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.