ADSORPTION AT LIQUID INTERFACES
The phenomenon of adsorption is the adhesion of atom, ions, or molecules from a gas, or liquid, a dissolved solid to the surface or interface viz; gas/liquid and liquid/liquid interface. The adsorption at the interface is of two types:
- Positive adsorption: This is the adsorption when certain solute molecules and ions when added to a liquid are partitioned in favor of surface or the interface. This resulted in the increase in the concentration of such molecules and ion at the interface than in the bulk of the liquid and lowers the surface free energy, the surface tension and interfacial tension.
- Negative adsorption: This is the adsorption when certain molecules like; inorganic electrolytes are partitioned in favor of bulk of the liquid. This resulted in the increase in the surface free energy and surface tension.
SURFACTANTS (SURFACE ACTING AGENTS)
Surfactants are solute molecules or ions that are adsorbed at surface/interface and reduce the surface/ interfacial tension. They are also called 'amphiphile' which means they have affinity for both polar and non polar solvents due to presence of polar head and non polar tail respectively. Thus, surfactant may be hydrophilic if it is predominately polar in nature and lipophilic if its predominately non polar. It is the amphiphilic nature of the surfactants that orient them towards polar solvents or away from polar solvents, lipophilic chains are oriented towards the air at air / water interface and towards oil phase at oil/water interface. Surfactants decrease the surface tension or interfacial tension due to their adsorption at the interface.
At lower concentration, surfactant molecules will be distributed at the surface and surface will not be completely filled. As the concentration of surfactant increases, the surface is filled up progressively and finally the surface will be completely saturated with surfactant with marked decrease in the surface tension. If more amount of surfactant is further added after saturation of the surface of the liquid, the surfactant molecules will further move onto the bulk of the liquid and associated into small aggregates called micelles. They are spherical in shape and having colloidal dimension (0.001-0.5 (M). Each micelle contains around 50-100 surfactant molecules.