Adsorption Definition (Chemistry)

Activated carbon
Activated carbon is a common material used for adsorption. Ken Brown/Getty Images

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 temperature has a significant effect on the process. The quantity of adsorbate bound to the adsorbent is expressed as a function of pressure of concentration at a constant temperature. Several isotherm models have been developed to describe adsorption, including the linear, Freundlich, Langmuir, BET (after Brunauer, Emmett, and Teller), and Kisliuk theories.

IUPAC Definition of Adsorption

The IUPAC definition of adsorption is the "Increase in the concentration of a substance at the interface of a condensed and a liquid or gaseous layer owing to the operation of surface forces."

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.

Adsorption vs Absorption

Adsorption is a surface phenomenon in which particles or molecules bind to the top layer of a 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.

Terms Related To Adsorption

Sorption: This encompasses both adsorption and absorption processes.

Desorption: The reverse process of sorption. The reverse of adsorption or absorption.

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-base 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.

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.


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