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The assembly of two aminosilanes on silicon dioxide surfaces is investigated in this work. It is found that for 3-aminopropyltrimethoxysilane (APS), a smaller concentration of the silane and trace amounts of water in the deposition medium, an optimum time, and a postdeposition thermal curing are necessary to obtain a high primary-amine content. By optimization of deposition conditions, uniform APS films with a primary-amine content of 88.6% were obtained. The dependence of the primary-amine content on the experimental parameters is related to the extent to which amines are lost to hydrogen bonding with each other or with the substrate surface. Whenp-aminophenyltrimethoxysilane (APhS) was used, the primary-amine content in the film reached 100% and the surface morphology was more uniform than that of APS films under the same conditions. This is attributed to the rigid phenyl component in APhS that reduces opportunities for hydrogen bonding. In a comparison of the immobilization capacities of the different aminosilane substrates for pyromellitic dianhydride (PMDA), it is observed that higher primary-amine content favors higher uptake, and the APhS film yields 100% PMDA coverage. We infer that primary-amine content could be a measure of the film morphology and accessibility of the substrate amine groups.

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The extraction of caffeine from whole coffee beans with supercritical carbon dioxide was studied in a continuous-flo w extraction apparatus. Decaffeination rates were determined as a function of CO, flow rate, temperature and pressure by continuously monitoring the caffeine in the effluent with a flame ionization detector. Soaking the raw beans in water prior to decaffeination enhanced the rate of extraction, which increased markedly with water content. Using CO, saturated with water also increased the rate of extraction. The rate of decaffeination increased with pressure and temperature and was influenced by both intraparticle diffusion in the water-soaked beans and external mass transfer. A mathematical model based on a linear-driving-force approximation of mass transfer and partitioning of caffeine between the water and the supercritical CO, describes the time-dependent process.The partition coefficient for caffeine distributed between water and supercritical CO, the only parameter determined from the dynamic extraction rate data, increases with temperature and pressure. IntroductionDecaffeination of coffee beans has been carried out with a variety of conventional solvents such as dichloromethane (Stahl et al., 1988;McHugh and Krukonis, 1986), but recently supercritical C 0 2 has been the preferred solvent in large-scale commercial processes (Leyers et al., 1991). The selectivity of supercritical CO, for caffeine in green (unroasted) coffee beans is an important property that is exploited in the decaffeination process, as little else besides caffeine in the raw bean is thought to be soluble in supercritical COz (Stahl et al., 1988). Since extraction is performed on the raw beans, flavor ingredients that are developed only during roasting are not removed. Additionally, use of supercritical C 0 2 precludes any post-decaffeination treatment of the beans to remove the solvent, a necessary step if conventional organic solvents are used.This supercritical fluid process is representative of a wide range of possible extractions: (a) to remove unwanted chemicals from the desired matrix material or (b) to separate and isolate a desired chemical from the matrix. A fundamental understanding is needed of the chemical, physical and transport processes that affect such extractions. The aim of the present research is to develop general experimental and theoretical approaches to study these basic phenomena as well as the particular decaffeination process.Supercritical CO, extraction, including the patents for caffeine extraction, is discussed by McHugh and Krukonis (1986) and Paulaitis et al. (1983). Stahl et al. (1988) also provide useful information about the decaffeination of raw coffee beans, which may contain 0.6 to 3 wt Vo of caffeine. The beans are first soaked in water up to a water content of 45 wt. Yo. Essential for satisfactory extraction, this step may change the texture of the cell walls to enhance diffusion, which is considered a rate-determining step, or the water may free the bound caffeine from the coffee matr...

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