Diffusion Theory

Diffusion can be defined as a process by which molecules transfer spontaneously from one region to another in such a way as to equalize chemical potential or thermodynamic activity. Although diffusion is a result of random molecular motion, with a wide spectrum of physicochemical properties occurring in various conditions and situations, the diffusion process can be abstracted to a simple system involving molecules of interest, a dif-fusional barrier, and a concentration gradient. The migrating molecules are termed diffusants (also called permeants or penetrants). The membrane or matrix in which the diffusant migrates is called the diffusional barrier. The external phase is called the medium. The concentration gradient or profile of the diffusant within the diffusional barrier is the driving force for diffusion. The mathematics of diffusion are discussed briefly in this section, with emphasis on both diffusion across a barrier membrane and diffusional release from a preloaded matrix.1-4

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