B.E.T. Sorption mode.
Example 2, case of high trapping and strong condensation :

It may be the case of solvents and water in rubbery polymers when their affinity for these solvents is high. The number of strong interaction sites may be high. The sorption isotherm has got a sigmo´dal shape. Trapping on strong interaction sites is the most important process at low pressures. At high pressures, condensation is the main process. The linear transformation of the isotherm gives a straight line curve with a positive slope. In this case of the simulation, the fraction of free molecules may be sufficiently low in regard to the trapped or condensed fraction and then should not introduice too much bias.