How is it possible to calculate the pore size distribution from the gas adsorption isotherm? The answer is that within a pore, adsorbate molecules interact with the surrounding pore wall, and the condensation occurs below the saturation pressure. This condensation pressure is related to the pore size.
Relation between adsorption isotherm and pore size
It is similar to the phenomenon when a straw is immersed into liquid and the level of liquid inside the straw rises (capillary condensation). The narrower the straw is, the higher the level of liquid becomes. This phenomenon is caused by the lower vapor pressure in the straw compared to the atmospheric pressure. Because of the difference in the pressure, the liquid level is increased to equilibrate.
The graph shows the relationship between the adsorption isotherm and pore size range. Below the relative pressure of 0.2, the isotherm corresponds to the micropore. Above the relative pressure of 0.95, the isotherm corresponds to the macropore. Non-porous materials can also have the pore size distribution due to the intra particular void, which is measured above the relative pressure of 0.95.
Adsorption isotherm and pore size range
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