Surfactant types classification, identification, separation

Surfactant. For the purpose of this book, a surfactant is a substance which in solution, particularly aqueous solution, tends to congregate at the bounding surfaces, i.e. the air-solution interface, the walls of the containing vessel and the liquid-liquid interface if a second liquid phase is present. As a result, the various interfacial tensions are reduced. 

The reason for this behaviour is that the surfactant molecule contains two structurally distinct parts, one of which is hydrophilic while the other is hydrophobic. Oil-soluble surfactants have an oleophilic and an oleophobic part. In the great majority of surfactants, the hydrophobic part is a hydrocarbon chain, which usually has an average length of 12 to 18 carbon atoms and may include an aromatic ring. A single molecule in aqueous solution seeks the surface, because its hydrocarbon tail is repelled by the water, and it tends to remain there, with the hydrophobic part above the surface and the hydrophilic part below, i.e. in the water phase. Further molecules seek the surface or the walls of the vessel until both are full, at which point further additions result in the formation of micelles, which are clusters of molecules arranged with the hydrophobic parts towards the centre and the hydrophilic parts on the outside. 

Surfactants are classified according to the nature of their hydrophilic parts. 

Anionic surfactant a surfactant in which the hydrophilic part carries a negative charge. Examples soaps, RCOO alkyl sulphates, ROSOJ. For typical cations see section 2.2.1. 

Nonionic surfactant a surfactant in which the hydrophilic part is uncharged. Examples acyl diethanolamides, RC0N(C2H40H)2 ethoxy-lated fatty alcohols, R(OC2H4) OH. 

---