Diffusivity of gases

A diffusion mechanism is also used in dialysis as a means of separating colloids from crystalloids. The rate of diffusion of molecules in gels is practically the same as in water, indicating the continuous nature of the aqueous phase. The diffusion of gases into a stream of vapour is of considerable importance in diffusion pumps. 

The mobility or diffusion of die atoms over the surface of die substrate, and over the film during its formation, will occur more rapidly as the temperature increases since epitaxy can be achieved, under condition of ctystallographic similarity between die film and the subsuate, when the substrate temperamre is increased. It was found experimentally that surface diffusion has a closer relationship to an activation-dependent process than to the movement of atoms in gases, and the temperamre dependence of the diffusion of gases. For surface diffusion the variation of the diffusion coefficient widr temperature is expressed by the Anhenius equation... 

These three functions involve the movement of O2, CO2, and HjO through the epidermal layers of the leaf. The analogy to human inhalation is obvious. With the diffusion of gases into and out of the leaf, pollutant gases have a direct pathway to the cellular system of the leaf structure. Direct deposition of particulate matter also occurs on the outer surfaces of the leaves. 

Diffusion of gases into the polymer under pressure with subsequent expansion of the composition at elevated temperatures after decompression. Such a process can be employed with a wide variety of polymers. 

In the cases we discussed above, students revealed their lack of knowledge of the random distribution of particles, which was consistent with much previous research. This study not only revealed the mis-representation of the diffusion of gases, but also showed the inconsistent mental models that the students held while solving the problems. The result provided some evidence in favor of research that attributes students learning in relation to the context while facing various types of questions. However, this result does not support Vosniadou s framework theory (1994), which implies a consistent mental model used by learners in her study. 

Mass transport may constitute another problem. Since many catalysts are porous systems, diffusion of gases in and out of the pores may not be fast enough in comparison to the rate of reaction on the catalytic site. In such cases diffusion limits the rate of the overall process. 

Diffusion is the term used to describe the movement of one material through another. The diffusion of gases can be explained by —... 

Parenteral products should be formulated to possess sufficient buffer capacity to maintain proper product pH. Factors that influence pH include product degradation, container and stopper effects, diffusion of gases through the closure, and the effect of gases in the product or in the headspace. However, the buffer capacity of a formulation must be readily overcome by the biological fluids thus, the concentration and ratios of buffer ingredients must be carefully selected. 

The thickness of the blood-gas interface is normally less than 0.5 (im. This extremely thin barrier promotes the diffusion of gases. The thickness may increase, however, under conditions of interstitial fibrosis, interstitial edema, and pneumonia. Fibrosis involves the excess production of collagen fibers by fibroblasts in the interstitial space. Edema is the movement of fluid from the capillaries into the interstitial space. Pneumonia causes inflammation and alveolar flooding. In each case, the thickness of the barrier between the air and the blood is increased and diffusion is impaired. 

The adsorbent—a powder generally, but it could be a metal or oxide film— is placed in a glass tube (the adsorption cell C in Fig. 15) which is connected to the volumetric and vacuum lines. The bottom part of the tube, which contains the adsorbent and is located in the calorimeter cell, is made of thin-walled (0.2-0.3 mm) blown tubing (A in Fig. 18). In order to avoid the slow diffusion of gases through a thick layer of adsorbent (see Section VII.A), the sample is often placed in the annular space between the inner wall of the adsorption cell and the outer wall of a cylinder made of glass,... 

The flame retardant mechanism for phosphorus compounds varies with the phosphorus compound, the polymer and the combustion conditions (5). For example, some phosphorus compounds decompose to phosphoric acids and polyphosphates. A viscous surface glass forms and shields the polymer from the flame. If the phosphoric acid reacts with the polymer, e.g., to form a phosphate ester with subsequent decomposition, a dense surface char may form. These coatings serve as a physical barrier to heat transfer from the flame to the polymer and to diffusion of gases in other words, fuel (the polymer) is isolated from heat and oxygen. 

In this chapter, to keep the material compact, only the relationship between diffusion and defects in solids will be discussed. Moreover, the diffusion coefficient will be considered as a constant at a fixed temperature, and attention is focused upon the movement of atoms and ions rather than the equally important diffusion of gases or liquids through a solid. Discussion of diffusion per se, the extensive literature on classical theories of diffusion, and diffusion when the diffusion coefficient is not a constant will be found in the Further Reading section at the end of this chapter. 

Penetration theories (Higbie, 1935 Danckwerts, 1951 Dobbins, 1956) according to the penetration theory, diffusion of gases takes place into elements of water transported by turbulence to the surface. 

The rate of diffusion of gases is very high compared to many conventional solvents and this leads to increased reaction rates in catalysed reactions involving gaseous substrates such as hydrogenation, hydroformylation and oxidation. 

Diffusion.—The separation of hydrogen from the ler constituents of blue water gas has been proposed, iploying diffusion for the purpose. Graham expressed 2 law of diffusion of gases as. —... 

In dense membranes, no pore space is available for diffusion. Transport in these membranes is achieved by the solution diffusion mechanism. Gases are to a certain extent soluble in the membrane matrix and dissolve. Due to a concentration gradient the dissolved species diffuses through the matrix. Due to differences in solubility and diffusivity of gases in the membrane, separation occurs. The selectivities of these separations can be very high, but the permeability is typically quite low, in comparison to that in porous membranes, primarily due to the low values of diffusion coefficients in the solid membrane phase. 

Feng, C., V. V. Kostrov and W. E. Stewart. 1974. Multicomponent diffusion of gases in porous solids. Models and experiments. Ind. Eng. Chem. Fundam. 13(1) 5-9. 

---