Mesoscopic theory for a double layer capacitance in concentrated ionic systems

The effect of oscillatory decay in charge density in concentrated ionic solutions and ionic liquids on the double-layer capacitance is studied within the framework of a mesoscopic theory. Only Coulomb and steric forces between the ions that are present in all ionic systems are taken into account. We show that the charge oscillations lead to a rescaled distance between the electrode and the virtual monolayer of counterions in the Helmholtz capacitance, and the scaling factor depends on the period of the charge oscillations. Our very simple formula for large density of ions and small voltages can serve as a reference point for the double layer capacitance in concentrated ionic solutions and ionic liquids, and can help to disentangle the universal and specific contributions to the capacitance in particular systems.