The surface excess 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 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, $c_i^{\alpha }$ 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 Gibbs surface.