| |
The molecules at a surface of a material experience imbalanced forces of intermolecular interaction which contribute to the surface energy. It causes accumulation of molecules of a solute or gas in contact with the substance. This preferential accumulation of substrate molecules at the surface is called adsorption which is purely a surface phenomenon.
The surface active material is referred to as the adsorbent and the molecules which are accumulated on the adsorbent called adsorbate molecules. The strength by which adsorbate molecules are attached with the adsorbents determines the nature of adsorption. Normally, release of energy in the range of 8 to 25 kJ/mole due to adsorption is termed as physisorption whereas a much larger energy comparable to chemical bonding energy leads to chemisorption. There are always some exceptions to these values. The prescribed value of energy differentiating physisorption and chemisorption are based on general experience.
When an adsorbed molecule receives energy equal to or greater than the energy of adsorption, it will leave the surface. This phenomenon is the reverse of adsorption and is called as desorption. When the number of molecules striking the surface and staying there is equal to the number of molecules that are leaving (evaporating) the surface the system is said to be in equilibrium.
All the atoms or molecules adsorbed on the surface do not have identical environment since distribution of free energy on the surface is not always smooth because of the differences in the energy of the molecular orbitals of the adsorbent and also due to other internal interactions. |
