where NA is the total flux in mol/s. It should be noted that in diffusion in solids, 2?ab ^ DBA and both Dab and DBA are independent of the partial pressure of the gas or liquid outside the solid. However, the solubility in the solid is directly proportional to the partial pressure.

Structure-Dependent Diffusion in Solids. The presence of interconnected pores and capillaries in porous solids does not allow direct application of Fick's law, because the diffusing substance follows a tortuous path, which is not equal to the distance between two points. To correct for that tortuous path, it is necessary to define a factor t, called the tortuosity, and rewrite the diffusion equation 27

where e is the porous fraction of the solid. Usually the porous fraction and tortuosity factor are combined into an effective diffusivity as where D is concentration dependent. Solution of this equation requires knowledge of the actual function of D and the corresponding boundary conditions. Later, D will be expressed as a function of concentration, temperature, viscosity of solution, and the properties of the medium.

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