Adsorption

Not to be confused with Absorption.

Particles adsorbed to a surface.

Adsorption is the sticking of atoms or molecules to a surface (called "adsorbent"). The particles which get stuck on can be from a gas, liquid or a dissolved solid. An example would be the way water sticks to the surface of sand grains on a beach, or to particles of soil.

The exact nature of the force which holds them on varies according to the nature of the surface and the adsorbed molecules. The force may be a weak physical force, or weak chemical bonds, but strong enough to keep the layer of molecules stuck on. Supplying more heat usually breaks the bonds and causes the adsorbed molecules to evaporate. Only "free water" (that is not trapped inside solid particles) can evaporate.

Adsorption Media

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  Brunauer, Emmett and Teller's model of multilayer adsorption is a random distribution of molecules on the material surface.

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  Langmuir (blue) and BET (red) isotherms

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  Two adsorbate nitrogen molecules adsorbing onto a tungsten adsorbent from the precursor state around an island of previously adsorbed adsorbate (left) and via random adsorption (right)

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  Activated carbon is used as an adsorbent

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  Silica gel adsorber for NO2, Fixed Nitrogen Research Laboratory, ca.1930s

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  Activated carbon nitrogen isotherm showing a marked microporous type I behavior

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  A schematic diagram of an adsorption chiller: (1) heat is lost through evaporation of refrigerant, (2) refrigerant vapour is adsorbed onto the solid medium, (3) refrigerant is desorbed from the solid medium section not in use, (4) refrigerant is condensed and returned to the start, (5) & (6) solid medium is cycled between adsorption and desorption to regenerate it.