Adsorption

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid onto a surface, forming a thin film of adsorbate on the adsorbent rather than being absorbed into its bulk. It is a consequence of surface energy: atoms at a material's surface are not fully surrounded by other atoms and so can attract and hold adsorbates, through processes classified as physisorption, driven by weak van der Waals forces, or chemisorption, driven by covalent bonding. Adsorption occurs throughout natural, biological and chemical systems and is used industrially in heterogeneous catalysts, activated charcoal and water purification, and in capturing waste heat to provide cold water for air conditioning through adsorption chillers. The word was coined in 1881 by the German physicist Heinrich Kayser.