a multilamellar structure), controlled diffusion (dictated by pore size and orientation) and even gated diffusion of solutes. If controlled, these properties can lead to applications in targeted drug delivery, high-capacity fuel storage, enzyme encapsulation, and multifunctional catalysis. We have recently developed several methods to produce useful hollow mesoporous structures. Here, we will describe the use of partially fluorinated cationic surfactants to spontaneously form hollow particles from alkoxysilane-based precursors in solution, without needing a secondary template for the hollow cavity. Fluorinated surfactants are useful for this purpose because their large tail volume favors the formation of low-curvature aggregates such as cylinders and bilayers (rather than spheres and cylinders). One type of hollow structure is formed by a short, nearly perfluorinated surfactant, 1H, 1H, 2H, 2H-perfluorodecylpyridinium chloride (HFDePC). This surfactant forms ceramic particles with hollow cavities when either on its own or when it is mixed with cetyltrimethylammonium chloride (CTAC). Another partially fluorinated surfactant which we recently synthesized, 10-perfluorooctyldecylpyridinium bromide, promotes the formation of multilamellar hollow particles by precipitation with ethoxysilanes in dilute aqueous ammonia. Because the fluorinated surfactant is lipophobic, we are able to produce not only silica but also organosilica multilamellar hollow particles.