|University Federico II
of Naples, Italy
Monolayer of a fatty acid on water
Several classes of molecules have simultaneously both hydrophilic and hydrophobic characteristics (amphiphilic molecules), with a polar head, interacting favourably with water, and a non polar tail which escapes the contact with water. Fatty acids, bearing very long chains of carbon atoms (alkyl chains), form surface films on water: the polar acid group of each molecule is in the water phase, while the long non polar alkyl chain escapes the aqueous phase, and protrudes in the air above the liquid surface.
A micelle in water
Upon entering water, amphiphilic molecules avoid the contact of the non polar regions with the solvent by forming "micelles", namely aggregates of molecules. In micelles, non polar chains point towards the interior, while the polar, hydrophilic or charged, heads stay on the surface, interacting with water. Hydrophobic interactions determine the favorable association of the hydrocarbon, non polar tails in the interior of the micelle. The shape of the formed structures and the number of molecules in the micelle depends on the kind of interacting molecules.
Structure of a phospholipid and its schematization
Phospholipids, for instance, are among the substances forming micelles. They have a terminal, charged phosphoric group, soluble in water, and two long hydrocarbon tails.
Soaps are sodium or potassium salts of long-alkyl chain fatty acids. Their action consists in removing dirt, bringing it in solution, and eliminating it when washing repeatedly with water. Dirt is usually constituted of greasy substances, as oil, that are non polar and not soluble in water.
Soap molecules have dual characters: the polar, electrically charged head participates to the attractive interactions with water molecules around it, while the non polar tail can interact with dirt. In aqueous solution, soaps form micelles. Oil or grease are sequestered in the internal, non polar region of the micelle, yet remaining dispersed in the aqueous phase through hydration of the external surface formed by the ionic heads. Further washing with water will take away micelles including dirt. To conclude, you can understand now why greasy hands have to be washed with water and soap, not with water only. Water and grease do not dissolve each other, and a carrier substance, the soap, is needed to bring grease in the aqueous phase.