Pore Expansion in Fluorinated and Hydrocarbon Surfactant Templated Silica Thin Films Due to Supercritical Carbon Dioxide Processing

Kaustav Ghosh1, Stephen E. Rankin1, Barbara L. Knutson1, and Hans-Joachim Lehmler2. (1) Chemical and Materials Engineering, University of Kentucky, 177 F. Paul Anderson Tower, Lexington, KY 40506-0046, (2) University of Iowa, 222 IREH , Department of Occupational and Environmental Health, Iowa City, IA 52242-5000

The synthesis of surfactant templated thin films in the presence of supercritical (sc) CO2 can be used to capture and interpret the pressure-tunable effects of CO2 on surfactant mesophases. Significant differences in the penetration of sc CO2 in fluorocarbon and hydrocarbon surfactant mesophases has been observed through processing of the respective surfactant templated films in sc CO2 immediately after casting. The ability to tailor CO2-induced pore expansion is compared for fluorinated and hydrocarbon pyridinium chloride surfactants (cetyl pyridinium bromide (CPyB); 1-(3,3,4,4,5,5,6,6,7,7,8,8, tridecafluorooctyl) pyridinium chloride (HFOPC) and 1-(11,11,12,12,13,13,14,14,15,15,16,16, tridecafluorocetyl) pyridinium chloride (HFCPC)). CPyB and HFCPC have equal number of carbons in the surfactant chain. HFOPC and HFCPC have an identical number of fluorocarbon groups (-C6F13) but differ by the number of hydrocarbon spacers (-C2H4- and C10H20-, respectively). Well-ordered thin films with 2D hexagonal pore structure are obtained for both unprocessed films and CO2-processed films (69 172 bar and 25oC - 45oC for 72 hours). HFOPC templated films underwent large pore expansion (varying between 20% - 45%) compared to unprocessed films with increase of CO2 pressure while a slightly lesser degree of pore expansion was observed for HFCPC templated films. However for both hydrocarbon surfactant templates, negligible pore size increase was observed on CO2 processing. The large degree of pore expansion observed for fluorinated surfactant templated films results from significant penetration of small CO2 molecules in the fluorinated tail due to favorable interactions between CO2 and CO2-philic' fluorinated moieties arising from their similar cohesive energy densities and low surface tension. Simulation and experimental studies have previously compared CO2 penetration in fluorinated and hydrocarbon tail in microemulsion systems and at CO2-water interfaces. However, the current experiments are the first to directly compare CO2-induced swelling in fluorinated and hydrocarbon surfactant mesophases. This work also demonstrates the ability to tailor the pore size in mesoporous silica thin films without loss of ordering by combining the tunable solvent strength of compressed and sc CO2 and fluorinated surfactants.