Chemistry in your cupboard

Greasy Stains

Most cleaning is done with water and the problem with greasy stains is that they do not mix with water. This is because the main interactions between water molecules are hydrogen bonding and those between molecules of oils and fats (which constitute grease) are van der Waals forces.

To get water and grease to mix we use molecules called surfactants or detergents. These two terms refer to essentially the same thing - molecules that are 'tadpole shaped' in that they have a non-polar 'tail' and a polar or ionic 'head'. The 'tail' can form van der Waals bonds with non-polar grease molecules whilst the 'head' can form hydrogen bonds with water. This is an example of the 'like dissolves like' rule.

There are essentially three types of surfactants - anionic, cationic and non-ionic. The ones which are used in Vanish are anionics and non-ionics.

Anionic surfactants have a negatively charged head group. Common types include soaps and alkylbenzene sulfonates.

Figure 13: Sodium stearate (a soap) - an anionic surfactant (Click to enlarge)

Figure 14: Sodium dodecylbenzene sulfonate - an anionic surfactant

Cationic surfactants have a positively charged head. Common types include alkyl ammonium chlorides.

Figure 15: Trimethylhexadecyl ammonium chloride - a cationic surfactant

Non-ionic surfactants have a polar, but uncharged, head. Common types include polyethylene ethoxylates.

Figure 16: A polyethylene ethoxylate - a non-ionic detergent

When dissolved in water, surfactants tend to cluster at the surface (hence the name), so that their non-polar tails can stick out of the water. Surfactant molecules can form structures called micelles: these are small spheres made of surfactant that trap oil molecules and enable them to dissolve in the water. One of the best known and used surfactants is soap.