Field measurement, sorption processe

Because it is widely recognized that a significant amount of reactivity takes places rapidly during sorption processes, it has become necessary to employ new experimental techniques that can adequately measure these processes with high temporal sensitivity. Techniques such as rapid-scan infrared spectroscopy and quick-EXAFS have shown great promise in the early stages of their development and application in our field. It will be over the next decade(s) that these... 

The results of numerous investigations on the kinetics of sorption of pure substances in zeolites have since then appeared in the literature and the field has been reviewed recently by Walker et al. 42). The total uptake or loss of sorbate in a large number of crystallites is commonly observed, and it is generally assumed that the rate of these processes is controlled by diffusion in the solid. Variable diffusion coefficients were sometimes observed by this method, and it appears possible that other processes than diffusion in the solid had some influence on the rate in these cases. The apparent diffusivity will depend only on concentration (besides temperature) if the migration of sorbate particles in the solid is rate controlling. A simple criterion whether this condition exists can be obtained by measuring sorption or desorption rates repeatedly for various initial concentrations and boundary conditions, as described by Diinwald and Wagner 43). 

Pulsed field gradient (PFG)-NMR experiments have been employed in the groups of Zawodzinski and Kreuer to measure the self-diffusivity of water in the membrane as a function of the water content. From QENS, the typical time and length scales of the molecular motions can be evaluated. It was observed that water mobility increases with water content up to almost bulk-like values above T 10, where the water content A = nn o/ nsojH is defined as the ratio of the number of moles of water molecules per moles of acid head groups (-SO3H). In Perrin et al., QENS data for hydrated Nation were analyzed with a Gaussian model for localized translational diffusion. Typical sizes of confining domains and diffusion coefficients, as well as characteristic times for the elementary jump processes, were obtained as functions of A the results were discussed with respect to membrane structure and sorption characteristics. ... 

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