Coaching Centre
Soaps and detergents
How soap is made by saponification of fats, the structure of soap and detergent molecules, how surfactants lower surface tension and form micelles to lift grease, why soap fails in hard and acidic water, and anionic, cationic and non-ionic detergents and their uses and environmental effects.
Crush a handful of leaves from the soap tree, Alphitonia excelsa, rub them with water and they foam. Aboriginal people across eastern and northern Australia have long used them to wash. The leaves contain saponins, natural molecules with one end that loves water and another that doesn't. Every soap, shampoo and dishwashing liquid works the same way. Water alone can't remove grease because grease is non-polar; you need a molecule that can hold on to both at once.
Making soap: saponification
Fats and oils are triglycerides: triesters made from glycerol (propane-1,2,3-triol) and three long-chain carboxylic acids called fatty acids. Boiling a fat or oil with sodium hydroxide hydrolyses the three ester links. This base hydrolysis is called saponification ("soap making"):
triglyceride + 3NaOH → glycerol + 3 sodium carboxylates (soap)
For a typical animal fat, glyceryl tristearate: (C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → C₃H₅(OH)₃ + 3C₁₇H₃₅COONa. Adding concentrated salt solution afterwards makes the soap less soluble so it separates out as a solid curd ("salting out").
The soap molecule
A soap anion has two very different ends:
- a long hydrocarbon tail (12–18 carbons), non-polar, which forms dispersion forces with grease and oil: it is hydrophobic (water-fearing) and lipophilic;
- an ionic carboxylate head, –COO⁻, which forms ion–dipole attractions and hydrogen bonds with water: it is hydrophilic.
A molecule like this is a surfactant (surface-active agent).
How soap cleans
- It lowers surface tension. Surfactant molecules crowd the water's surface with their tails pointing out, disrupting the hydrogen bonding between surface water molecules. Water then spreads and soaks into fabric instead of beading.
- The tails dissolve in the grease. They burrow into the oily dirt while the heads stay in the water.
- Agitation breaks the grease into micelles. Each droplet ends up coated in soap anions, tails in and heads out.
- The micelles stay suspended. Every micelle has a negatively charged surface, so they repel each other and don't merge back into a grease layer. The result is an oil-in-water emulsion that is rinsed away.
- Add soap to the greasy water in soft water. Watch where the tails and heads go. How many micelles form?
- Switch to hard water (Ca²⁺). What appears, and what happens to the cleaning score?
- Switch to acidic water (pH 3). Why does the soap stop working this time?
- Try each detergent in hard water. Which ones still clean, and why?
Why soap fails in hard and acidic water
Hard water contains dissolved Ca²⁺ and Mg²⁺ ions (common in groundwater from limestone areas). These form insoluble salts with soap anions, a grey precipitate called scum:
2C₁₇H₃₅COO⁻(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s)
The soap is wasted as scum, so more is needed before any lather forms, and the scum leaves rings on baths and dulls clothes. In acidic water, the carboxylate head is protonated (C₁₇H₃₅COO⁻ + H⁺ → C₁₇H₃₅COOH). The fatty acid has no charge, so it is insoluble and no longer acts as a surfactant.
Synthetic detergents
Detergents are synthetic surfactants, usually made from petrochemicals, designed to work where soap doesn't. They all have a long hydrocarbon tail; they differ in the head.
| Type | Head group | Example | Properties | Typical uses |
|---|---|---|---|---|
| Anionic | negative, usually sulfonate –SO₃⁻ or sulfate –OSO₃⁻ | sodium dodecylbenzenesulfonate; sodium lauryl sulfate | cheap, high foaming; calcium and magnesium salts are soluble, so no scum in hard water | laundry powders, dishwashing liquid, shampoo, toothpaste |
| Cationic | positive, quaternary ammonium –N⁺(CH₃)₃ | cetyltrimethylammonium bromide | bind strongly to negatively charged surfaces such as hair and fabric; kill bacteria; poorer cleaners | fabric softener, hair conditioner, disinfectants, antiseptic mouthwash |
| Non-ionic | uncharged, a chain of –OCH₂CH₂– units ending in –OH that hydrogen bonds to water | alcohol ethoxylates | no charge, so unaffected by hard water; low foam; mild | dishwasher detergent, front-loader powders, glass cleaner, emulsifiers in food and cosmetics |
Cationic and anionic detergents can't be mixed, because their oppositely charged heads attract and both are neutralised. That is why shampoo (anionic) and conditioner (cationic) come in separate bottles.
Environmental effects
The first detergents of the 1950s had branched tails that bacteria couldn't break down, and rivers and sewage works filled with foam. They were replaced by linear alkylbenzene sulfonates, which are biodegradable. Detergents also used to contain phosphate "builders" to soften water; phosphates fertilise algae and cause algal blooms (eutrophication), so most Australian laundry products are now low-phosphate or phosphate-free. Soap itself is biodegradable, being made from natural fats.
Test yourself
Saponification is
Base hydrolysis of triglycerides makes glycerol and soap.
Micelles of soap in water do not join back together because
Each micelle is coated with carboxylate heads, all negative.
Which surfactant would be best in a hair conditioner?
Cationic heads cling to negatively charged hair, smoothing it.
Soap forms a scum in water containing
Calcium carboxylates are insoluble.
Label the soap molecule.
Which kind of detergent fits each description?
A molecule that lowers the surface tension of water is a . The hydrocarbon tail of a soap is . Soap particles surround grease to form . Fats and oils are . Water containing calcium and magnesium ions is called water.
Explain how soap removes grease from fabric, and why it is less effective in hard water. Include a relevant equation.
A soap anion, such as stearate, C₁₇H₃₅COO⁻, has a long non-polar hydrocarbon tail and an ionic carboxylate head. Grease is non-polar, so it doesn't dissolve in water, but the soap's tails dissolve in the grease because they form dispersion forces with it, while the heads stay in the water, attracted by ion–dipole forces and hydrogen bonds. Agitation breaks the grease into small droplets, each surrounded by soap anions with their tails inside and their heads outside. These micelles all have negatively charged surfaces, so they repel each other and stay suspended as an emulsion that is rinsed away with the water.
Hard water contains Ca²⁺ and Mg²⁺ ions, which react with soap anions to form insoluble salts (scum): 2C₁₇H₃₅COO⁻(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s). Soap that has precipitated can't act as a surfactant, so much more is needed before the grease is lifted, and the scum deposits on fabric and baths.