Effect of specific interactions on the double layer capacitance of concentrated ionic systems

The effect of non-coulombic short-range (SR) interactions on the double-layer capacitance in concentrated ionic systems is studied on a general level within mesoscopic density-functional theory. We improve the formula for the capacitance obtained in our previous work by adding a factor representing the charge of the electrode consistent with the formalism of the mesoscopic theory. Perfect agreement with the Debye capacitance for dilute electrolytes, and a fair agreement with simulations for concentrated electrolytes is obtained for the restricted primitive model (RPM). Explicit formulas for the potential of zero charge (PZC) and the capacitance at the PZC are obtained for the RPM with additional attractive SR interactions between like ions and with different SR cation-electrode and anion-electrode interactions. The capacitance at the PZC has the form of the original Helmholtz capacitance, with the distance of the virtual monolayer of counterions from the electrode that has the same dependence on the lengths characterizing the oscillatory decay of the charge density as in the RPM. These lengths, however, depend significantly on the SR interactions.