Soaps and detergents
Soaps are sodium salts of long-chain fatty acids; detergents are sodium salts of sulphonic acids. Both clean by forming micelles around grease.
Making soap: saponification
Oils and fats are esters of glycerol with long-chain fatty acids. Heating them with **sodium hydroxide** gives soap and **glycerol**. This is **saponification**.
- 1An oil or fat (for example coconut oil or groundnut oil) is heated with sodium hydroxide solution.
- 2★(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃Glyceryl stearate (a fat) gives sodium stearate (soap) and glycerol
- 3Common salt is added so that the soap separates and floats (salting out). It is then cooled and cut into cakes.
- 4Sodium salts give hard soaps (washing soaps). Potassium salts give soft soaps (toilet soaps, shampoos, shaving cream).
★ key step
Cleansing action of soap
A soap molecule has two ends. Its long **hydrocarbon tail** is **hydrophobic** (water-hating) and dissolves in oil. Its ionic **head**, -COO⁻Na⁺, is **hydrophilic** (water-loving) and dissolves in water.
- 1When a dirty cloth is soaked in soap water, the tails of soap molecules dissolve in the oily dirt. The heads stay in the water.
- 2★The molecules surround each oil drop, tails inside and heads outside. This cluster is a micelle.
- 3Every micelle carries a negative charge on its surface, so micelles repel each other and do not join up. The oil stays spread in the water as an emulsion.
- 4Rubbing or beating the cloth loosens the dirt. Rinsing with water carries away the micelles, and the dirt with them.
★ key step
Why soap fails in hard water
Hard water contains calcium and magnesium salts, such as CaCl₂ and MgSO₄. Soap reacts with them to form an insoluble, sticky scum: 2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl. Much soap is wasted, and the scum sticks to clothes. Detergents are sodium salts of long-chain sulphonic acids (for example sodium alkyl benzene sulphonate). Their calcium and magnesium salts dissolve in water, so they form no scum and clean well even in hard water.
Soaps vs detergents
| Point | Soap | Detergent |
|---|---|---|
| Chemical nature | Sodium salt of long-chain fatty acids | Sodium salt of long-chain sulphonic acids |
| Ionic head | -COO⁻Na⁺ | -SO₃⁻Na⁺ |
| Made from | Vegetable oils and animal fats | Hydrocarbons from petroleum (crude oil) |
| In hard water | Forms scum; soap is wasted | No scum; works well |
| Foam | Less foam | More foam |
| Effect on environment | Biodegradable | Many are non-biodegradable and pollute rivers and lakes |
How to draw a micelle
- 1Draw a small irregular blob in the centre and label it oil or grease drop.
- 2Draw about ten soap molecules around it like the spokes of a wheel: each a zigzag line (tail) with a small circle (head) at the outer end.
- 3Point every tail into the oil and every head outwards into the water.
- 4Write a minus sign in each head. Label hydrophobic tail, hydrophilic head, micelle and water.
Look at a bathing soap wrapper: Indian soaps print their TFM (total fatty matter). A higher TFM, as in Grade 1 soap, means more real soap and better cleaning. Detergents are handy with hard bore-well water, but non-biodegradable ones cause foam in city lakes.
1 marks: “What is the process of making soap called?” Saponification.
4 marks: “Explain the cleansing action of soap.” Two-part molecule (hydrophobic tail, hydrophilic head); tails dissolve in grease; micelle with heads outside; charged micelles repel and form an emulsion; rinsing removes dirt; draw a micelle.
7 marks: “What is saponification? Explain the cleansing action of soap with a diagram. Give three differences between soaps and detergents.” Saponification equation; cleansing action with a labelled micelle; three differences such as ionic head, behaviour in hard water and biodegradability.
A soap molecule is a tadpole: oil-loving tail, water-loving head. Tail in oil, head in water.