Monodisperse Mesoporous Microparticles Prepared by Evaporation-Induced Self Assembly Within Aerosols
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Monodisperse Mesoporous Microparticles Prepared by Evaporation-Induced Self Assembly Within Aerosols Shailendra Rathod, G. V. Rama Rao, Brett Andrzejewski, Gabriel P. López, Timothy L. Ward, C. Jeffrey Brinker and Abhaya K. Datye University of New Mexico, Department of Chemical and Nuclear Engineering Albuquerque, NM 87131
ABSTRACT Evaporation induced self assembly (EISA) within microdroplets produced by a vibrating orifice aerosol generator (VOAG) has been used to produce monodisperse mesoporous silica particles. This process exploits the concentration of evaporating droplets to induce the organization of various amphiphilic molecules, effectively partitioning the silica precursor (TEOS) to the hydrophilic regions of the structure. Promotion of silica condensation, followed by removal of the surfactant, provides ordered spherical mesoporous particles. Using the VOAG we have produced highly monodisperse particles in the 5 to 10 µm diameter range. The cationic surfactant CTAB typically leads to hexagonal mesostructure with mean pore size of about 2 nm and specific surface area around 900 m2/g. We have also shown that the pore size in CTABtemplated particles can be increased to 3.8 nm by incorporating trimethylbenzene as a swelling agent. The TMB prefentially locates inside and swells the hydrophobic regions of the surfactant mesostructure. Pore size can also be varied by the choice of amphiphile. Hexagonally ordered particles have been produced using the nonionic surfactant Brij-58 and block copolymer F127. These powders possessed mean pore size 2.8 nm and 6.9 nm, respectively. The uptake of alkyl pyridinium chloride molecules have recently been measured, revealing an uptake capacity that is explained by surface adsorption (as opposed to simple pore infiltration). Kinetics of the uptake process are still be analyzed.
INTRODUCTION The self-assembly of amphiphilic molecules has been widely investigated to template inorganic precursors in the synthesis of highly ordered mesoporous materials, most notably silica. The traditional process, first pioneered by researchers at Mobil [1], is based on thermodynamically-driven assembly of liquid crystalline phases from a bulk solution, typically leading to precipitation of the ordered material from solution. We have recently explored the use of evaporation-driven concentration of solutions as a means to drive the self-assembly process in thin films [2,3] and small (micron to submicron) droplets [4,5]. We have demonstrated that a variety of highly ordered mesostructures in spherical submicron silica particles can be produced using this “evaporation-induced self assembly” (EISA) process with aerosol droplets [4,5]. As in the traditional bulk synthesis, the mesostructure is controlled by choice (geometry) of the amphiphile, in conjunction with the solution properties. Mesoporous particle synthesis based on EISA has several significant advantages over traditional bulk synthesis. It typically produces spherical particles, whose size and size distribution is governed by the
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