surface excess energy

Defined by:

$U σ = U − U α − U β = U − V α U m α V m α − V β U m β V m β$

where $V α$ and $V β$ satisfy the condition $V α + V β = V$ , the total volume of the system. ( $U m α V m α$ ) and ( $U m β V m β$ ) are the energy densities in the two bulk phases where $U m α$ and $U m β$ are the mean molar energies and $V m α$ and $V m β$ are the mean molar volumes of the two phases.

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