Multicomponent
We can treat adsorption from a mixture of gases (at low pressures) by extending the Langmuir isotherm:
These reactions are not independent because both species compete for the same surface sites. The two equilibria must be solved simultaneously:
As for the one-component case, we define surface coverages and , and note that
[S] + [A(ads)] + [B(ads)] = [S]_0$, or $[S]/[S]_0 = 1 - \theta_A - \theta_B
Then,
and dividing, we get
The ratio of A:B on the surface is therefore different than the ratio of A:B in the gas phase, by precisely , so the more strongly adsorbed component is enriched on the surface. This is referred to as the thermodynamic selectivity, the preference of the surface for one species of a mixture, and is the basis for many techniques to separate gas mixtures.