Absorption isotherm
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An absorption isotherm is an analytical method for estimating the surface physical and chemical values of materials.
It is well known that the amount of absorbent on a surface depends on two factors: the pressure of the absorbent and the temperate. An experimental absorption isotherm plot is obtained in isothermal conditions by measuring the transition of a particular gas that is absorbed on the surface. Usually, the X-axis is the weight increment of the objective material in terms of the absorption amount, and the Y-axis is the gas pressure (sometimes the relative pressure is used instead of the absolute pressure).
When an inert gas (such as nitrogen or argon) is used as an absorbent gas, the absorption isotherm indicates the surface area and/or the pore size distribution of the objective material by applying data to the theoretical absorption isotherm for gas absorption on the surface. Some typical theoretical systems for gas absorption include the Langmuir's absorption isotherm, where monolayer absorption is hypothesized, and the Brunauer-Emmett-Teller (BET) absorption isotherm, where multilayer absorption is hypothesized .
The chemical properties of a material surface are revealed if the used absorbent has acidity or basicity. In some experiments, a liquid absorbent like water is used in the same way.
The desorption isotherm plot based on the desorption from the objective material is also acquired and used for the same purpose. But the absorption isotherm plot and desorption isotherm plot sometimes draw a hysteresis loop, since the absorbed gas may cause transformation of the surface structure.
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