Gas adsorption is one of many experimental methods available for the surface and pore size characterization of porous materials. These include small angle x-ray and neutron scattering (SAXA and SANS), mercury porosimetry, electron microscopy (scanning and transmission), thermoporometry, NMR-method, and others. Each method has a limited length scale of applicability for pore size analysis. An overview of different methods for pore size characterization and their application range was recently given by IUPAC. Among these methods gas adsorption is the most popular one because it allows assessment of a wide range of pore sizes (from 0.35 nm up to 100 nm), including the complete range of micro- and mesopores and even macropores. In addition, gas adsorption techniques are convenient to use and are not that cost intensive as compared to some of the other methods. A combination of mercury porosimetry and gas adsorption techniques allows even performing a pore size analysis over a range from 0.35nm up to 400 um.

Adsorption can be understood as the enrichment of one or more components in an interfacial layer; in gas adsorption we consider the gas/solid interface. The solid is called the adsorbent and the gas, which is capable of being adsorbed, is called the adsorptive. The fluid in the adsorbed state is called adsorbate.

Invariable the amount adsorbed on a solid surface will depend upon the absolute temperature T, the pressure P, and the interaction potential E between the vapor (adsorbate) and the surface (adsorbent). Therefore, at some equilibrium pressure and temperature the weight W of gas adsorbed on a unit weight of adsorbent is given by

W = F (P, T, E)

Usually the quantity adsorbed is measured at constant temperature and equation reduces to

W = F (P, E)

A plot of W versus P, at constant T, is refereed to as the sorption isotherm of a particule gas-solid interface.