Simple thermodynamic derivation of the electrocapillary equations

Debate on the equations for surface excess stress and charge density on a solid continues in the literature. The various points of views are supported by complex mathematical argumentations. This hampers forming a unified understanding on the issue. To alleviate the situation, a concise but rigorous thermodynamic analysis is presented here. It shows that the Shuttleworth equation and the second Gokhshtein equation are invalid. The validity of the Lippman equation and the first Gokhshtein equation depends on the independent variable of the system.