Synthesis of micro-mesoporous bimodal silica nanoparticles using lyotropic mixed surfactant liquid-crystal templates

Abstract

Micro-mesoporous bimodal silica nanoparticles with a particle diameter of as small as 40–90 nm have been synthesized by a two-step reaction based on the polymerization of silicate (TEOS) species confined to the mixed surfactant hexagonal-structured liquid-crystal (LC) templates of nonaethyleneglycol dodecylether (C 12EO 9) and polyoxyethylene (20) sorbitan monostearate (Tween60) or eicosaethyleneglycol octadecyl ether (C 18EO 20). After pre-aging for water-insolubilization, the LC phase was kept in contact with dilute aqueous solution of ammonium acetate to achieve full condensation of silicate species. The catalyst-free pre-aging treatment induces the separation of the LC phase into domains and the subsequent water-phase-in-contact process serves to maintain the hexagonal framework by removing the ethanol evolved through hydrolysis. On calcination the hexagonal array of mixed surfactant cylindrical micelles in the LC templates is converted into a hexagonal structure of mesopores with some irregularity in pore arrangement, separated by silica walls with irregularly arranged micropores. Throughout aging and calcination, the hexagonal framework of the silicate-loaded LC or condensed particles in the Tween60 based mixed surfactant system contract to yield silica nanoparticles with mesopores of ∼5 nm diameter and ∼2 nm-thick silica walls. In contrast, the framework in the other system remarkably expands to form mesopores of 4.6 nm diameter and silica walls of as thick as 4.7 nm, along with 1.5–2 times larger micropore volume and specific surface area. The striking difference between the structural parameters in both systems is arisen from the opposing effects due to the hydrophilic triple-branched or linear polyoxyethylene (PEO) chains of Tween60 and C 18EO 20 molecules.