Pore size engineering in fluorinated surfactant templated mesoporous silica powders through supercritical carbon dioxide processing Kaustav Ghosh a , Sarah Bashadi a , Hans-Joachim Lehmler b , Stephen E. Rankin a , Barbara L. Knutson a, * a Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506-0046, United States b Department of Occupational and Environmental Health, University of Iowa, Iowa City, IA 52242-5000, United States Received 9 July 2007; received in revised form 1 November 2007; accepted 6 November 2007 Available online 24 November 2007 Abstract Pore expansion of fluorinated surfactant templated mesoporous silica powders is demonstrated as a function of pressurized CO 2 pro- cessing conditions. Mesoporous silica powder is synthesized by sol–gel reaction induced precipitation in a base-catalyzed medium using 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyl)-pyridinium chloride (HFOPC) as a template and, immediately after filtration, the precip- itated material is processed in gaseous and supercritical CO 2 (88–344 bar, 45 °C) for 48 h. Characterization of the silica powders by XRD, TEM and N 2 adsorption reveals the formation of well-ordered materials with 2D hexagonal close-packed pore structure before and after CO 2 processing. An optimal aging time (time from addition of silica precursor to the sol until the filtration of the hydrolyzed sol) of 20 min prior to CO 2 processing is identified. Proper aging time results in silica powder with significant pore expansion at all pro- cessing pressures while retaining the long-range structure of the material. The pore diameter of the mesoporous material increases with increasing CO 2 pressure (from 2.60 nm (unprocessed) to 3.21 nm at 344 bar), but appears to level off above 100 bar. The pore expansion behavior is attributed to favorable CO 2 penetration in the ‘CO 2 -philic’ fluorinated tails of the surfactant template. The CO 2 expansion of base-catalyzed silica powders is significantly less than we previously observed for acid catalyzed, evaporation-driven thin film synthesis using fluorinated cationic surfactant templates. The effect of pH on self-assembly and increased silica condensation in basic conditions may inhibit pore expansion by CO 2 . Ó 2007 Elsevier Inc. All rights reserved. Keywords: Mesoporous silica; Pore expansion; Supercritical carbon dioxide; Self-assembly; Aging time 1. Introduction The base-catalyzed sol–gel synthesis of mesoporous sil- ica proceeds through a co-assembly process, in which the surfactant molecules form well-ordered supramolecular templates and the hydrolyzed silica precursor associates with the surfactant head group [1]. Under basic conditions, hydrolyzed silica precursors initially precipitate with sur- factant micelles to form composite particles that are solid- ified by condensation of the silica network. Finally, removal of the surfactant templates results in mesoporous silica with the desirable properties of uniform pore size dis- tribution and high surface area. The ordered nanostruc- tured materials have diverse applications in the fields of separations, adsorption, catalysis, biomimetics and sensing [2–6]. The ability to tailor the mesoporous materials has further increased their applications, as recently demon- strated in drug delivery, chromatographic and electrode applications [7–9]. Traditionally, the synthesis of pore expanded surfactant templated materials is accomplished through the addition of an inert swelling agent (e.g., poly- propylene oxide or dodecane) in the micellar solution. The swelling agent interacts favorably with the surfactant tails or forms an inner core in the micelle [10–13]. However, 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.11.017 * Corresponding author. Tel.: +1 859 257 5715; fax: +1 859 323 1929. E-mail address: bknutson@engr.uky.edu (B.L. Knutson). www.elsevier.com/locate/micromeso Available online at www.sciencedirect.com Microporous and Mesoporous Materials 113 (2008) 106–113