Adsorption is the taking up of a gas, vapor, or liquid by a surface or interface. This term, rather than "absorption," is used when no surface penetration occurs. When doubt exists as to whether adsorption or absorption takes place, the term "sorption" is used.

Adsorption results from unbalanced forces associated with surface molecules of a solid or liquid. The high potential energy of these molecules is reduced by the attraction of foreign substances. The adsorbed substances show increased reactivity.

Solids that have large surface areas compared with their bulk volumes are adsorbents on which molecules of gas, vapor, or liquid (adsorbate) are adsorbed. A large surface-to-volume ratio is found either in finely divided solids or in large bodies that contain a network of pores.

In physical adsorption, the bonding between adsorbent and adsorbate is caused by Van der Waal forces. Chemical adsorption, or chemisorption, is characterized by exchange or sharing of electrons between adsorbent and adsorbate.

Adsorption on a liquid surface occurs when the adsorbate dissolves in the liquid. If the substance dissolved in the liquid is more concentrated at the surface than in the bulk of the liquid, the surface tension of the liquid is lowered. Such solutes are called surface-active agents.

Solid adsorbents can be polar or nonpolar. lon-dipole or dipole-dipole interactions between the solid and adsorbate predominate on polar surfaces, such as alumina, barium sulfate, calcium carbonate, glass, ion-exchange resins, silica gel, and zeolites. Dispersion forces predominate on nonpolar surfaces, such as carbon black, graphite, charcoal, organic resins, plastics, paraffin, and talc.

The amount of adsorption depends on the surface area, the adsorbent, the adsorbate, and the temperature. The adsorption can be very great: one volume of boxwood charcoal adsorbs 90 volumes of ammonia. Gas-mask canisters contain layers of charcoal that remove toxic gases by adsorption. Charcoal also is used to purify water and to decolorize sugar solutions.

Trace materials can be separated from biological samples by taking advantage of the different effect of powdered adsorbent on each component of the mixture. This method of separation is called adsorption chromatography.

More by this Author

  • What is Hypnotism?

    Hypnotism the study of hypnosis or the art of inducing it. Hypnosis is an induced state of extreme suggestibility. The hypnotized subject concentrates exclusively on whatever is presented to him by the hypnotist and is...

  • Acoustics Definition

    Sound enters a room in three ways: through the air, through the structure, or by the diaphragm action of floors, walls, and ceilings. Airborne sounds are absorbed by using thick and absorbent walls in which there are no...

  • The White House

    The White House as it appears from the north. Photograph by David Lat. The White House is the official residence of the President of the United States, located in Washington, D.C. It is on Pennsylvania Avenue facing...


No comments yet.

    Sign in or sign up and post using a HubPages Network account.

    0 of 8192 characters used
    Post Comment

    No HTML is allowed in comments, but URLs will be hyperlinked. Comments are not for promoting your articles or other sites.

    Click to Rate This Article