Gas Phase Isotherm Types

When molecules in gaseous state adsorb onto the surface of a condensed phase, usually a solid, different behaviors can be observed depending on the adsorbate, the substrate, and the experimental conditions such as temperature. Brunauer et al. (1940) gave a classification with five main fypes which remains in use today, albeit extended with a sixth type (Sing 1985). These types cover the majority of situations, except in some cases supercritical fluids show different behavior (Donohue and Aranovich 1998). Here we discuss the six classical types. 

FIGURE 4.1 Schematic plot of a type I isotherm from lUPAC classification (Sing 1985) as a function of pressure (a) and the logarithm of pressure (b). The dashed line shows the point of monolayer saturation that is, when the surface is covered by a layer just one molecule thick. In (c), a schematic drawing of the (ideal) situation is presented, showing an almost complete monolayer. 

FIGURE 4.6 Schematic plot of a type VI isotherm from lUPAC classification (Sing 1985) as a function of pressure (a) and the logarithm of pressure (b). 

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