Demixed micelle morphology control in hydrocarbon/fluorocarbon cationic surfactant templating of mesoporous silica

We report a study of the coassembly behavior of mixed hydrocarbon surfactant cetyltrimethylammonium chloride (CTAC) and the fluorocarbon surfactant 1H,1H,2H,2H-perfluorodecylpyridinium chloride (HFDePC) with precipitated silica at room temperature. This pair of surfactants is known to segregate into demixed fluorocarbon-rich and hydrocarbon-rich micelles in dilute solution for a range of compositions. Synthesis parameters (including the molar composition of the surfactant mixture, the ammonia concentration, and the addition of NaCl or ethanol to the initial sols) are varied to show how the mixed or demixed micelles found in dilute aqueous solutions act as templates to form mesoporous silica. Four distinct types of pore structures are found. The first are particles with a single mesopore size and structure whose morphology is influenced by the addition of a second surfactant. Adding HFDePC to CTAC for this series of samples induces a transition from 2D hexagonal mesopores to disordered mesopores to mesh phase pores. At intermediate HFDePC compositions, large voids (templated by HFDePC vesicles) are introduced. A second class of particles, with a biphasic 2D hexagonal/mesh phase structure, is formed when the amount of ammonia increases. The third class of particles, with separate fluorocarbon and hydrocarbon micelles combined in a single disordered phase, forms when a moderate amount of salt is added, most likely due to a reduction in the rates of coassembly and precipitation. Finally, a sample consisting of two types of particles with differing shape and pore structure is prepared by adding ethanol to enhance the rate of precipitation. The conditions explored here can be used for the design and synthesis of mesoporous silica with controlled pore structures and morphologies.