Counter-diffusion measurements

Very few measurements have been reported thereafter due to experimental complications [42]. In order to overcome these difficulties, a new simplified relative permeability technique has been demonstrated recently by Steriotis et al. [10]. A similar technique, known as permporometry, has been developed [43-45], based on counter diffusion measurement through the partially blocked membrane. 

Formulae (49) and (89) have been experimentally tested by some investigators. R. Neihof and K. Sollner (109) compared the transport ratios obtained from B.I.P. s with those obtained from diffusion measurements. In their experiments two counter-ions 1 and 2 occurred at the a side of the membrane, and at the m side there was a third counter-ion. In this case,... 

The ZLC method offers advantages of speed and simplicity and requires only a very small adsorbent sample thus making it useful for characterization of new materials. The basic experiment using an inert carrier (usually He) measures the limiting transport difiiisivity (Do) at low concentration. A variant of the technique using isotopically labeled tracers (TZLC) yields the tracer diffiisivity and counter diffusion in a binary system may also be studied by this method. To obtain reliable results a number of preliminary experiments are needed, e.g. varying sample quality, nature of the purge gas, the flow rate and, if possible, particle size to confirm intracrystalline diffusion control. 

The ramifications of these findings are many. Unlike Knudsen difusion, the rate of diffusion in one direction is affected markedly by the opposite flux. Adsorption or desorption measurements cannot be used to approximate counter diffusion rates these must be determined independently. They are a function of the nature of the zeohte, the type of cation within the pore structure, and the nature of the counterdiffusing species. 

The isobaric counter-current diffusion measurements in the modified Wicke-Kallenbach cell employ the validity of the Graham law which states that under isobaric... 

Fig. 9.17. Experimental ( ) and theoretical flux ratio of H2 and N2 at 25°C on a nonsupported Y-Al203 layer modified with 17 wt% silver, measured in counter-diffusion configuration. After Uhlhom et...

Abstract Infrared spectroscopic methodsfor the measurement of adsorption and adsorption kinetics of some aromatics (benzene, ethylbenzene, p-xylene), pyridine, and paraffins in solid microporous materials such as zeolites (MOR, ZSM-5, silicalite-1) are described as well as the evaluation of the spectroscopically obtained data. The adsorption isotherms are of the Langmuir-Freundlich type. Isosteric heats of adsorption, transport diffusivities, and activation energies of diffusion as deduced from the spectroscopic measurements are compared with literature data as far as available, and they are found to be in reasonable agreement with results provided by independent techniques. Special attention is paid to sorption and sorption kinetics of binary mixtures, especially the problems of co- and counter-diffusion. ... 

In Fig. 8, a set of uptake curves for ethylbenzene adsorbed and desorbed at various temperatures after various partial pressure changes has already been shown. The diffusivities of benzene and p-xylene derived from IR measurements in the same way as described for ethylbenzene are collected in Tables 3 and 4. Table 3 includes data for single-component diffusion as well as for co-and counter-diffusion (see Sect. 2.2.4.1)... 

The FTIR technique has proven to be a powerful method for investigating adsorption, desorption, and diffusion of single components or binary mixtures in microporous solids such as zeolites. In the latter case of mixtures, the phenomena of codiffusion and counter-diffusion became accessible to measurement, which was not possible with methods of investigation based on changes of weight, volume, or pressure. Even with the powerful and most important NMR techniques (see Chap. 3 of the present volume), the study of multicomponent (e.g., H2-D2) self-diffusion rather than co- and counterdiffusion experiments is possible (see Sect. 1 and [6]). The only prerequisite for the IR method is that the IR spectra, which are contributed by the components of the mixture, can be sufficiently decomposed. This, however, was easily achieved for all systems studied so far, owing to appropriate computer programs nowadays available. Certainly, the computational methods... 

Besides measurements, there are potential benefits in the observation of other nuclides for diffusion measurements. There may be fewer problems with resonance overlap of different species or less interference from solvent or impurity resonances, for example, or the compound of interest may simply lack hydrogen as is often the case with counter-ions. In this case, one should be aware that optimum parameters for measurement may not be suitable for nuclides of lower magnetogyric ratio, 7. This is because the total effective gradient strengths employed also depend on this parameter and, as can be seen from the Stejskel-Tanner equation (Eq. (9.6)), the degree of attenuation of resonance intensity will be reduced as 7 becomes smaller. This is illustrated in Fig. 9.23 where the decay profiles for four different nuclides are shown, having been calculated with identical diffusion parameters but with the appropriate 7... 

Diffusion measurements (Hartley and Runnicles ) show up the like influences of the counter ions on the micelle size. 

Ruthven, D.M., and Stapleton, P., Measurement of liquid phase counter-diffusion in zeolite crystals by the ZLC method, Chem. Eng, Sci., 48(1), 89-98 (1993). 

Lindman and co-workers [21-23] demonstrated that the organisation and structure of microemulsions can be elucidated from self-diffusion measurements of all the components (using pulse-gradient or spin-echo NMR techniques). Within a micelle, the molecular motion of the hydrocarbon tails (translational, reorientation and chain flexibility) is almost as rapid as in a liquid hydrocarbon. In a reverse micelle, water molecules and counter ions are also highly mobile. 

Impedance spectroscopy (IS) measurements were carried out by using an impedance analyzer (Solartron 1260) controlled by a computer, which was connected to the solution in both half-cells via Ag/AgCl or platinum electrodes, which were larger that the membrane area to ensure the uniformity of the electric field (border effect can be neglected) and small holes to permit solution counter-diffusion and reduce concentration polarization near the membrane. The... 

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