and free sites on the surface are captured by Γ 𝑖 . The number of available surface sites Γ 𝑎𝑣 fulfills the same role as 𝑐 ∘ and 𝑝 ∘ in that it links chemical activities to coverages. Differences to the ideal behavior of the species can be captured by an activity coefficient 𝛾 𝑖 which we assume to be unity moving forward but can be retrospectively introduced into every equation. We can now define the surface coverage, 𝜃 𝑖 , (Eq. 6b), with the sum of coverages, including sites potentially blocked by an innocent observer specie 𝑋, 𝜃 𝑥 , and free sites, 𝜃 * , being constrained to unity (Eq. 6c).