A generalized form of the Langmuir isotherm for gas adsorption in glassy polymers

Glassy polymers contain sites where excess free volume makes adsorption of gases possible; they also contain sites where ordinary absorption governed by Henry's law can occur. In this paper, we analyze the possibility of a continuous distribution of site types, characterized by values of adsorption capacity, affinity constant, and sorption constant which span a continuous range. Experimental data are easily fitted with great accuracy by the resulting equations; indeed, different sets of parameter values may fit any given set of data equally well, indicating that evaluation of parameter values from experiments should be regarded with careful skepticism.