A nonlinear equation for ionic diffusion in a strong binary electrolyte

TL;DR

This paper derives a nonlinear integro-differential equation to model ionic diffusion in strong binary electrolytes, improving upon classical linear models by accounting for nonlinear effects and providing more accurate solutions.

Contribution

The authors develop a general nonlinear theory for ionic diffusion that extends the classical ambipolar diffusion model using asymptotic analysis.

Findings

The nonlinear equation offers a better approximation to the exact solution than the classical linear model.

Numerical integration confirms the nonlinear model's improved accuracy.

The theory applies to other physics problems like semiconductor junctions and plasma diffusion.

Abstract

The problem of the one dimensional electro-diffusion of ions in a strong binary electrolyte is considered. In such a system the solute dissociates completely into two species of ions with unlike charges. The mathematical description consists of a diffusion equation for each species augmented by transport due to a self consistent electrostatic field determined by the Poisson equation. This mathematical framework also describes other important problems in physics such as electron and hole diffusion across semi-conductor junctions and the diffusion of ions in plasmas. If concentrations do not vary appreciably over distances of the order of the Debye length, the Poisson equation can be replaced by the condition of local charge neutrality first introduced by Planck. It can then be shown that both species diffuse at the same rate with a common diffusivity that is intermediate between that of the slow and fast species (ambipolar diffusion). Here we derive a more general theory by exploiting the ratio of Debye length to a characteristic length scale as a small asymptotic parameter. It is shown that the concentration of either species may be described by a nonlinear integro-differential equation which replaces the classical linear equation for ambipolar diffusion but reduces to it in the appropriate limit. Through numerical integration of the full set of equations it is shown that this nonlinear equation provides a better approximation to the exact solution than the linear equation it replaces.