Defined by: \[A^{\sigma }=U^{\sigma }- T\ S^{\sigma }\] where U σ is the surface excess energy, T is the thermodynamic temperature, and S σ is the surface excess entropy.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 599 (https://doi.org/10.1351/pac197231040577)