Diffusion film/layer

Fig. 17. Schematic cross section of Agfachrome-Speed film. During processing of this single-sheet film, dyes released by reduction in unexposed areas diffuse from layers of the negative to the image-receiving layer to form the positive color image (5).

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

The basic assumption for a mass transport limited model is that diffusion of water vapor thorugh air provides the major resistance to moisture sorption on hygroscopic materials. The boundary conditions for the mass transport limited sorption model are that at the surface of the condensed film the partial pressure of water is given by the vapor pressure above a saturated solution of the salt (Ps) and at the edge of the diffusion boundary layer the vapor pressure is experimentally fixed to be Pc. The problem involves setting up a mass balance and solving the differential equation according to the boundary conditions (see Fig. 10). 

The ions of the solution are diffused into the surface and react with the adsorbed cations of the inner layer. The first solid thin-film layer has now been formed on the substrate surface. Proceeding in this way, the growth of the thin film occurs only on the surface of the substrate. In a final step, the substrate is rinsed to remove the ions from the diffusion layer. A second growth cycle can now commence. The overall reaction occuring during one SILAR cycle can be presented as follows ... 

The parameter K is a measure of how fast the molecules diffuse across the stagnant liquid film layer. It is assumed that local equilibrium occurs at the exterior solid particle surface. The average solid phase loading, which is only a function of time, is given by ... 

Fig. 5.1 Interfacial diffusion films. 5 and 5 are the thickness of the organic and aqueous films, respectively. The presence of an adsorbed layer of extractant molecules at the interface is also shown.

These two thin liquid films, which are also called diffusion films, diffusion layers, or Nernst films, have thicknesses that range between 10 and 10 cm (in this chapter centimeter-gram-second (CGS) units are used, since most published data on diffusion and extraction kinetics are reported in these units comparison with literature values is, therefore, straightforward). 

The description of the diffusion films as completely stagnant layers, having definite and well-identified thicknesses, represents only a practical approximation useful for a simple mathematical description of interfacial diffusion. A... 

In a hydrodynamically free system the flow of solution may be induced by the boundary conditions, as for example when a solution is fed forcibly into an electrodialysis (ED) cell. This type of flow is known as forced convection. The flow may also result from the action of the volume force entering the right-hand side of (1.6a). This is the so-called natural convection, either gravitational, if it results from the component defined by (1.6c), or electroconvection, if it results from the action of the electric force defined by (1.6d). In most practical situations the dimensionless Peclet number Pe, defined by (1.11b), is large. Accordingly, we distinguish between the bulk of the fluid where the solute transport is entirely dominated by convection, and the boundary diffusion layer, where the transport is electro-diffusion-dominated. Sometimes, as a crude qualitative model, the diffusion layer is replaced by a motionless unstirred layer (the Nemst film) with electrodiffusion assumed to be the only transport mechanism in it. The thickness of the unstirred layer is evaluated as the Peclet number-dependent thickness of the diffusion boundary layer. 

In rate-based multistage separation models, separate balance equations are written for each distinct phase, and mass and heat transfer resistances are considered according to the two-film theory with explicit calculation of interfacial fluxes and film discretization for non-homogeneous film layer. The film model equations are combined with relevant diffusion and reaction kinetics and account for the specific features of electrolyte solution chemistry, electrolyte thermodynamics, and electroneutrality in the liquid phase. 

Because the consecutive stacked film layers are miscible, it is expected that a typical two-layer sample can be represented morphologically, as shown on Fig. El 1.2a. The thickness of the interface layer, <5/, increases with time, provided that the adjacent layers are molten, as is the case during the residence in the die (220°C), as well as during the time of thermal conditioning and stretching in the RME (140°C). Assuming an Arrhenius-type temperature dependence of the diffusivity (57),... 

However, the mass transfer coefficients found are clearly lower than those reported for distillation packings [26, 27]. This can be explained by the flow patterns in distillation packings, where the films constantly are disturbed and remixed, and therefore a completely developed laminar profile is never present. The mass transport is dominated by convection, not diffusion. It would be expected that remixing of the film layers, as accomplished by the stacking of monoliths (see Section 8.23) improves not only the RTD but also the mass transfer performance of monoliths. 

Fig. 5. Calibration measurement of solid hydrogen film layer thickness via energy loss of a particles [27,28]. Americium a source is embedded on the surface of the gold-plated copper substrate, onto which hydrogen thin film is deposited by releasing the gas through porous sintered metal (diffuser). Silicon detector, mounted on the vertically movable diffuser, measured the a particle energy loss in the film, which is converted the thickness using the stopping power...

Hull and Kitchener (2) measured the rate of deposition of 0.3- an-diameter polystyrene latex particles onto a rotating disk coated with a film of polyvinyl formaldehyde. In electrolytes of high ionic strength (where the double-layer repulsion is negligible), they found close agreement between experiments and the prediction of Levich s boundary-layer analysis (Eq. 3]), indicating that a diffusion boundary layer exists and that its thickness is large compared to the domain of van der Waais and hydrodynamic interactions. These are neces-... 

Denitrification, 340,473 Desorption, 221,362 Diffuse double layer, 141 Model, 142-146 Thickness, 145 Variable-charge surfaces, 146 Constant-charge surfaces, 143-146 Diffusion, 298, 398 Film diffusion, 398 Particle diffusion, 398 Solution diffusion, 398 Dioctahedral silicates, 122 Dispersion, 367... 

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

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