Soap and detergent
Cleaning agents remove oily dirt from surfaces. A soap is the sodium salt of a fatty acid, made by boiling animal fat or vegetable oil with concentrated sodium hydroxide — a reaction called saponification: fat + 3NaOH → soap + glycerol. A detergent is the sodium salt of a sulfonic acid (an alkyl sulfate or sulfonate) made from petroleum fractions.
How they clean
Both soap and detergent molecules have the same design: a long hydrophobic (water-hating) hydrocarbon tail that dissolves in grease, and an ionic hydrophilic (water-loving) head that dissolves in water. During washing the tails bury themselves in the grease while the heads face the water. The grease is lifted off and broken into tiny droplets called micelles, which are washed away — this is emulsification.
Key idea
Cleaning action = hydrophobic tail into grease + hydrophilic head into water. Agitation forms micelles that carry grease away in the rinse water.
Hard water and additives
In hard water (containing Ca2+ and Mg2+ ions) soap forms an insoluble scum, wasting soap. Detergents do not form scum, so they clean well in hard water — a key advantage. Detergents also contain useful additives.
| Additive | Function | |
|---|---|---|
| Biological enzymes | Digest protein and fat stains | |
| Optical brightener | Makes fabric look whiter | |
| Sodium tripolyphosphate | Softens water (a builder) | |
| Perfume | Gives a pleasant smell |
Worked example
Why does soap clean poorly in seawater? Seawater is hard, rich in Ca2+ and Mg2+. Soap reacts with these ions to form insoluble scum, so less soap remains to clean. A detergent avoids this problem.
Remember
- Soap = salt of fatty acid; detergent = salt of sulfonic acid.
- Tail = hydrophobic; head = hydrophilic.
- Detergent works in hard water; soap forms scum.