Self-Organization Kinetics of Mesoporous Nanostructured Particles Leon Gradon ´ Dept. of Chemical Engineering, Warsaw University of Technology, 00-645 Warsaw, Poland Stanislaw Janeczko Dept. of Mathematics and Information Sciences, Warsaw University of Technology, 00-661 Warsaw, Poland Mikrajuddin Abdullah, Ferry Iskandar, and Kikuo Okuyama Dept. of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi Hiroshima 739-8527, Japan DOI 10.1002/aic.10257 Published online in Wiley InterScience (www.interscience.wiley.com). Spray drying a mixture of silica and polystyrene latex (PSL) colloids above the decomposition temperature of PSL results in the formation of porous silica particles. By appropriate selection of the experimental conditions, such as the use of a tubular temperature, the weight fraction of colloids, and the flow rate of carrier gas, the resulting silica particles contain organized pores with a hexagonal close-packing arrangement. The process by which organized mesoporous silica particles are formed by the spray-drying method was examined using elementary laws of topology. Although a direct test using experimental data was not performed, at least qualitatively, the existence of a “stationary state” at which organized mesoporous particles can be produced with hexagonal close packing could be defined. Deviation from this stationary state results in the formation of unorganized pores as well as a deformed particle shape, rather than spherical ones. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2583–2593, 2004 Keywords: mesoporous particles, nanostructure, polystyrene latex, topology, stationary state Introduction Catalysis, chromatography, the controlled release of drugs, low dielectric constant fillers, sensors, pigments, microelec- tronics, and electro-optics all represent examples of applica- tions of mesoporous materials (see Huo et al., 1997; Ozin, 1992; Velev et al., 1997). Such materials are frequently pre- pared using a colloidal crystal template on a plate substrate (see Park et al., 1998; Velev and Kaler, 2000; Velev et al., 1997; Zakhidov et al., 1998). The procedure typically consists of three main steps: (1) the formation of colloidal crystals; (2) infiltration of the voids between the colloidal beads with other materials [a second colloid, or reactant gases in the case of a chemical vapor deposition (CVD) reactor, or reactant ions in the case of an electrochemical cell]; and (3) the removal of the colloidal beads chemically or thermally, leaving behind a po- rous material that is an inverse replica of the mesostructure of the template. This, however, requires numerous processing steps and is time consuming. The entire process requires sev- eral hours or longer for completion. We recently reported on an aerosol-assisted spray-drying method for producing powder particles, in an organized mesoporous state. As shown by Iskandar et al. (2001a, 2002), this procedure requires only several seconds to reach completion. Both the controllability of the pore size and the morphology Permanent address of M. Abdullah: Dept. of Physics, Bandung Institute of Tech- nology, Jalan Ganeca 10, Bandung 40132, Indonesia. Correspondence concerning this article should be addressed to K. Okuyama at okuyama@hiroshima-u.ac.jp. © 2004 American Institute of Chemical Engineers MATERIALS, INTERFACES, AND ELECTROCHEMICAL PHENOMENA AIChE Journal 2583 October 2004 Vol. 50, No. 10