If Γ i σ and Γ 1 σ are the Gibbs surface concentrations of components i and 1, respectively, with reference to the same, but arbitrarily chosen, Gibbs surface, then the relative adsorption of component i with respect to component 1, is defined as \[\mathit{\Gamma}_{i}^{\left(1\right)} = \mathit{\Gamma}_{i}^{\sigma}- \mathit{\Gamma}_{1}^{\sigma}\ \frac{c_{i}^{\alpha}- c_{i}^{\beta}}{c_{1}^{\alpha}- c_{1}^{\beta}}\] and is invariant to the location of the Gibbs surface. Alternatively, Γ i 1 may be regarded as the Gibbs surface concentration of i when the Gibbs surface is chosen so that Γ i σ is zero, i.e. the Gibbs surface is chosen so that the reference system contains the same amount of component 1 as the real system. Hence Γ 1 1 ≡ 0.