Supercritical carbon dioxide processing of mesoporous silica templated by fluorinated surfactants

The tailoring of nanoporous silica materials (thin films and powders) synthesized using perfluoroalkylpyridinium chloride surfactants as templating agents is achieved as a function of carbon dioxide processing conditions and surfactant tail structure. Processing the sol gel materials with CO₂ (69-172 bar, 25 °C and 45 °C) immediately after coating (thin film formation) or after onset of precipitation (base-catalyzed particle formation) results in significant increases in pore diameter relative to the unprocessed materials. Well ordered mesoporous material is also achieved using a traditional cationic surfactant template, hydrocarbon analog cetyl pyridinium bromide (CPB), in the presence of CO₂. However, the pore expansion of the hydrocarbon templated material is negligible at the CO₂ pressures investigated in this study. The dramatic pore expansion achieved in fluorinated surfactant templated silica suggests a favorable penetration of CO₂ molecules into the fluorinated tails compared to hydrocarbon surfactants. In addition to tailoring the structure of nanoporous silica, materials synthesis in the presence of supercritical CO₂ can be used to capture and interpret the pressure-tunable effects of CO₂ on surfactant mesophases.