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Adsorption isotherms: concept, types, examples

By observing this isotherm, one can get an idea of ​​how the adsorption process proceeds; and therefore, how are the particle-surface interactions, and the characteristics of the surface. Analyzing the isotherm it is deduced if the surface is smooth, porous, or microporous, as well as possible condensations.

The image above helps to clarify the aforementioned. The adsorbed particles (purple circles) are called adsorbates. While the adsorbent is the one that has a surface on which the adsorbates will be adsorbed. As its pressure or concentration increases, the greater the volume adsorbed on the adsorbent.

Types of adsorption isotherms

Shown above are five of the main experimental isotherms used by S. Brunauer to classify adsorption of gaseous particles on solids. Each describes a different adsorption process. Likewise, each one has mathematical models that attempt to deduce the behavior of its curves.

Regardless of the units or variables used in the X (p / po) and Y (X) axes, the X axis indicates how much pressure or concentration of adsorbate “acts” on the solid; while the Y axis indicates how much of that adsorbate was actually adsorbed on the surface of said solid or adsorbent.

Thus, as we move to the right of the X axis, we see how the amount of adsorbed particles increases as a function of the increase in their pressures or concentrations. This leads to the observation of a maximum, a decay, a valley, etc., which in some way or another allow us to interpret how adsorption takes place.

Isotherm type I

Type I isotherm. Source: Daniele Pugliesi / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Above we have the type I isotherm, which is also known as the Langmuir isotherm, since your model predicted the shape of this curve. When viewed, it is immediately interpreted that there is a maximum amount (Xmax) of adsorbed particles, which will not vary no matter how much the pressures are increased.

This maximum adsorption value can be due to several reasons. One of them is that a chemisorption is occurring, which means that the particles or adsorbates are strongly attached to the surface of the solid or adsorbent. Once there is no more space on the surface to accommodate more particles, there will be no more adsorption.

Another reason to justify the type I isotherm is that a physisorption occurs, which means that the particle-surface interactions are very weak (they do not imply formation of chemical bonds ).

In this case, the particles end up entering micropores, which once filled, the surface will not have more sites for subsequent adsorption; that is, it has little external area available (as if it were a very fine trellis). This behavior is observed when microporous powders are analyzed.

Isotherm type  II

Type II isotherm. Source: Daniele Pugliesi / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Above we have the type II isotherm, also known as the sigmoid isotherm. It describes physisorption processes for both non-porous and macroporous solids.

Note that it initially resembles the above isotherm, which means that the adsorbed particles are forming a monolayer on the surface of the adsorbent. Once the monolayer is ready, the other particles will adsorb on top of the first ones, giving rise to multilayers. It is here that we see the characteristic increase of this isotherm (on the right).

Another reason why the type II isotherm is obtained is because the particles have a greater affinity for the surface than for themselves. In other words, the monolayer (particle-surface) will be more stable and durable than the multilayers (particle-particles) formed later.

Isotherm type  III

Type III isotherm. Source: Daniele Pugliesi / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The type III isotherm is similar to the type II in terms of its interpretation: multilayers and a physisorption. However, this time the interactions between the multilayers are stronger than those of the monolayer with the surface of the solid. Therefore, it is an irregular adsorption, with mounds of adsorbed particles and free surface parts.

Isotherm type  IV

Type IV isotherm. Source: Daniele Pugliesi / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The type IV isotherm also describes physisorption and multilayer processes, resembling the type II isotherm; but now, in porous (and mesoporous) solids, where the condensation of gaseous particles in small volumes of liquid is possible. Until the pore is “clogged” with liquid, the monolayer is not complete.

Isotherm type  V

Type V isotherm. Source: Daniele Pugliesi / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

The type V isotherm is similar to the type IV, only this time multilayer formations are more prone than the respective monolayer. That is, it resembles the adsorption described by the type III isotherm. Here the multilayer reaches a maximum thickness, where there are no longer places for more particles to adsorb.

Examples

Some examples of gas-solid systems will be mentioned below together with the type of isotherms that have been obtained in their experimental studies:

-Ammonia-carbon (type I)

-Nitrogen-zeolites (type I)

-Hydrogen-carbon at high temperatures (type I)

-Oxygen-carbon black (type I)

-Nitrogen-silica gel (type II)

-Nitrogen-iron (type II)

-Bromo-silica gel (type III)

-Iodine vapor-silica gel (type III)

-Nitrogen-polyethylene (type III)

-Krypton-carbon black (type IV)

-Benzene-ferric oxide gel (type IV)

-Water-carbon steam (type V)

Note that the solids mentioned were carbon, carbon black, metallic iron, iron oxide, zeolites, and silica gel. All of them are good examples of adsorbents with various industrial applications.

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