Effect of adsorption on the electrostatic behavior of an electrolytic cell

Abstract

The coupling between bulk electrostatics and surface processes is studied. In particular, the influence of the ionic adsorption, both with and without an external bias voltage, on the ionic distribution and the associated electric field and electric potential, is investigated in the static equilibrium state using the Poisson-Nernst-Planck model. The analysis is performed assuming that the kinetic equation describing the adsorption-desorption phenomenon is well approximated by Langmuir isotherm, containing an adsorption term proportional to the bulk density of adsorbate particles just in front of the adsorbent surface, and a desorption term proportional to the adsorbed particles. The sample is assumed to have the shape of a slab, and only ions of a given sign are supposed mobile, as for hydrogels. We consider both symmetric and asymmetric electrolytic cells under open-circuit, short-circuit and externally applied bias conditions. The equilibrium bulk ionic distribution, electric field and electric potential profiles are determined. In addition, the surface density of adsorbed ions, as well as the electric field and potential at the surfaces are also calculated.The limit of non interacting surfaces was investigated.