Diffusion interference microscopy

The design and application of a very promising novel technique for the measurement of sorbate transport in porous materials, viz. diffusion interference microscopy, is presented and pertinent results obtained by this technique are reported. 

In a recent experimental study of the adsorption of methanol in a large crystal of CrAPO by interference microscopy, Lehmann et al. [36] observed that, even at equilibrium, the distribution of sorbate through the crystal is far from uniform. It seems clear that access is controlled lai ely by the defect structure and the growth planes of the crystal. This observation may provide a plausible explanation for the discrepancies observed between different diffusion measurements. The impact of the defect structure... 

Interference microscopy as a tool of choice for investigating the role of crystal morphology in diffusion studies... 

Measurements by interference microscopy are, under favorable conditions, capable of yielding both internal diffusivities and apparent diffusivities based on overall sorption rates. The former tend to approach the values obtained from microscopic measurements while the latter yield values similar to those obtained by other macroscopic methods. Of necessity these studies have been carried out in large zeolite crystals. One may expect that smaller crystals may be less defective, although the influence of surface resistance may be expected to be greater. The extent to which these conclusions are applicable to the small zeolite crystals generally used in commercial zeolite catalysts and adsorbents remains an open question. 

Fig. 17 Transient concentration profiles in y-direction (i.e., along 8-ring channels) measured by interference microscopy for a adsorption and b desorption of methanol in a large crystal of ferrierite for pressure steps 5 -> 10 and 10 5 mbar. The form of the profiles shows that both surface resistance and internal diffusion (along the 8-ring chan-...

Karge HG, Karger J (2008) Application of IR Spectroscopy, IR Microscopy and Optical Interference Microscopy on Diffusion in Zeolites, in this volimie... 

Keywords Diffusion Co- and Counterdiffusion Adsorption IR techniques Interference microscopy... 

Karger et al. [18,19] and later on developed by Schemmert [20] and Karger et al. [21] to a very sophisticated method, when employing optical interference microscopy (IFM). The design of the experiments using interference microscopy for diffusion studies (DIFM), the data acquisition through this type of experiment, and the evaluation of selected data obtained by DIFM are described in Sect. 3. 

If we abandon the very unlikely case of anisotropic diffusion with principal tensor axes which are not perpendicular to the crystal faces normal, molecular fluxes may quite generally be assumed to be directed perpendicular to the crystal surfaces. Hence, molecular uptake and release may be considered to proceed via one-dimensional diffusion quite generally, as long as the fluxes stemming from different crystal faces do not superimpose upon each other. This includes in particular the initial phases of uptake and release. We shall see that due to this reason, by measuring surface permeabilities, interference microscopy is in general able to quantify the intensity of surface resistances. 

Fig. 45 Comparison of the transient concentration profiles during methanol uptake by the MOF-type crystal as recorded by interference microscopy (symbols) with the corresponding profiles recalculated from the measured diffusivities with surface permeabilities (full line in Fig. 44) which lead to the best fit to the experimental points...

As schematically shown by Fig. 46a, ferrierite contains two mutually intersecting arrays of channels. In comparison with the strictly one-dimensional MOF crystals considered in the previous section, their analysis is additionally complicated by the existence of two rooflike parts on either side of the platelike main crystal body. It turned out, however, that these features did in no way complicate the method of analysis. Contrary to the MOFs, which required an additional activation step after each uptake experiment, methanol in ferrierite proved to be an ideal host-guest system, where one and the same crystal could alternately be subjected to adsorption and desorption without any perceptible change in the sorbate profiles. It were these special conditions under which interference microscopy could be developed to a technique of diffusion measurement in nanoporous materials of unprecedented power [63,65,70,71,88,89]. 

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