Diffusion thin layer-like

Dispersion equations, typically the van Deemter equation (2), have been often applied to the TLC plate. Qualitatively, this use of dispersion equations derived for GC and LC can be useful, but any quantitative relationship between such equations and the actual thin layer plate are likely to be fraught with en or. In general, there will be the three similar dispersion terms representing the main sources of spot dispersion, namely, multipath dispersion, longitudinal diffusion and dispersion due to resistance to mass transfer between the two phases. 

The multipath dispersion on a thin layer plate is the process most likely to be described by a function similar to that in the van Deemter equation. However, the actual mobile phase velocity is likely to enter that range where the Giddings function (3) applies. In addition, as the solvent composition is continually changing (at least in the vast majority of practical applications) the solute diffusivity is also altered and thus, the mobile phase velocity at which the Giddings function applies will vary. 

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Problems in designing cells for semi-infinite diffusion techniques are well known to electrochemists. When the same considerations must be applied to a layer of solution less than 100 pm thick, the difficulties are significantly enhanced. Like most real-world components, thin-layer cells do not always conform to theory (Chap. 3), and design trade-offs are necessary to optimize performance in a particular application. 

The flat, conformal, CVD-like growth of a-Si H is important both for the material properties and for the device technology. Invariably, PVD conditions result in films with a high density of electronic defects, which are associated with the internal surfaces of voids. The columns also oxidize rapidly when exposed to air, because their open structure allows the diffusion of oxygen from the atmosphere. CVD films have a much lower defect density and no oxidation except for a thin layer on... 

Another kind of cell, made by Graham and Curran, was based on an internal reflection crystal [80]. A gold minigrid was mounted directly on a prism (9 x 9 x 45 mm) and on top of this was a zinc selenide prism. The distance (observation) between the minigrid and the prism is typically 13-15 pm, which results in a very short response time. For a potential-step experiment, maximum absorbance is achieved within a couple of seconds. The cell is especially well-suited for potential-scan experiments because the intermediate generated at the electrode will rapidly fill out most of the observation distance even when moderately fast sweep rates (50 mV s ) are applied. Some memory effect is, however, present, because the diffusion layer will not be completely evolved on this timescale. At smaller sweep rates (2 mV s ) all of the observation layer behaves like a thin layer, where the concentrations are in equilibrium with the electrode surface concentrations. The cell has been used to study the reduction process of Fe(CO)s by CV, where it was pos-... 

The diffusional resistance of a thin sheet of material is primarily governed by the solubility of the diffusing species in the material of the sheet and on its thickness. Very thin sheets, like adsorption layers, tend to cause negligible resistence to most solutes. 

We assume that deposition on the sphere is ideal, that is, each collision of a particle with the sphere results in the particle being captured. The factor of Brownian diffusion Dj,r = kTwhere Oj, is the particle s radius, is much smaller than the factor of molecular diffusion, therefore the Peclet diffusion number is Peo = Ua/Dhr 1- By virtue of this inequality (see Section 6.5), the diffusion flux of particles toward the sphere can be found by solving the stationary equation of convective diffusion with a condition corresponding to a thick or thin diffusion-boundary layer. Particles may then be considered as point-like, and the diffusion equation will become ... 

On no account must the chambers be exposed to sunlight but should be placed in diffuse daylight. It is advisable in analytical work to cover the inside of the laboratory window with sheets opaque to UV-light (Firm 34) or an equivalent varnish (Firm 141). For the thin-layer chromatography of substances like carotenes which are sensitive to light, the outside of the chamber is covered with black foil or one works in a dark room in red or green light. 

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