Tailoring porous silica films through supercritical carbon dioxide processing of fluorinated surfactant templates

The tailoring of porous silica thin films synthesized using perfluoroalkylpyridinium chloride surfactants as templating agents is achieved as a function of carbon dioxide processing conditions and surfactant tail length and branching. Well-ordered films with 2D hexagonal close-packed pore structure are obtained from sol-gel synthesis using the following cationic fluorinated surfactants as templates: 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl) pyridinium chloride (HFOPC), 1-(3,3,4,4,5,5,6,6,7,8,8,8-dodecafluoro-7- trifluoromethyl -octyl)pyridinium chloride (HFDoMePC), and 1-(3,3,4,4,5,5,6,6,7, 7,8,8,9,9,10,10,10-heptadecafluoro-decyl)pyridinium chloride (HFDePC). Processing the sol-gel film with CO₂ (69-172 bar, 25 and 45°C) immediately after coating results in significant increases in pore diameter relative to the unprocessed thin films (increasing from 20% to 80% depending on surfactant template and processing conditions). Pore expansion increases with CO₂ processing pressure, surfactant tail length, and surfactant branching. The varying degree of CO₂ induced expansion is attributed to the solvation of the "CO₂-philic" fluorinated tail and is interpreted from interfacial behavior of HFOPC, HFDoMePC, and HFDePC at the CO₂-water interface.