Diffusion and Mass Transport

Hydration can be an important factor in diffusion and mass transport phenomena in pharmaceutical systems. It may alter the apparent solubility or dissolution rate of the drug, the hydrodynamic radii of permeants, the physicochemical state of the polymeric membrane through which the permeant is moving, or the skin permeability characteristics in transdermal applications. 

J. Philibert, Atom Movements Diffusion and Mass Transport, translated by S. J. Rothman, Les Editions de Physique, F-91944 Les Ulis, 1991. 

These values can be correlated with the heat of adsorption of hydrogen on the catalytic metal since the oxidation mechanism, apart from diffusion and mass transport limitations, is controlled by an adsorption step in a two consecutive step mechanism ... 

Philibert, J. (1991) Atom Movement, Diffusion and Mass Transport in Solids,... 

Equation (5.15) concerns self-diffusion, and mass transport involves mutual diffusion if the solute diffuses in one direction, then the solvent does so in the opposite one. A kind of average diffusion coefficient must be taken, and only for low solute concentrations is it about equal to the self-diffusion coefficient of the solute, taking the viscosity of the solvent. This is primarily because solute concentration will generally affect the viscosity of the solution in most cases it is higher than that of the solvent. 

G.I. Taylor, Diffusion and mass transport in tubes, Proc. Phys. Soc. London 67 (1954) 857. 

Taylor, G. 1950. The Instability of Liquid Surfaces when Accelerated in a Direction Perpendicular to their Planes. L, Proc. Roy. Soc. (London). Ser. A 202, 192-196. Taylor, G. 1. 1954. Diffusion and Mass Transport in Tubes, Proc. Phys. Soc. 67b, 57%9. 

Taylor, G. I. Diffusion and Mass Transport in Tubes. Proc. Physical Society (London) B67 (1954) 857. 

One of the most important parameters is the temperature, the starting temperature, and the initial reaction rate. One can determine experimentally the initial reaction rate after elimination of diffusion and mass transport effects and then determine the Arrhenius constants, which depend on the temperature. The collision factor (ko) and activation energy (E) parameters influence significantly the activity pattern and selectivity. Figure 3.1 illustrates the influence of the temperature on these parameters for different reactions and metallic catalysts. This effect is known as compensation effect, although empirically there are attempts on theoretical interpretations for different heterogeneous systems [1, 2]. 

Several studies have investigated empirically the flux of chemicals within snow or between snow and the atmosphere (Guimbaud et al., 2002 Albert and Shultz, 2002 Herbert et al., 2006). In particular, measured concentration gradients within the atmospheric boundary layer or within the snow pack have been used to calculate a chemical s flux into or out of the snow pack. This approach has resulted in miscellaneous parameterizations to calculate fluxes of, for example, carbonyl compounds and NO c species from the snow pack as a result of photochemical processes in snow (Domind and Shepson, 2002 Hutterli et al., 1999 Guimbaud et al., 2002 Grannas et al., 2002). However, flux measurements can only be used to derive kinetic transport parameters, such as diffusivities and mass transport coefficients, if the chemicals involved are reasonably persistent and do not undergo rapid conversions within the snow pack. For example, measurements of the flux of carbonyl compounds out of snow are more likely to reflect the kinetics of formation in the snow pack than the kinetics of snow-air gas exchange. As a result, there is a very limited number of experimental studies that provide quantitative information on the rate of chemical transport in snow. 

This book is divided into three parts developments in high-temperature corrosion theories and processes, oxide scales and coatings, and practical case studies. In the second chapter. Professor Pieraggi describes the role of diffusion and mass transport processes in scale growth, and in particular, the potential influences of interfacial reactions and structures on the mechanical... 

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