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

$n i σ = n i − V α c i α − V β c i β$

where $n i$ is the total amount of the component $i$ in the system, $c i α$ and $c i β$ 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.

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 588