Characterization

Adsorption isotherm data is the basis of several materials characterization techniques. For a homogeneous material (or surface), the amount of gas adsorbed at given conditions should be proprtional to the surface area. If the same experiment has been performed on a reference material of known surface area, the surface area (per unit volume, say) of the sample material can be determined.

In practice, determination of surface area from adsorption at a single pressure and temperature is not very reliable. Instead, one analyzes adsorption over a particular range of pressure in order to extract the monolayer capacity (or related quantity) of the material. The monolayer capacity is the total adsorptive capacity of the material at a coverage (in molecules/area) corresponding to a dense adsorbed layer one molecule thick. In practice, the monolayer capacity is a parameter in various adsorption models, which can be determined by fitting the models to experimental data. Then,

surface area = monolayer capacity / monolayer density

where the inverse monolayer density is the area per molecule in the monolayer. This last quantity is mostly a property of the adsorbing gas, and mostly independent of the nature of the adsorbent. For instance, the area occupied by an adsorbed nitrogen molecule is frequently taken to be 14 $\text{\AA}^2$ (14 square Angstroms). For a typical adsorbent with a surface area of 100 $m^2/g$, the monolayer capacity would be approximately $8.5\times10^{-5}$ mol/g.

In experimental work, amounts of gas are often given in units of cc(STP) - cubic centimeters of gas at Standard Temperature and Pressure (1.0 atm, 273.15 K); the monolayer capacity in these units would be 1.9 cc(STP)/g.

One can also extract a pore size distribution from gas adsorption data by fitting to the Kelvin equation or other thermodynamic model, but this is a more advanced topic.