Ionic capacitors revive the Helmholtz model at polarised soft interfaces:
The discrete nature of the charge carriers at polarised soft interfaces strongly impacts the shape of their electrostatic potential profile. Indeed, at the interface between two immiscible electrolyte solutions, the potential drops between two closely interacting ionic monolayers, which induces sharp and concentration independent profiles as well as a constant capacitance (A). This behaviour resembles that predicted by Helmholtz, in 1853, who assumed that the double layer behaved like a planar capacitor, hence “ionic capacitors”. This is in opposition with the diffuseness of the potential and ion concentration profiles observed at metal/electrolyte interfaces (B). This suggests that highly-structured interfaces, as used in supercapacitors, may also present the same discreteness, such that this model may find other applications in energy storage or fundamental studies of biological membranes.