Mediated diffusion

In SECM feedback studies of FT reactions at ITIES, four main processes contribute to the magnitude of the tip current (Fig. 4) (1) mediator diffusion in phase I between the tip and the ITIES, (2) mediator diffusion in phase 2, (3) the rate of the ET reaction at the ITIES, and (4) charge compensation by ion transport across the ITIES. 

Under conditions where mediator diffusion in phase 2 and ion transport across the ITIES are nonlimiting [conditions (2) and (4)], the steady-state tip current response has been found to be accurately described by the following equations [44,47,48] ... 

Further evidence for the OHb pairs is given in an STM movie recorded at approximately 187 K. The movie shows the OHb pairs separating across the Ob rows via proton exchange with water molecules [19]. Key frames from the movie are shown in Figure 8.5. This water-assisted OHb diffusion mechanism is supported by calculations, which show that the barrier to diffusion is lowered by the exchange with water. The same diffusion mechanism is also observed for isolated OHb and because of the misassignment of OHb and Ob-vacs by Schaub et al. [15], this water-mediated diffusion of OHb was incorrectly reported as oxygen-mediated diffusion of Ob-vacs [32, 33] via a mechanism inconsistent with subsequent isotope studies [34],... 

An argument against the defect mediated diffusion model is the same one used earlier that is, there are not enough defects as determined by ESR (Brodsky and Title, 1969, 1976) or Deep Level Transient Spectroscopy measurements (Johnson, 1983) to account for the motion of all of the bonded hydrogen in a-Si H. This objection is removed if the floating bonds are 104-106 times more mobile than the hydrogen atoms. However, such highly mobile defects would rapidly self-annihilate via the process. 

Substances can be transported across epithelial membranes by simple passive diffusion, carrier-mediated diffusion, and active transport, in addition to other specialized mechanisms, including endocytosis. 

Bilge, S. S., Clausen, C. R., Lau, W., and Moseley, S. L. (1989). Molecular characterization of a fimbrial adhesin, F1845, mediating diffuse adherence of diarrhea associated Escherichia coli to HEp-2 cells. /. Bacteriol. 171, 4281-4289. 

In case II, the mediator is exhausted within the film before reaching the outside. The reaction occurs only in a first-order layer and the current becomes independent of film thickness, half-order in enzyme concentration, k and (mediator diffusion coefficient within the film). 

Fig. 1. Illustration of the predominance of surface-mediated diffusion of Xe in NaY zeolite. The abscissa and ordinate axes give the distance between a Xe atom and the center of a parent and daughter supercage, respectively, during an intercage diffusion event. Reprinted with permission from Ref. 13. Copyright 1991 American Chemical Society.

This situation is very different from that representing Xe in zeolite Y, whereby the surface-mediated diffusion is characterized by a potential barrier to cage-to-cage crossings, consistent with the lower activation energy to diffusion of Ar in zeolite A. As was found for Xe, the surface-mediated... 

The calculations led to predictions of adsorption sites for the nonpolar compounds that are in good agreement with those determined experimentally. The cation site is preferred over the window site. The activation barrier for movement between two cation sites was calculated to be 30 kJ/ mol and that for movement between a cation and a window site 43 kJ/mol. Experimental measurements of activation barriers to diffusion of benzene in faujasites are between 17 and 27 kJ/mol (24). The calculations provide strong support for the mechanism of surface-mediated diffusion for all guest molecules in the limit of infinite dilution and 0 K. The MEPs show that molecules slide along the wall of the supercage, with the plane of the aromatic ring almost parallel to the pore wall. 

The activation volume for diffusion, as measured by the pressure dependence of the diffusivity, is zero to within experimental accuracy [13, 14]. This is unexpected for defect-mediated diffusion, as in such cases, the activation volume for diffusion should consist of the sum of the volume of formation of the defect and the activation volume for the defect migration, and this is usually measurable. 

J.S. Schultz, J.D. Goddard and S.R. Suchdeo, Facilitated Transport via Carrier-mediated Diffusion in Membranes, AIChE 7. 20, 417 (1974). 

In the SECM measurements shown schematically in Fig. 4a, four stages of the overall process may influence the tip current organic mediator diffusion... 

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