ADSORPTION OF PURE GASES AND THEIR MIXTURES ON HETEROGENEOUS SURFACES.

The thermodynamics of adsorption of mixtures is guided by the Gibbs adsorption isotherm. In addition, boundary conditions place mathematical constraints on allowable forms for the adsorption isotherm of the pure gas. There exists a family of three-constant equations which incorporate surface heterogeneity in the model but ignore adsorbate-adsorbate interactions. Any of these three-constant equations provides a basis for predicting multicomponent equilibria by means of ideal-adsorbed-solution (IAS) theory. Here a specific algorithm for the Langmuir-uniform-distribution (LUD) equation is provided. Agreement with experimental mixture data is excellent up to 50% of saturation, but at higher coverage negative deviations from Raoult's law and enhanced loading are observed. These deviations are not due to adsorbate-adsorbate intermolecular forces. Sites of different energies possess different selectivities, and the resultant segregation in composition is primarily responsible for the nonideal behavior in the adsorbed phase.