The @[email protected] amount or Gibbs adsorption of component $$i$$, $$n_{i}^{\sigma }$$, 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 @[email protected] 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, $$c_{i}^{\alpha }$$ and $$c_{i}^{\beta }$$ are the concentrations in the two bulk phases $$\alpha$$ and $$\beta$$, and $$V^{\alpha }$$ and $$V^{\beta }$$ are the volumes of the two phases defined by the @[email protected]