Dynamic Theory of Liquid Junction Potentials

Abstract

A Nernst-Planck-Poisson finite difference simulation system is used to model the dynamic evolution of a liquid junction from a nonequilibrium initial condition to a condition of steady potential difference, in a linear semi-infinite space. Liquid junctions of Lingane's type 1 (monophasic, unequal concentration) and type 2 (bi-ionic potential; biphasic, equal concentration) are considered, for the sake of simplicity. Analysis of the results shows consistency with known and novel asymptotic solutions. A comprehensive dynamic theory of the free liquid junction potential is presented, having considered the simulated concentration profiles and electric field in the system. This reveals a dynamically relaxing junction in which a diffuse layer continues to expand. This is advocated as physically realistic and shown to be consistent with a steady state potential difference, which arises after 10-1000 ns for typical aqueous systems, when the expanding diffuse layer has a corresponding size of 10-1000 nm. Hence, Planck's concept [Wied. Ann. 1890, 40, 561-576] that a steady state potential difference exclusively implies a static junction with equal fluxes of all species is shown to be false, for an unconstrained system.