Defined by:
where
is the surface excess Helmholtz energy,
is the surface excess Gibbs energy is the interfacial Helmholtz energy,
is the interfacial Gibbs energy, and
is the surface area. The quantities thus defined can be shown to be identical, and
the conditions of equilibrium of component
in the system to be
where
and
are the
chemical potentials of
in the bulk phases
α and β.
(
or
have to be omitted from this equlibrium condition if component
is not present in the respective bulk phase.) The surface chemical potentials are
related to the
Gibbs energy functions by the equations

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 602