Why do we use soap to wash ourselves? Why don’t we just use water? If you’ve ever asked yourself these questions, you may have decided that using soap gets us cleaner. So why does soap get us cleaner? Not many people ask themselves this question. But it is by asking ourselves questions like this that we learn and discover new ideas.
Oil, grease and fats all stick dirt to us and to our clothes. So, to get rid of dirt, we need to get rid them. In post 16.45 we saw that water molecules attract each other because they have a small positive electric charge at one end of the molecule and a small negative electric charge on the other end; this negative charge (post 16.25) is less than the charge on an electron (post 16.27).
Water molecules can attract other molecules that have unevenly distributed electrical charges in the same way. Regions of molecules that attract water are said to be hydrophilic (from Greek words meaning “loving water”). However, oils and fats consist of molecules that have long chains of carbon atoms covalently bonded to hydrogen atoms (post 16.47) that don’t mix with water; these long chains are said to be hydrophobic (from Greek words meaning “hating water”) because they don’t attract water. So, oil and water don’t mix. If we make a mixture of cooking oil and water, by whisking the two together, they will soon separate and the oil will float on the water, because it is less dense (post 16.44).
But if we add a few drops of washing-up liquid to the oil and water, before we mix them, the mixture is much more stable. We have little drops of oil suspended in the water. A mixture of little oil drops in water is called an emulsion. Washing-up liquid stabilizes the emulsion and is called an emulsifying agent or a surfactant.
Soap is like washing-up liquid – it acts as an emulsifying agent that stabilizes little drops of oil in water so that dirt drifts off what was previously an oily surface.
How does soap stabilize an emulsion? Let’s represent a hydrophobic molecule by a red blob and a hydrophilic molecule by a yellow blob; as in the picture below. Don’t forget that water molecules are hydrophilic because they mix with each other.
Soap is a mixture of positively charged sodium or potassium ions with negatively charged ions, like the stearate ion (post 16.40). If you look at the chemical formula of the stearate ion (in post 16.40) you will see that it has a long chain of carbon atoms, covalently bonded to hydrogen atoms, (hydrophobic) and has a negative charge on an oxygen atom (hydrophilic) at the end. So, one end of the stearate ion is hydrophobic and the other end is hydrophilic. A molecule or complicated ion like this is called an amphiphile – we say that the stearate ion is amphiphilic. So, we can represent the stearate ion by a hydrophobic part and a hydrophilic part; as in the picture below.
Now suppose we have a little drop of oil surrounded by water. The hydrophobic end of the stearate ion can bury itself in the oil drop; they mix because both are hydrophobic. The hydrophilic end of the stearate ion is left on the surface of the drop, because it doesn’t mix with oil. But this hydrophilic end does attract water molecules. The result is that the suspension of little oil drops in water is stabilized by stearate ions; as in the picture below.
These little drops of oil that are stabilized in water by amphiphilic molecules (or complicated ions like stearate) are called micelles. Micelles occur in emulsion paint, cream and mayonnaise; in mayonnaise, the emulsifying agent is a substance called lecithin that occurs in egg yolks. So thinking about how soap gets us clean helps us to understand the nature of a range of everyday substances and foods.