LOCALIZED ADSORPTION WITH LATERAL INTERACTION ON RANDOM AND PATCHWISE HETEROGENEOUS SURFACES

An analytical expression for an overall adsorption isotherm was derived which accounts for both lateral interaction of the Bragg-Williams kind and surface heterogeneity via a random distribution of sites described by a uniform distribution of energies. The isotherm reduces to the Langmuir, Fowler and Guggenheim, and Langmuir uniform distribution isotherms when the appropriate limits are taken. In addition, a parametric study was made which reveals the following features of localized monolayer adsorption with lateral interaction on both random and patchwise heterogeneous surfaces: (1) Adsorption with favorable lateral interaction on a patchwise heterogeneous surface, compared to a random heterogeneous surface, results in higher fractional surface coverages but only at low pressures, whereas at high pressures or for unfavorable lateral interaction, there is little difference between the isotherms. For the case of no lateral interaction, random and patchwise heteroeneous surfaces are indistinguishable. (2) Irrespective of heterogeneity, unfavorable and no lateral interaction result in concave isotherms, whereas favorable lateral interaction results in sigmoidal isotherms with the convex region occurring at low pressures. (3) Irrespective of lateral interaction and heterogeneity, all the isotherms intersect at 6 = 0.5. Furthermore, as lateral interaction varies from unfavorable to none to favorable, the characteristic pressure at which the isotherms intersect decreases exponentially. © 1990 American Chemical Society.