The previous derivations assumed that there is only one species, A, adsorbing onto the surface. This section considers the case when there are two distinct adsorbates present in the system. Consider two species A and B that compete for the same adsorption sites. The following hypotheses are made here:
# All the sites are equivalent.
# Each site can hold at most one molecule of A, or one molecule of B, but not both simultaneously.
# There are no interactions between adsorbate molecules on adjacent sites.
As derived using kinetic considerations, the equilibrium constants for both A and B are given by
and
The site balance states that the concentration of total sites [S] is equal to the sum of free sites, sites occupied by A and sites occupied by B:
Inserting the equilibrium equations and rearranging in the same way we did for the single-species adsorption, we get similar expressions for both θ and θ: