adsorption

Taking up of a gas or liquid at the surface of another substance, most commonly a solid (for example, activated charcoal adsorbs gases). It involves molecular attraction at the surface, and should be distinguished from absorption (in which a uniform solution results from a gas or liquid being incorporated into the bulk structure of a liquid or solid).

There are several categories of adsorption.

Liquid films on liquids Long-chain acids such as stearic acid, CH3(CH2)12COOH, will spread over the surface of water and be adsorbed by it. The -COOH or polar part of the molecule will enter the water, while the hydrophobic ‘tail’ will remain on the surface.

Gases and solids There are two types of adsorption of gases by solids: physical adsorption and chemisorption. Physical adsorption takes place if essentially physical forces, such as condensation, hold the gas to the solid. In these cases the heat of adsorption is less than 40 kJ mol−1 and adsorption is only appreciable at temperatures below the boiling point of the adsorbate. Physical adsorption is more a function of the adsorbate than the adsorbent, and no activation energy is involved in the process. The converse is true for chemisorption. As a rule chemical bonds play a part in the process. The heat of adsorption is greater than 80 kJ mol−1, the process occurs at high temperatures, the adsorbent itself is significant, and activation energy may be involved.