B.E.T. Sorption mode.
Example 4, particular case of an almost linear sorption isotherm at low pressures, dry diffusion is fast :

In this example, a particular value of the equilibrium trapping constant in regard to the saturation vapor pressure leads to an almost linear sorption isotherm like in the case of Henry's law. That can be easily verified by changing the applied vapor pressure (but not the saturation vapor pressure) in a range from 0 to 0.090 (here Psat = 0.200). Therefore, one notices that the sorption process in the membrane is composed both on trapping and condensation phenomena. A theorical calculation gives : DeltaG condensation - DeltaG trapping = 3.4 KJ. (if the equilibrium constant of trapping was equal to 5, the free energy of trapping would be equal to the free energy of condensation and their difference would then be nil). One would notice that if the sorption kinetic curve remains a Fickian one, the measured diffusion coefficient cannot be simply related to the diffusion jump speed like in a Fickian case and it is in fact much lower than the attempted value.