The interface between a metal and an electrolytic solution

Abstract Measurements of the capacity of the electrical double layer at the interface between a polarizable mercury electrode and an aqueous solution of a strong electrolyte are interpreted theoretically. Attention is concentrated on the layer of charges at the actual phase boundary, whose behaviour has not been satisfactorily explained before. It is suggested that for anodic polarization an increasing number of mercury atoms is situated at adsorbed sites on an otherwise flat surface. These carry a charge, and lead to a rise in the measured capacity as the polarization becomes anodic. It is thus shown why the capacity as a function of polarization has a form for fluoride electrolytes, where there is no specific adsorption of the anion, so similar to that for other electrolytes, where there is. The behaviour of the water molecules in contact with the mercury is considered in some detail, and compared with the behaviour of bulk water and of a monolayer of water adsorbed to a mercury surface. It is shown that the hump, which appears in most capacity curves slightly to the anodic side of the electrocapillary maximum, may be due to the high polarizability of the water layer between the metal and the outer Helmholtz plane, when the field there is small. It is not necessary to assume as Grahame does that there is any appreciable chemical or imago force polarizing the surface molecules, or that the water takes up a rigid ice-like structure for low fields. The suggestion that the rise in the capacity for cathodic polarization is duo to compression of the double layer by electrostatic forces is also considered, with the implication of this idea on the mechanism of overvoltage in the discharge of cations onto mercury.