1. Chemical structures and different classes of surfactants

Surfactant molecules have two parts of different polarities.

— An apolar, hydrophobic, lipophilic part, with an affinity for oils (oil-soluble).

— A polar, hydrophilic part with an affinity for water (water-soluble).

The lipophilic part consists of one or more aliphatic, linear or branched, or aromatic or alkylaromatic hydrocarbon chains. The hydrophobic character of the hydrocarbon part varies with the number of carbon atoms, the number of unsaturations and the branching. As a general rule, hydrophobicity increases with the number of carbon atoms and decreases with the number of unsaturations.

The hydrophilic part, or polar head, is made up of one or more polar, ionic or non-ionic groups.

Surfactants are classified according to the nature of the hydrophilic part,...