Gibbs adsorption
The surface excess amount or Gibbs adsorption of component i, n i σ, which may be positive or negative, is defined as the excess of the amount of this component actually present in the system over that present in a reference system of the same volume as the real system and in which the bulk concentrations in the two phases remain uniform up to the Gibbs dividing surface. That is \[n_{i}^{\sigma }=n_{i}- V^{\alpha }\ c_{i}^{\alpha }- V^{\beta }\ c_{i}^{\beta }\] where n i is the total amount of the component i in the system, ciα and ciβ are the concentrations in the two bulk phases α and β, and V α and V β are the volumes of the two phases defined by the Gibbs surface.
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 588 (