Soap is a sine qua non in everyday life that has a very ancient tradition.
Its use dates back to the Babylonians and the Egyptians, who used to combine animal and vegetable oils with alkaline salts to produce it. According to the roman legend, the word “soap” comes from Mount Sapo, where the animals were sacrificed. The fat from sacrificed animals together with wooden ashes were washed by the rain to the Tiber River, where people used to wash their clothes and found out that this mixture could help to clean the clothes. A similar word was also present in German language for a mixture of ash and fats that was utilized to dye the hair red. Gallic women have been the first to realize that this mixture could easily remove stains. This recipe didn’t change for centuries and had a great diffusion all over the world, however it evolved during the years, finally allowing the industrial production of soap from the mid-nineteenth century. Since then soap, which was initially considered a luxury, became widely available to everyone.
CHEMISTRY OF SOAP
From a chemical point of view, soaps are water-soluble sodium or potassium salts of fatty acids. They are obtained by mixing a strong base with fats and oils from animal or plant sources. When these are heated up together, they react in the so-called “saponification reaction”. In the past, this reaction was performed using fat and lye, which was obtained from leaching ashes through a purification process by filtration and heating. Nowadays, the principle is the same, but some variations have been introduced to make the process easier and quicker.
In the saponification reaction fats and oils comes usually from animal or plant sources. These are used in the form of triglycerides, whose structure is composed of a glycerol molecule to which are linked three molecules of fatty acids. When these react with a base, the alkaline conditions hydrolyse the triglyceride allowing the formation of the corresponding salt (which constitutes the soap) and glycerin, which is reused for other purposes in industrial processes.
Each soap molecule is constituted by two moities:
– An hydrophobic moiety or “tail”, which is a long hydrocarbon chain
– An hydrophilic “head”, which is represented by the carboxylate group that forms the sodium or potassium salts and is negatively charged.
This hydrophilic part of the molecules interacts with water molecules through hydrogen bonding and ion-dipole interactions, whereas the hydrophobic parts attract dirt and do not interact with water molecules. However, this long hydrocarbon chains interact with each other by dispersion forces and form structures called micelles. In each micelle, the molecules are oriented in a way that allows the hydrophobic moieties of the molecules to hide from water, giving rise to a spherical structure. The hydrophilic heads of the molecules, instead, interact with water, but since they are negatively charged, they repel other micelles, which remain dispersed in water.
THE INDUSTRIAL PRODUCTION
The industrial manufacture of soap has a long tradition and it can be performed through discontinuous or continuous processes.
The discontinuous processes include different phases:
- Saponification, which is carried out in boilers heated with direct fire or steam, by stirring the fat mixture and the base that is gradually added. This operation takes about 3-4 hours keeping the temperature around 100°C. However, the same process can be carried out in few minutes under pressure and at higher temperatures (250°C).
- Salting: from the saponification a colloidal solution is obtained and it contains soap, glycerin and base residues. These two components can be separated from the lye simply by adding sodium chloride (kitchen salt). During this stage, also soap and glycerin are separated and the glycerin can be recovered and reused for other industrial purposes (cosmetic and pharmaceutical industries).
- Strong change: This process allows removing the fat that has not saponified by adding a strong caustic solution. In this way, all the fat is converted into soap. This step can be followed by a second salting treatment.
- Pitching: During this process, the soap is boiled again with water, to form two different layers
– “Neat soap”, which is formed by 70% soap and 30% water
– “Nigre”, which contains most of the impurities (salts and dirt) and most of the water
The neat soap is collected and cooled and finally treated with other substances such as emollients, colorants and perfumes.
The continuous processes are the most common applied production systems and also the most convenient, as the prices are consistently reduced and the obtained products have consistent characteristics.
MAKE YOUR OWN SOAP AT HOME
Soap can be easily made at home with some nice experiments that will help you to understand the chemistry and to create your own unique soap.
Some nice recipes are described in this website: http://candleandsoap.about.com/od/soaprecipes/tp/basicsoaprecipes.htm
Or you can simply follow this youtube video: https://www.youtube.com/watch?v=qGfXLznJJY0