Interphase gradient, description

Mass transport processes - diffusion, migration, and - convection are the three possible mass transport processes accompanying an - electrode reaction. Diffusion should always be considered because, as the reagent is consumed or the product is formed at the electrode, concentration gradients between the vicinity of the electrode and the bulk solution arise, which will induce diffusion processes. Reactant species move in the direction of the electrode surface and product molecules leave the interfacial region (- interface, -> interphase) [i-v]. The - Nernst-Planck equation provides a general description of the mass transport processes. Mass transport is frequently called mass transfer however, it is better to reserve that term for the case that mass is transferred from one phase to another phase. 

Solution of this equation requires not only various physicochemical and reaction kinetics data, but also a detailed knowledge of the fluid mechanics near the interface which is however not available in any but artificially simplified agitated system (see, for instance, reference (9) for a detailed review of interphase mass transfer models and description of interfaces). However, the concentration gradient V A and the velocity vector 7 are approximately perpendicular to each other, hence the scalar product of them is always negligible. Furthermore, diffusion is usually unidimensional. Therefore Eqn (1) reduces to ... 

Figure 24.2 Nanostructure of bicontinuous nanocomposites. Top Pictorial description of interconnected organic and inorganic domains with gradient density interphase. Bottom TEM micrograph of an epoxy/silica bicontinuous nanocomposite.

Fig. 1. Two h3rpothetical real materials A and B (a) prior to contact and (b) after contact. Surface and near surface components form the interphase, which contains the interface) s) as well as (gradient) compositions and structures different from the bulk materials. In general, the interphase is quite complex. See text for further description.

The Uquid product water that is formed on the cathode side dilutes the electrolyte and diffuses in part through the membrane as shown by Fig. 18.4. The water diffusion results from a concentration gradient of the phosphoric acid caused by the phase change of water at the interphase in the catalytic layers and the water production at the cathode. In general, the mathematical description of the net-mass flux could be accomplished by an interphase mass transfer theory. The two-film theory [219],... 

The full description of the gradient in surface-to-bulk properties would include a density profile, the macromolecular configurations (center of mass of the molecules), and the positions of chain ends—both parallel and perpendicular to the surface. These details of the state of the polymer at the surface can affect both the mechanical aspects of its performance, such as scratch and mar resistance, and the diffusion coefficient of coatings and of solvent into the surface. Additionally, chemical characteristics such as surface speciation (chain-end concentration) can affect specific interactions with a coating. Over the last 10 to 15 years new surface analytical techniques have been developed that significantly increase our understanding of these characteristics of polymer surfaces and interphases. Some of these analytical techniques are listed in Table 6. 

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