Sorption and desorption processes of cationic dyes occurring in single silica gel microparticles in an aqueous solution were kinetically studied by microcapillary manipulation/injection and microabsorption methods. The sorption and desorption rates of rhodamine 6G were limited by diffusion of the dye in the particle interior. For methylene blue, the desorption rate was slow compared with the sorption rate, and the rate-determining step was not the diffusion of the dye. The sorption and desorption processes of the dyes were discussed in terms of intraparticle diffusion as well as of that accompanied by slow desorption.

Experimental Section Materials. 2-(Dimethylamino)ethyl methacrylate (DMAEMA) and 2-hydroxyethyl methacrylate (HEMA) were purchased from Nakalai Tesque Inc. (Kyoto, Japan) and used following purification by vacuum distillation. N-Isopropylacrylamide (NIPAM) was purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) and recrystallized three times from dichloromethane. 2-Methacryoyloxyethyl phosphorylcholine (MPC) was kindly gifted from Prof. Ishihara, Tokyo University and used without further purification. α,α´-Azobisisobutyronitrile (AIBN) from Kanto Chemical Co., Inc.

The sorption of rhodamine 6G into single silica gel microparticles possessing pore sizes of 3, 6, 15, and 30 nm was analyzed using single microparticle injection and microabsorption methods. The intraparticle diffusion of the solute was highly influenced by the pore size of the silica gel. The relationship between the intraparticle diffusion coefficient and a hindrance parameter for the diffusion in the pores is discussed on the basis of a pore and surface diffusion model.

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