According to IUPAC (International Union of Pure and Applied Chemistry), pores are classified as macropores for pore widths greater than 500 A, mesopores for the pore range 20 to 500 A and micropores for pore widths less than 20 A. Because of the intense potential fields in very narrow pores (overlapping fields from opposite pore walls), the mechanism of pore filling is different as in mesopores. Mesopores fill via pore condensation, which represents a first order gasliquid phase transition. In contrast the filling of micropores reflects in most cases a continuous process. The micropore range is subdivided into those smaller than about 7 A (ultramicropores) and those in the range from 7 to 20 A (supermicropores). The filling of ultramicropores (pore width smaller < 7 A) occurs at very low relative pressures and is entirely governed by the enhanced gas-solid interactions. However, in addition to the strong adsorption potential a cooperative mechanism may play a role in the pore filling process of so-called supermicropores. The relative pressure where micropore filling occurs is dependent on a number of factors including the size and nature of the molecules of the adsorptive, the pore shape and the effective pore width. The pore filling capacity depends essentially on the accessibility of the pores for the probe molecules, which is determined by the size of the molecule and the chosen experimental conditions.

In an ideal case microporous materials exhibit type I isotherms. However, many microporous adsorbents (e.g., active carbons) contain pores over a wide range of pore sizes, including micro- and mesopores. Accordingly, the observed adsorption isotherm reveals features from both type I and type IV isotherms.  Below figure 1 it shows the nitrogen isotherm (at ~77 K) on a disordered active carbon sample. 

Figure 1: Nitrogen adsorption at 77.35 K on an active carbon sample, which contains, in addition to its microporosity, some mesoporosity indicated by the occurrence of hysteresis and the fact that the adsorption isotherm does not reveal a truly horizontal plateau at relative pressures > 0.1; the observed slope being associated with the filling of mesopores.

The observed hysteresis loop is indicative of mesoporosity, whereas the type I behavior is clearly visible in the lower relative pressure range. Another example is shown in figure 2 , which shows the adsorption isotherm at 87 K (i.e., liquid argon temperature) in a faujasite zeolite. In order to reveal details of the adsorption isotherm (in particular in the range of the low relative pressures where micropore filling occurs), the isotherm is favorably represented in a semi-logarithmic scale of the relative pressure. The strong increase of the adsorbed amount close to saturation pressure results from pore condensation into large meso- and macropores.

Figure 2: Semi-logarithmic isothenn plot of argon at 87K on a faujasite zeolite which clearly resolves the micropore filling in the low relative pressure range. The steep increase close to the saturation pressure represents the pore filling oflarge meso- and macro-pores.

In order to interpret sorption isotherms measured on microporous materials various methods and theories have been developed. The so-called 'classical methods' are based on macroscopic, thermodynamic assumption, i.e., they assume that the adsorbed pore fluid is liquid-like and that it reveals essentially the same properties as a bulk liquid at the same temperature.

Such classical approaches are, for instance, the theories for micropore characterization by Polanyi, Dubinin, Stoeckli including the more recent approaches by Horvath-Kawazoe (HK) and related methods. In contrast to these macroscopic approaches, methods like the Density Functional Theory (DFT) or methods of molecular simulation (e.g., Monte Carlo simulation methods (MC), Molecular Dynamics methods provide not only a microscopic model of adsorption but lead also to a better assessment of the thermodynamic properties of the pore fluid . These theories, which are based on statistical mechanics, connect macroscopic properties to the molecular behavior allowing a much more realistic description of micropore filling, which is the prerequisite for an accurate and comprehensive pore size analysis.