https://doi.org/10.1351/goldbook.G02627
The , , 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 where is the total amount of the component in the system, and are the concentrations in the two bulk phases and , and and are the volumes of the two phases defined by the
surface excess
amount or Gibbs adsorption of component Gibbs dividing surface
. That is Gibbs surface
.