Concept 05 / 05Chemistry in Everyday Life
Cleansing Agents: Soaps and Synthetic Detergents
Soaps as sodium or potassium salts of fatty acids, saponification and types of soaps, why soaps fail in hard water, the cleansing action through micelles, and synthetic detergents (anionic, cationic and non-ionic) with examples, uses and biodegradability.
Soaps are the sodium or potassium salts of long-chain fatty acids such as stearic, oleic and palmitic acids. They are made by saponification: heating a fat or oil (a glyceryl ester of fatty acids) with aqueous sodium hydroxide. For glyceryl tristearate:
The soap is precipitated by adding sodium chloride (salting out), and the glycerol remaining in solution is recovered by fractional distillation. Potassium soaps are softer than sodium soaps.
Types of soap. Toilet soaps are made from better-grade fats and oils, with excess alkali removed, and colour and perfume added. Floating soaps have air bubbles beaten in before setting. Transparent soaps are made by dissolving the soap in ethanol and evaporating the excess solvent. Medicated soaps contain medicinal substances; shaving soaps contain glycerol to prevent rapid drying, and a gum called rosin, which forms sodium rosinate that lathers well. Laundry soaps contain fillers such as sodium rosinate, sodium silicate, borax and sodium carbonate. Soap chips and soap powders are other forms.
Soaps in hard water. Hard water contains calcium and magnesium ions, which form insoluble calcium and magnesium soaps that separate as scum: . The soap is wasted, and the sticky precipitate clings to cloth and hair, making them dull.
Cleansing action. A soap anion has a polar, water-loving (hydrophilic) head, , and a long, non-polar hydrocarbon tail that repels water (hydrophobic). Above a certain concentration, the critical micelle concentration (CMC), the anions cluster into micelles (see figure): the tails dissolve in the oily dirt while the heads face the water. The dirt is emulsified into tiny droplets surrounded by negative charges, which repel each other and are rinsed away.
Synthetic detergents have all the cleansing properties of soaps but can be used in hard water (their calcium salts are soluble) and some even in ice-cold water. Three types:
Anionic detergents: sodium salts of sulfonated long-chain alcohols or hydrocarbons, such as sodium lauryl sulfate (from lauryl alcohol with concentrated , then neutralised with NaOH) and sodium dodecylbenzenesulfonate (alkylbenzene sulfonates). The anion is the active part. Used in household washing products and toothpastes.
Cationic detergents: quaternary ammonium salts of amines with acetate, chloride or bromide anions, such as cetyltrimethylammonium bromide. The cation is the active part. They are germicidal and used in hair conditioners, but are expensive.
Non-ionic detergents: no ions, such as the ester of stearic acid with polyethylene glycol. Used in liquid dishwashing detergents; they remove grease by the same micelle mechanism.
Biodegradability. Detergents with highly branched hydrocarbon chains are not degraded by bacteria and cause foam in rivers, ponds and streams. Branching is now kept to a minimum, since unbranched chains are more easily biodegraded.
Key Takeaways:
- Soap = Na or K salt of a fatty acid; saponification releases glycerol; NaCl salts out the soap.
- Hard water (, ) forms insoluble scum with soap but not with detergents.
- Micelles form above the CMC: tails in the grease, heads in the water.
- Anionic (sulfonates, sulfates), cationic (quaternary ammonium, germicidal) and non-ionic detergents.
- Pitfall: branched-chain detergents are non-biodegradable; linear chains degrade.
Brain-Teaser 1: of glyceryl tristearate () is completely saponified. Find the masses of soap and glycerol formed.
Solution: of fat gives of sodium stearate (), that is , and of glycerol (), that is . Check: fat + NaOH = = .
Brain-Teaser 2: Why do synthetic detergents work in hard water while soaps do not?
Solution: The calcium and magnesium salts of sulfonic acids and alkyl hydrogensulfates are soluble in water, so no scum forms and the detergent stays in solution. Soap anions (carboxylates) form insoluble calcium and magnesium salts that precipitate.
Brain-Teaser 3: Why does soap not cleanse well in acidic water?
Solution: In acid, the carboxylate anion is protonated to the free fatty acid, . The fatty acid is insoluble and has no ionic head, so it separates as an oily layer and cannot form micelles.
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