The layers of molecular ions and charged dipoles at a solid/solution interface. It is electrically analogous to a capacitor in that there is charge separation between the solid (electrode) and the charge center of the oriented ions or dipoles. Next to an electrode there may be an adsorbed fixed layer of ions called the inner Helmholtz double layer. A diffuse layer (outer Helmholtz double layer) in the electrolyte contains an excess of ions that is usually of the same charge as the electrode but opposite to that of the fixed layer. The thickness of the double layer is less than 100 Ångstroms (10–8 m). The double-layer capacitance is effectively in parallel with the Warburg impedance.