Diffusion pathway

Atomic fluctuation Temporal diffusion pathways onds (ps) (10 -10 s) less... 

Reactions of this type can also occur when the conductivity of one of the phases is very low or practically zero. In these reactions, the sites of reactant lattice destruction and product lattice formation are spatially separated. During the reaction, dissolved species diffuse from the dissolution sites to sites where they undergo further reaction and form the nuclei of the new phase. The length of the diffusion pathway in the sofution depends on the degrees of dispersion of the originaf reactant and resufting product, and most often is between 10 and 10 m. 

Figure 20.2 A skin model showing possible diffusion pathways and its equivalent circuit diagram, (a) Permeation pathways in a two-dimensional skin model, (b) Resistances in a two-dimensional skin model.

Scheuplein RJ (1966) Analysis of permeability data for the case of parallel diffusion pathways. Biophys J 6 1-17. 

Won t a reference electrode (RE) impede the diffusion pathways if placed directly in front of the working electrode ... 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

Schematic diagram of the coupled iron and phosphate cycles during early diagenesis in marine sediments. Light gray ovals and circles represent solid phases black arrows are solid-phase fluxes. White outlined black arrows Indicate reactions white arrows are diffusion pathways. Source From Ruttenberg, K. C. (2003). Treatise on Geochemistry, Elsevier Ltd. pp. 585-643.

The mechanism of action of inert barrier pigments is commonly stated to be to Increase the diffusion pathway to the substrate it is also possible that pigments may tend to block or prevent the formation of pathways for direct ionic conduction to the substrate. 

Fig. 24. Snapshots of a possible CO2 diffusion pathway into the active site of carbonic anhydrase II, as calculated in a molecular dynamics simulation. Tbe CO2 molecule is represented as a stick, and its diffusion pathway is shown by dashed arrows. [Reprinted with permission from Liang, J.-Y., Lipscomb, W. N. 1990) Proc. Natl. Acad. Sci. U.S.A. 87, 3675-3679.1...

Figure 24 shows a possible CO2 diffusion pathway into the carbonic anhydrase II active site, and Fig. 25 shows the binding of CO2 in the hydrophobic pocket (Liang and Lipscomb, 1990). Also in Fig. 25 is the binding of CO2 in this pocket as independently calculated by Merz (1990,... 

The absolute concentration of blood-derived proteins is modulated by the CSF flow rate. In general, the actual CSF protein concentration is determined by the following variables individual serum concentration and individual diffusion pathway, age, site of puncture, and volume of extraction. 

A blood-CSF barrier dysfunction means decreased CSF flow rate. The structures along the diffusion pathway for proteins between blood and CSF are conservative (no leakage ). 

We interprete the above effects as conventional product shape selectivity inside the pore system of zeolite ZSM-5 or ZSM-11, and part of our arguments were presented earlier, in a preliminary note [28]. While the catalyst is on stream, coke is gradually formed and deposits, in part, inside the channel system. As a consequence, the diffusion pathways for product molecules increase. Slim molecules, such as 2,6-dimethylnaphthalene are less affected than... 

DMN in the DMN isomers being greater (84% ) on the precoked catalyst than on the fresh catalyst (70%). This is a well known effect in zeolite catalysis due to the partial deactivation of the acid sites by carbonaceous deposits. The authors explain these results by a product selectivity mechanism inside the pore system of ZSM-5 or ZSM-11. Even though the diffusion pathways for the product molecules decreased by the formation of carbonaceous deposits, the narrow molecules such as 2-MN, 2,6-DMN, and 2,7-DMN were less affected than bulkier ones such as 1-MN and 1,6-DMN. 

Fig. 15. Cluster network model for highly cation-permselective Nafion membranes126). Counterions are largely concentrated in the high-charge shaded regions which provide somewhat tortuous, but continuous (low activation energy), diffusion pathways. Coions are largely confined to the central cluster regions and must, therefore, overcome a high electrical barrier, in order to diffuse from one cluster to the next...

The picture is less clear for molecular crystals when the molecules deviate strongly from a globular form. NMR data and tracer diffusion data are then often in disagreement. Diffusion profiles (In c, vs. distance) are found to be curved, which is usually attributed to additional heterogeneous and fast diffusion pathways. For plastic crystals, this could indicate that many of them possess a highly defective structure. Even for the aromatic ring molecule benzene, which forms a non-plastic crystal, one finds a D (NMR)/D (tracer) ratio on the order of 103. This cannot be understood unless one invokes other than bulk lattice mechanisms of diffusion. 

Bulk rather than film imprinting can be adopted towards increasing the number of the recognition sites in MIP. In the former imprinting, the template molecules determine both the diffusion pathways and interaction sites. Application of a bulk... 

Application of MIP chemosensors imprinted with two different templates can also improve detectability. For instance, polycyclic aromatic hydrocarbons (PAHs) have been determined using polymers molecularly imprinted with two different PAH templates [155]. Compared with MIPs of a single-template imprint, the detection signal of the resonant frequency change was enhanced by a factor of five and LOD for pyrene was as low as 60 pg L 1. This two-template largely improved detectability can be attributed to easier accessibility of the recognition sites through different diffusion pathways. 

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