If
and
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
and is invariant to the location of the Gibbs surface. Alternatively,
may be regarded as the Gibbs surface concentration of when the Gibbs surface is chosen so that
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
.

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 591