Ionic Competition over Adsorption into Charged Spherical Cavities Affecting the Shape of Electric Double Layer Capacitance Curve and Zeta Potential: A Density Functional Theory Study

Abstract

Modified fundamental measure theory in framework of restricted primitive model is employed to investigate the formation of the electric double layer at the concave wall of a charged spherical cavity when two positive ions, A + and C 2+, are in competition. Results indicate that, compared to A + at a bulk concentration 1M, the presence of even low concentrations (0.01 M) of C 2+ leads to a remarkable change in the capacitance curve’s shape. The concentration plays an important role in ionic adsorption at low values of electric potential, EP, while the ion charge has a dominant role at high enough EPs. The mole fraction of A + ions in the cavity, shows an initial increase with EP because of their higher concentration but they are later desorbed from the cavity. Hence, shows a maximum at EP at which their substitution with C 2+ ions start. This maximum shifts to lower EP with increasing C 2+ concentration and cavity curvature. This substitution of ions leads to a remarkable change in the capacitance curve’s shape, converting the symmetric capacitance curve to an asymmetric one. The zeta potential whose value is effectively changed by this substitution, is investigated along with its role in the diffuse capacitance.