Gas sorption analysis takes advantage of the attraction between an open surface and any gas molecules it encounters. Interactions between the surface molecules of the sample (the adsorbent) and a gas molecule (adsorbate) lead to the gas molecules becoming weakly attached to the surface as liquids. A surface free of interactions (as
one is under vacuum) is in an unfavorable state, so when gas molecules are introduced to the adsorbent at low pressures, the molecules interact with the surface to form a film, as this is a more favorable state. Upon adsorption to the surface, these molecules are known as the adsorptive. This technique was originally developed to analyze mesoporous (pore diameter 2-50 nm) materials, though it can also be extended to microporous (< 2 nm diameter) and small macroporous (> 50 nm diameter) materials with the appropriate selection of analysis parameters.