Electrocapillary Phenomena

physical phenomena associated with the dependence of the surface tension at the interface between an electrode and an electrolyte on the electrode potential. Such phenomena result from the existence, on a metal surface, of ions that produce a surface charge ∊ and that give rise to a double layer in the absence of an applied electromotive force.

The mutual repulsion of ions with the same charge along a phase interface counteracts the molecular forces of attraction, resulting in a surface tension σ that is less than the surface tension in the case of an uncharged surface. The introduction of charges with a sign opposite that of ∊ reduces the value of ∊ (seePOLARIZATION, ELECTROCHEMICAL) and increases ∊. When the attractive forces are completely counteracted by electrostatic forces, σ reaches a maximum. The further introduction of charges leads to a decrease in σ as a consequence of the production and buildup of a new surface charge. An experimental plot of the relation between σ and the electrode potential φ at a constant solution composition is well described by Lippmann’s equation: ∊ = –dσ/dφ. The equation makes it possible to calculate both ∊ and the capacitance of the double layer.

Electrocapillary phenomena are affected by the specific adsorption of ions, especially ions of surfactants, making it possible to determine the surface activity of surfactants. Electrocapillary phenomena in molten metals—such as aluminum, gallium, cadmium, and zinc—are used to determine the adsorptive power of the metals. The theory of such phenomena is used to explain the maxima observed in polarography.

Effects associated with the dependence of an electrode’s hardness, wettability, and coefficient of friction on its potential are also considered electrocapillary phenomena.