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Diffusion medium layer

The parameter refers to the thickness of a stagnant diffusion medium layer at the surface of the solid, where the drug concentration reacBgat steady-state dissolution. Indeed, it is preferable to consideCsto be the solubility ofthe drug in the diffusion layer, since it is the maximum concentration possible in that layer that controls the dissolution rate. Nevertheless, on the basis of this equation, it can still be seen that if the solubility in the dissolution medium was increased, the dissolution rate would also increase. [Pg.429]

The MEA consists of an anodic electrode, PEM, and a cathodic electrode. Because the electrode reactions take place in the MEA, it is the heart of a H2/air PEM fuel cell. The components of an MEA include the anode gas diffusion medium (A-GDM), anode microporous layer (A-MPL), anode catalyst layer (A-CL), PEM, cathode catalyst layer (C-CL), cathode microporous layer (C-MPL), and cathode gas diffusion medium (C-GDM), as shown in Fig. 2.1. A commonly used term is gas diffusion layer (GDL), which actuaUy contains the gas-diffusion-medium layer and the microporous layer. Each component shown in Fig. 2.1 has specific characteristics and functions in fuel cell operation and performance. Therefore, they differ significantly in their design and fabrication. These topics wUl be addressed in the following sections. [Pg.52]

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... [Pg.253]

Figure 1. Schematic outline of the typical dimensions of the various physically relevant layers at the organism/medium interphase cell membrane, cell wall layer, electric double layer, diffusive depletion layer... Figure 1. Schematic outline of the typical dimensions of the various physically relevant layers at the organism/medium interphase cell membrane, cell wall layer, electric double layer, diffusive depletion layer...
While the lipid bilayer has a very low water content, and therefore behaves quite hydrophobically, especially in its core (see Chapter 2 of this volume), the cell wall is rather hydrophilic, with some 90% of water. Physicochemically, the cell wall is particularly relevant because of its high ion binding capacity and the ensuing impact on the biointerphasial electric double layer. Due to the presence of such an electric double layer, the cell wall possesses Donnan-like features, leaving only a limited part of the interphasial potential decay in the diffuse double layer in the adjacent medium. For a detailed outline, the reader is referred to recent overviews of the subject matter [1,2]. [Pg.115]

According to the Gouy-Chapman model, the thickness of the diffuse countercharge atmosphere in the medium (diffuse double layer) is characterised by the Debye length k 1, which depends on the electrostatic properties of the... [Pg.117]

The diffusion boundary layer thickness depends on D, and consequently the viscosity of the medium, r, and the geometry of the microorganism. In the absence of flow, the diffusion boundary layer of large or planar surfaces (n> > (5) can be defined by [40,43] ... [Pg.453]

For example, if fhe DL is used on the side of fhe cell where fhe fuel or oxidant is in gas phase, then this part can be referred to as gas diffusion layer (GDL). When bofh fhe CL and the DL are mentioned as one component, then the name "diffusion electrode" is commonly used. Because the DL is of a porous nature, it has also been called "diffusion medium" (DM) or "porous transporf layer" (PTL). Sometimes the DL is also referred to as fhe component formed by an MPL and a backing layer. The MPL has also been called the "water management layer" (WML) because one of its main purposes is to improve the water removal inside the fuel cell. In this chapter, we will refer to these components as MPL and DL because these names are widely used in the fuel cell indusfry. [Pg.196]

J. H. Nam and M. Kaviany. Effective diffusivity and water-saturation distribution in single- and two-layer PEMEC diffusion medium. International Journal of Heat Mass Transfer 46 (2003) 4595-4611. [Pg.296]

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. [Pg.448]

A more sophisticated and more common treatment of the catalyst layers still models them as interfaces but incorporates kinetic expressions at the interfaces. Hence, it differs from the above approach in not using an overall polarization equation with the results, but using kinetic expressions directly in the simulations at the membrane/diffusion medium interfaces. This allows for the models to account for multidimensional effects, where the current density or potential changes 16,24,46-48,51,52,54,56,60-62,66,80,82,87,107,125 although... [Pg.464]

Figure 11. Tafel plot of flooded porous-electrode simulation results for the cathode at three different values of xp = 2.2nFIfQ 2 02, z=dbK. The z coordinate ranges from 0 (catalyst layer/membrane interface) to L (catalyst layer/diffusion medium interface), the dimensionless overpotential is defined as // = —o FIRT r]oRR, - ), and the ORR rate constant is defined as A = hFFq 2 (Reproduced with permission from ref 36. Copyright 1998 The Electrochemical Society, Inc.)... Figure 11. Tafel plot of flooded porous-electrode simulation results for the cathode at three different values of xp = 2.2nFIfQ 2 02, z=dbK. The z coordinate ranges from 0 (catalyst layer/membrane interface) to L (catalyst layer/diffusion medium interface), the dimensionless overpotential is defined as // = —o FIRT r]oRR, - ), and the ORR rate constant is defined as A = hFFq 2 (Reproduced with permission from ref 36. Copyright 1998 The Electrochemical Society, Inc.)...
These are typically cathode models that include the diffusion medium and perhaps a membrane water flux. Next are the models that treat all of the layers of the sandwich and are only... [Pg.471]

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... [Pg.492]

Often it is necessary to treat diffusion between different layers as three dimensional diffusion. For isotropic minerals such as garnet and spinel (including magnetite), diffusion across different layers may be considered as between spherical shells, here referred to as "spherical diffusion couple." Oxygen diffusion in zircon may also be treated as isotropic because diffusivity c and that Tc are roughly the same (Watson and Cherniak, 1997). If each shell can be treated as a semi-infinite diffusion medium, the problem can be solved (Zhang and Chen, 2007) as follows ... [Pg.534]

This equation was derived above for the movement of a liquid through a stationary solid phase. Its application here to the movement of colloidal particles under experimental conditions that render the liquid medium immobile implies that the solid particle is large compared with the dimensions of the diffuse double layer k 1. It is customary to term this movement of the solid phase electrophoresis. The phenomenon is observed with particles suspended in a liquid (Fig. 6.139). [Pg.295]

The treatment of the diffuse double layer outlined in the last section is based on an assumption of point charges in the electrolyte medium. The finite size of the ions will, however, limit the inner boundary of the diffuse part of the double layer, since the centre of an ion can only... [Pg.181]

The experiments demonstrate the development of a streaming potential in consolidated bentonite clay when flushed by a NaCl-solution of either low or high concentration. The streaming potential measured in our experiments is at least 5 to 10 times larger than values reported for bentonite in the literature. Apparently this is caused by a very low electric conductivity of the bentonite samples studied. This low conductivity might be ascribed to overlapping diffuse double layers on the clay particles, caused by the high compaction and the presence of monovalent ions in the equilibrium solution. The bentonite, thus compacted, will be a very effective medium for active application of electroosmosis. Compared with electrically shorted conditions, chemical osmosis will be reduced when the clay is not short-circuited. [Pg.288]

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. [Pg.65]


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See also in sourсe #XX -- [ Pg.7 , Pg.55 , Pg.286 , Pg.288 , Pg.290 , Pg.291 , Pg.292 ]




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