Soap. It’s a household staple and comes in practically every scent, colour and texture that you could ever desire. It saves your favourite clothes from the dreaded tomato stains you inflict on them at lunch and keeps you safe from the germs that hitch a ride on your hands. I’m sure you’ve all asked yourselves at least once, well how is this magical substance made? And if you haven’t, then I’m glad I’ve piqued your curiosity enough to click on the link.
The History of Soap
The oldest mention we have of soap production is from 2800 BC in ancient Babylon. The soap was made using a mixture of oil and wood ash and used to wash woollen clothing. Other examples of historical soaps were the Egyptian soaps (made of animal fat or vegetable oil mixed with soda ash), or the Roman soaps (made of tallow and ashes).
However, before discovering the milder soaps used by the Gauls around 58 BC, the Romans used oil and a strigil to scrape off dirt. Soap made of animal fat didn’t appear in Ancient China until the modern era, so plant-based detergents were the norm. Some soap varieties produced in 16th century Europe are still around today- such as Castile soap, which is made of pure olive oil. The ubiquitous liquid soap was only invented in 1865, with popular iterations being produced in the late 19th and early 20th centuries, such as Palmolive, Pine-Sol and Tide.
How is soap made?
In order to make a good bar of soap, you must combine a lipid (a fat or oil) and a base (a substance that neutralises an acid to produce a salt and water). A lipid is made up of fatty acids attached to a backbone of glycerol.
Most toilet soaps are made using triglycerides,which consist of three fatty acids and a glycerol backbone and are found in vegetables and animals. This triglyceride is saponified using an alkaline solution, causing the lipid to convert into an alcohol (the glycerol backbone) and soaps (the salts of the fatty acids).
The alkali you use will determine the kind of soap you get. Alkalis like potassium hydroxide will yield a soft or liquid soap, while sodium hydroxide makes a hard one. The texture is also determined by the lipid you use. Castile soap is said to be very mild and is quite a soft option. Other vegetable-based oils used include coconut, palm and seed oils, and the most common animal fat used is tallow.
And now, the moment you’ve all been waiting for:
How does soap work?
First, we must understand how water works.
As shown by this beautiful hand drawn image, water is an oxygen atom and two hydrogen atoms covalently bonded together. This means that each hydrogen shares a pair of electrons with the oxygen molecule. However, oxygen has more positive protons than the hydrogen atoms, so it pulls more strongly on the electrons. As a result, the oxygen is slightly negative (δ-) while the hydrogens are slightly positive (δ+). The molecule thus has two different charged regions, making it dipolar, and this influences a lot of the properties that water has.
The polar molecules present in water make it an extremely good solvent. For example, salt dissolves in water because the water makes the positive sodium atoms (Na+) and negative chloride ions (Cl-) split apart. They do this because collectively the water molecules have a stronger attraction for the ions than they have for each other. However, non-polar molecules, such as oils and fats are unable to do this and are therefore hydrophobic.
But soap is a jack of all trades, and contains both an oil-soluble section, and a water-soluble section. When you wash something with soap, the water-insoluble lipids get trapped inside spheres called micelles, which are hydrophilic on the outside, and lipophilic on the inside. Now when the water washes the oil stain on your clothes, it washes the micelles, and the lipids inside them, away. This emulsification of oil is the same mechanism used when you wash your hands with soap and hot water after baking, to get all the butter off. Soap is also used to sanitise objects, such as your hands. It can kill microorganisms by disorganising their outer membrane, which is made of two layers of lipid molecules, and denaturing their proteins (spoiler alert: this is bad. Without proteins you’ve got no enzymes, which keep the reactions in your body speedy enough to keep you alive).
And there you have it. Even if you don’t click off this tab with a newfound inspiration to start your own soap business, or try your hand at some amateur soap making, I hope you spare a little more thought for the hard working micelles next time you’re washing your hands.
