(81e) In-Situ Fluorescence Spectroscopy Investigation of Supercritical Carbon Dioxide Swelling of Surfactant Templates in Porous Silica Thin Films

Surfactant templated mesoporous thin films have generated much interest because of their potential application in membrane separation, adsorption, catalysis, biomimetics, and chemical and optical sensors. Tailoring the pore size and structure of these thin films, while maintaining the narrow pore size distribution and high surface area, further increases their function. Fluorinated and hydrocarbon surfactant templated silica thin films are processed in supercritical (sc) CO₂ (69 – 172 bar, 45°C) immediately after casting, with a goal of using the penetration of CO₂ molecules in the tails of the surfactant template to tailor the final pore size. CO₂ processing results in significant increase in pore diameter relative to the unprocessed thin films for the perfluoroalkylpyridinium chloride surfactant templates due to the favorable free energy of mixing of the CO₂ molecules with the ‘ CO₂-philic' fluorinated tail. However, for the hydrocarbon surfactant templates, cetyl trimethyl ammonium bromide (CTAB) and cetyl pyridinium bromide (CPyB), negligible degree of pore expansion is observed with CO₂ processing.

In-situ fluorescence spectroscopy is used to monitor the dynamic changes in the self-assembly process of the surfactant mesophases on CO₂ penetration and also to interpret the difference in pore size increase observed for hydrocarbon and fluorocarbon templates. The preferential partitioning of the pyrene-based probe in either the hydrophobic interior of the micelles or at the ionic silicate-tail interface is demonstrated as a function of surfactant template tail. The responsiveness of these probes to CO₂ processing of the sol gel thin films is also demonstrated. CO₂ processing of surfactant templated films results in flexible modulable steady state (MSS) for a much longer period than unprocessed film synthesis, as indicated by the continuous change in emission spectra of pyrene for more than 24 hours in CO₂ processed CTAB templated film. The present work is the first to use in-situ fluorescence spectroscopy experiments to observe dynamic structural changes in surfactant mesophases due to sc CO₂ penetration.