On the nature of the diffusion potential

Abstract

The 'diffusion' potential concept is examined critically within the conditions imposed by Maxwell's equations and the Thermodynamics of Irreversible Processes. It is demonstrated that under the zero current condition during non-steady state diffusion the self-field associated with the diffusion potential gradient is due entirely to a diffusion-induced distribution of dipoles. The general failure to recognize that the charge associated with the field is due to dipoles has led to an improper use of Poisson's equation and to certain oft-remarked-upon paradoxes. The dipoles appear in systems where counter-current ion mobilities differ greatly. In seeking electroneutrality the ion trajectories become 'curved' in such a way as to generate an instantaneous dipole current and field. The magnitudes have nothing to do with the dielectric properties of the medium.It is demonstrated that the zero current condition represents a quasi-steady state of minimum entropy production. Such a state of stability can be sustained provided the process of relaxation of the field to the pure dipole state by real charge neutralization is fast compared with the relaxation process for the chemical potentials of the neutral salts in solution. The energetics of ionic solutions are usually such as to abet the neutrality condition during diffusion processes.