Langmuir Isotherm
The Langmuir isotherm describes adsorption onto a fixed number of equivalent, non-interacting sites, relating fractional coverage to gas partial pressure through an equilibrium constant. Because the equilibrium constant is tied to a free energy of adsorption, fitting measured coverages yields thermodynamic quantities that can be used to predict coverage at other temperatures and pressures; deviations from the model signal site heterogeneity or adsorbate-adsorbate interactions.
Single-site model for CO2 capture on supported amidines (2010)
Alesi et al. used a single-site, two-component Langmuir form in which CO2 and H2O react together at an amidine site to form bicarbonate, with coverage taken as the measured fraction of amidines bound to CO2 (Alesi et al. 2010). The resulting equilibrium constant gave a reaction Gibbs energy, corrected to the standard state with tabulated gas-phase data, and solving the isotherm at capture and regeneration conditions mapped out the range of reaction free energies that give useful working capacity. The model over-predicted the efficiency gain at higher CO2 pressure, which the authors suggest may come from repulsion between neighboring bicarbonates or a distribution of site energies, both violations of the Langmuir assumptions. Summary: alesi2010.