Soap 101 – What is Soap?
Soap consists of fats and oils that have been saponified by reacting them with lye or a similar chemical. So when you’re feeling grimy and oily, you use fat to clean the oil off? That sounds counterintuitive.
Here’s the simple explanation:
When fats and oils are mixed with a lye solution, they undergo a chemical reaction that changes them into a completely different substance that we call soap. Even though soap is made from fats and oils, the chemical change turns them into a substance that breaks up fats and oils, allowing water to wash them off easily. It doesn’t have a significant effect on dirt and dust, but since since fats and oils are the reason the dirt is sticking to a surface, breaking up the oils help wash the dirt away too.
Here’s the chemistry:
Fat consists of three fatty acids attached to a molecule of glycerol, forming a triglyceride. Fatty acids consist of a carboxylate ‘head’ attached to a long carbon chain ‘tail’. Fatty acids exist in different lengths. The longer the molecules, the more they tend to stick together. That’s why some fats are solid, while some (oils) are liquid at room temperature.
Lye is the common name for sodium hydroxide. When a lye solution is combined with fat, the triglycerides are split apart from the glycerol molecule, which remains free. This glycerol remains in the soap as glycerin (although commercial soap manufacturers often remove the glycerin to sell as a separate product). A sodium ion attaches to each fatty acid molecule. The new molecule is considered to be a salt of the fatty acid.
When soap and water encounter a layer of oil, the soap breaks up the oil, forming tiny spheres called micelles. The ‘tails’ of the fatty acid molecules move away from the water molecules, becoming hidden inside the spheres with the carboxylate ‘heads’ (which aren’t as hydrophobic, or ‘water-fearing’, as the tails) on the outside. The micelles can dissolve in water, allowing them to be washed away.
After being washed down the drain, the fatty acid micelles and the soap molecules will biodegrade into shorter carbon chains, and eventually into carbon dioxide, sodium ions, water, and whatever impurities existed in the soap or the oil.
(Chemical explanations thanks to the Wikipedia articles on Soap, Glycerol, Lye, and Fatty Acids.
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