What Adsorption Means in Chemistry

Adsorption is defined as the adhesion of a chemical species onto the surface of particles. German physicist Heinrich Kayser coined the term "adsorption" in 1881. Adsorption is a different process from absorption, in which a substance diffuses into a liquid or solid to form a solution.

In adsorption, the gas or liquid particles bind to the solid or liquid surface that is termed the adsorbent. The particles form an atomic or molecular adsorbate film.

Isotherms are used to describe adsorption because the temperature has a significant effect on the process. The quantity of adsorbate bound to the adsorbent is expressed as a function of the pressure of concentration at a constant temperature.

Several isotherm models have been developed to describe adsorption including:

• The linear theory
• Freundlich theory
• Langmuir theory
• BET theory (after Brunauer, Emmett, and Teller)
• Kisliuk theory

Terms related to adsorption include:

• Sorption: This encompasses both adsorption and absorption processes.
• Desorption: The reverse process of sorption. The reverse of adsorption or absorption.

IUPAC Definition of Adsorption

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Absorption Definition in the Chemistry Glossary

By Anne Marie Helmenstine, Ph.D.

The International Union of Pure and Applied Chemistry (IUPAC) definition of adsorption is:

"Adsorption vs. Absorption

Adsorption is a surface phenomenon in which particles or molecules bind to the top layer of material. Absorption, on the other hand, goes deeper, involving the entire volume of the absorbent. Absorption is the filling of pores or holes in a substance.

Characteristics of Adsorbents

Typically, adsorbents have small pore diameters so that there is a high surface area to facilitate adsorption. The pore size usually ranges between 0.25 and 5 mm. Industrial adsorbents have high thermal stability and resistance to abrasion. Depending on the application, the surface may be hydrophobic or hydrophilic. Both polar and nonpolar adsorbents exist. The adsorbents come in many shapes, including rods, pellets, and molded shapes. There are three major classes of industrial adsorbents:

• Carbon-based compounds (e.g., graphite, activated charcoal)
• Oxygen-based compounds (e.g., zeolites, silica)
• Polymer-based compounds

How Adsorption Works

Adsorption depends on surface energy. Surface atoms of the adsorbent are partially exposed so they can attract the adsorbate molecules. Adsorption may result from electrostatic attraction, chemisorption, or physisorption.

Examples of Adsorption

Examples of adsorbents include:

• Silica gel
• Alumina
• Activated carbon or charcoal
• Zeolites
• Adsorption chillers used with refrigerants
• Biomaterials that adsorb proteins

Adsorption is the first stage of a virus life cycle. Some scientists consider the video game Tetris a model for the process of adsorption of shaped molecules onto flat surfaces.

Uses of Adsorption

There are many applications of the adsorption process, including:

• Adsorption is used to cool water for air conditioning units.
• Activated charcoal is used for aquarium filtration and home water filtration.
• Silica gel is used to prevent moisture from damaging electronics and clothing.
• Adsorbents are used to increase the capacity of carbide-derived carbons.
• Adsorbents are used to produce non-stick coatings on surfaces.
• Adsorption may be used to extend the exposure time of specific drugs.
• Zeolites are used to remove carbon dioxide from natural gas, remove carbon monoxide from reforming gas, for catalytic cracking, and other processes.
• The process is used in chemistry labs for ion-exchange and chromatography.

Sources

• _{Glossary of atmospheric chemistry terms (Recommendations 1990)". Pure and Applied Chemistry 62: 2167. 1990.}
• _{Ferrari, L.; Kaufmann, J.; Winnefeld, F.; Plank, J. (2010). "Interaction of cement model systems with superplasticizers investigated by atomic force microscopy, zeta potential, and adsorption measurements." J Colloid Interface Sci. 347 (1): 15–24.}