Processing of Surfactant Templated Nano-Structured Silica Films Using Compressed Carbon Dioxide as Interpreted from In Situ Fluorescence Spectroscopy

• Published in: The journal of physical chemistry. B Vol. 116; no. 38; pp. 11646 - 11655
• Main Authors: Ghosh, Kaustav, Rankin, Stephen E, Lehmler, Hans-Joachim, Knutson, Barbara L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 27.09.2012
• Subjects: Carbon dioxide; Carbon Dioxide - chemistry; Fluorescence; Micelles; Models, Molecular; Molecular Structure; Nanostructures - chemistry; Penetration; Pyrenes; Pyrenes - chemistry; Silicon dioxide; Silicon Dioxide - chemical synthesis; Silicon Dioxide - chemistry; Spectrometry, Fluorescence; Surface-Active Agents - chemistry; Surfactants; Time
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp305113b

The local environment and dynamics of compressed carbon dioxide (CO2) penetration in surfactant templated silica film synthesis is interpreted from the in situ fluorescence emission spectra of pyrene (Py) and a modified pyrene probe. Pyrene emission in cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) templated silica films is monitored immediately after casting and during processing with gaseous and supercritical (sc) CO2 (17–172 bar, 45 °C). The solvatochromic emission spectra of pyrene in CTAB templated films suggest CO2 penetration in both the micelle interface and its interior. An anchored derivative of pyrene, 1-pyrenehexadecanoic acid (C16-pyr), is established for probing CPB films, where the pyrene moiety is preferentially oriented toward the micelle interior, thus limiting quenching by the pyridinium headgroup of CPB. CO2 processing of CPB templated silica films results in an increase in the time scale for probe mobility, suggesting an increased time scale of silica condensation through CO2 processing. The mobility of C16-pyr increases with pressure from gaseous to sc CO2 processing and persists for over 5 h for sc CO2 processing at 172 bar and 45 °C compared to about 25 min for the unprocessed film. The delivery of CO2 soluble solutes to specific regions of surfactant templated mesoporous materials is examined via the nonradiative energy transfer (NRET) between pyrene and CO2-solubilized naphthalene.