Double mode of sorption.
It may be the case of oxygen and carbone dioxide in oxygenated polymers. The existence of polar interactions between these molecules and polymer macromolecular chains may induce strong interactions. The absolute energy of sorption on strong interaction sites is lighty higher than in the other parts of the membrane. The kinetic sorption curve likes a Fickian kinetic curve but the total equilibrium concentration profile is higher than in the Henry sorption process because the fraction of trapped molecules on the Langmu´r sites is not negligeable. The measurement of the initial slope of the kinetic curve drawed with a square root of time scale gives a rather good estimation of the diffusion coefficient.
Example 2, Case of a high concentration of Langmu´r sites and weak interactions :