Supercritical carbon dioxide processing of fluorinated surfactant templated mesoporous silica thin films

The effect of processing mesoporous silica thin films with supercritical CO ₂ immediately after casting is investigated, with a goal of using the penetration of CO ₂ molecules in the tails of fluorinated surfactant templates to tailor the final pore size. Well-ordered films with two-dimensional hexagonal close-packed pore structure are synthesized using a cationic fluorinated surfactant, 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10- heptadecafluorodecyl)pyridinium chloride, as a templating agent. Hexagonal mesopore structures are obtained for both unprocessed films and after processing the cast films in CO ₂ at constant pressure (69-172 bar) and temperature (25-45°C) for 72 h, followed by traditional heat treatment steps. X-ray diffraction and transmission electron microscopy analysis reveal significant increases in pore size for all CO ₂-treated thin films (final pore diameter up to 4.22 ± 0.14 nm) relative to the unprocessed sample (final pore diameter of 2.21 ± 0.20 nm) before surfactant extraction. Similar pore sizes are obtained with liquid and supercritical fluid treatments over the range of conditions tested. These results demonstrate that combining the tunable solvent strength of compressed and supercritical CO ₂ with the "CO ₂-philic" nature of fluorinated tails allows one to use CO ₂ processing to control the pore size in ordered mesoporous silica films.