The shape of sorption isotherms of pure fluids on planar surfaces and porous materials depends on the interplay between the strength of fluid-wall and fluid-fluid interactions as well as the effects of confined pore space on the state and thermodynamic stability of fluids confined to narrow pores. The International Union of Pure and Applied Chemistry proposed to classify pores by their internal pore width (the pore width defined as the diameter in case of a cylindrical pore and as the distance between opposite walls in case of a slit pore), i.e., micropore: pore of internal width less than 2nm; mesopore: pore of internal width between 2 and 50 nm; macropore: pore of internal width greater than 50nm.

Schematic illustration of adsorption potential, έ, on (a) planar, nonporous surface; (b) mesopore; (c) micropore.

The sorption behavior in macropores is distinct from that of mesopores and micropore. Whereas macropores are so wide that they can be considered as nearly flat surfaces the sorption behavior in micropores is dominated almost entirely by the interactions between fluid molecules and the pore walls; in fact the adsorption potentials of the opposite pore walls are overlapping. Hence the adsorption in micropores (like micropore filling) is distinct from the adsorption phenomena occurring in mesopores. As illustrated in Fig. 3b, the pore potential in mesopores is not dominant anymore in the core of the pores. Hence, the adsorption behavior in mesopores does not depend only on the fluid-wall attraction, but also on the attractive interactions between fluid molecules, which may lead to the occurrence of capillary (pore) condensation. Pore condensation represents a phenomenon whereby gas condenses to a liquid-like phase in pores at a pressure less than the saturation pressure Po of the bulk fluid. It represents an example of a shifted bulk transition under the influence of the attractive fluid-wall interactions.