= Competitive adsorption
<Competitive adsorption> occurs when different <chemical species> seek the same <adsorption sites>. For intact <molecules> $A$ and $B$, retain identical <adsorption sites> and no interactions, but replace the vacant fraction by $1-\theta_A-\theta_B$. The same equilibrium balance gives
$$
\theta_A=\frac{b_AP_A}{1+b_AP_A+b_BP_B},\qquad
\theta_B=\frac{b_BP_B}{1+b_AP_A+b_BP_B}.
$$
Here $P_A$ and $P_B$ are <partial pressures>, not total <pressure>. Increasing $P_B$ can reduce $\theta_A$ at unchanged $P_A$: replacing $P_A$ by total <pressure> in a single-species <Langmuir isotherm> loses this competition.
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