Gibbs film elasticity

https://dx.doi.org/10.1351/goldbook.G02634
Pertains to a film element of a soap film changing in area at constant mass and is the differential change of its surface tension with relative change in area A,
E = A ( ∂ σ ∂ A ) T , p , n i
where T is the thermodynamic temperature, p is the pressure, and n i is the amount of substance of the species i.
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 615