Free Diffusion

Free diffusion is a random nonrestricted walk or shift of atoms, ions, or particles on distances higher than those of the interatomic ones. From the atomistic point of view, diffusion is considered as a result of the random walk of the diffusing particles, self-propelled by thermal energy (Brownian motion); from the phenomenological (macroscopic) point of view, such (diffusion) flux, which can be described by Fick’s laws, is proportional to the minus gradient of chemical potential and goes from regions of higher concentration to regions of lower concentration of diffusant in appropriate (motionless) environment or solvent till concentration equalizing. From thermodynamic point of view, diffusion (mixing) in chemical system increases its entropy (a rate of disorder) and tends to state with the lowest inner energy.

An example of free diffusion is as follows (see Fig. 1). If we consider infinite cylinder of unit cross section, not with an instantaneous plane source at x= 0, but with an...