Adsorption Isotherms Type III, IV and V

Why do many isotherms follow the same pattern like III, IV, or V?

Before we begin, it is advisable to understand few terms below so that you have better understanding towards the description.

Porosity: A measurement of empty spaces in a material, it is how much empty space per volume of material.

We will cover three pore sizes in this video which are the macropores, micropores and mesopores.
-Macroporous materials have widths measuring more than 50 nanometers.
-Microporous materials have widths measuring less than 2 nanometers.
-Mesoporous materials have widths measuring between 2 to 50 nanometers.

Adsorption is a surface phenomenon where the molecules condense on solid surfaces (adsorbent). It plays an important role in catalysis, crystallization, evaporation, condensation and many more. Adsorption isotherms are graphs plotted from adsorption experiments that are conducted at constant temperatures. The amount of molecules adsorbed is plotted against increasing pressure. There are many types of them but all essentially fit in 5 main categories. Usually adsorption will increase when pressure increases. In general, different porosity and pore size of the solid surfaces will give different results in the adsorption experiments and thus different isotherms.

"Models" are mathematical systems which are used to predict how something in the real world would behave. Langmuir isotherm model is used for Type I isotherms where only one layer is adsorbed (monolayer) while the BET isotherm model is used for Type II, III, IV and V where multiple layers of adsorption (multilayer) takes place.

Type III isotherms occur when the material being adsorbed (adsorbate) prefers to interact among themselves rather than the surface of non-porous or macroporous solids. The amount adsorbed is low at low pressures, but the adsorbate–adsorbate forces will increase the adsorption once some patches of adsorbate forms on the surface. The isotherm curve will increase in steepness with increasing pressure. Multilayer adsorption happens without the formation of a monolayer beforehand.

Type IV isotherms are monolayer and multilayer adsorption with capillary condensation. Capillary condensation is an incident where the gas condenses at a lower pressure inside capillaries compared to open surfaces. This happens when the pores have sizes of the mesoporous region. The curve first shows a “knee” which indicates a monolayer formation, increases, then flattens as capillary condensation fills the pores and stops adsorption.

Type V isotherms occurs on mesoporous solids as well. However, like type III isotherms, the adsorbates interact more easily with themselves than to the adsorbent. This causes the formation of multiple layers of adsorbate without the completion of one layer beforehand, unlike type II and type IV isotherms. The amount adsorbed increases with increasing pressure and flattens when adsorption is stopped due to capillary condensation fills the pores with condensed liquid.

Type IV and type V isotherms may also have different adsorption and desorption curves (also called hysteresis) due to the difference in how the substance fills up a pore and how it leaves it.

[SaidAZ-pfzo]

Thank you for the explanations .
is there a video on the correspondence between the type of hysterisis with the shape of the types of pores: cylindrical or lamellar or in bottle ...
?
cordially