Adsorption is a surface phenomenon where particles are attached to the top layer of material. It is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface. The adsorbing phase is the adsorbent, and the material concentrated or adsorbed at the surface of that phase is the adsorbate. Adsorption is different from absorption, which is a process in which material transferred from one phase to another (e.g. liquid) interpenetrates the second phase to form a “solution”.
Adsorption can be classified into two types: physisorption and chemisorption. Physisorption is due to weak Van der Waals forces between adsorbate and adsorbent, while chemisorption is due to strong chemical forces of bonding type between adsorbate and adsorbent. The adsorption of a gas on a solid is a spontaneous exothermic reaction, and the amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.
Adsorption is present in many natural, physical, biological, and chemical systems and is widely used in industrial applications such as heterogeneous catalysts, activated charcoal, synthetic resins, and water purification.
