Defined by \[G^{\sigma }=H^{\sigma }- T\ S^{\sigma }=A^{\sigma }- \gamma \ A_{\rm{s}}\] where \(H^{\sigma }\) is the surface excess enthalpy, \(T\) is the thermodynamic temperature, \(S^{\sigma }\) is the surface excess entropy, \(A^{\sigma }\) is the surface excess Helmholtz energy, \(\gamma \) is the surface tension, and \(A_{\rm{s}}\) is the area relative to a Gibbs surface.