Two faces of one molecule
A surfactant has a hydrophilic head that bonds with water and a hydrophobic tail that bonds with fats and oils. In water the molecules cluster into tiny spheres – micelles – with their tails pointing inwards. Greasy dirt is trapped in the centre of a micelle and the water carries it away.
Surfactants also lower the surface tension of water. The water then wets surfaces better, gets in between textile fibres and forms foam. Foam is visible, but it says little about cleaning power – some very effective surfactants foam only weakly.
Four groups by the charge of the head
| Group | Examples (INCI) | Where you find them |
|---|---|---|
| Anionic | Sodium Lauryl Sulfate, Sodium Laureth Sulfate, soaps | shampoos, gels, washing-up liquids, powders |
| Non-ionic | Coco-Glucoside, ethoxylated alcohols | gentle cleansers, laundry gels |
| Amphoteric | Cocamidopropyl Betaine | shampoos, shower gels (they soften anionic surfactants) |
| Cationic | quaternary ammonium compounds | fabric softeners, hair conditioners |
Cationic surfactants carry a positive charge, so they bind to negatively charged surfaces – textile fibres or damaged hair. They do not clean; they soften and reduce static. That is why they are in fabric softeners and conditioners, not in shampoo.
SLS and SLES
Sodium Lauryl Sulfate (SLS) and Sodium Laureth Sulfate (SLES) are related anionic surfactants. SLES is made from the same fatty alcohol, which is ethoxylated before sulfation – a short chain is inserted between the tail and the head. The result is a surfactant that is generally less irritating to the skin. That is why SLES dominates in shower gels, while SLS is more likely to turn up in toothpastes and cleaning products.
On a cosmetics label, their position in the list makes them easy to spot: the surfactant usually comes straight after water.
Surfactants and the environment
In the EU, surfactants in detergents must meet the criteria for ultimate aerobic biodegradability (Article 4 of Regulation No 648/2004). Surfactants that fail can only be granted a derogation under strict conditions, and only for industrial use.
Where fragrance comes in
Surfactants themselves do not smell pleasant – many have a soapy or fatty odour. The fragrance in laundry and cleaning products masks it and adds a “clean” scent, which is in fact composed mainly of synthetic musks, floral and citrus materials. How it is labelled is covered in the article Fragrance in laundry detergents.