How soap is actually made and how it acts as a cleansing agent?

“Lux, Dove, Palmolive, Nivea…” all  these are very common brands and products of soap that can be found everywhere in the market. Soap is an essential cleansing agent that we used for bathing, cleaning and washing in our daily life. It helps us to get rid of stain and wash away oil, grease and dirt. Nowadays, there are many different types of soap produced by the soap industries for various usages such as personal use soap, kitchen soap, laundry soap and medicated soap. We know soap is necessary in our life and we use it every day. However, have you ever wondered how soap is made and how it acts as a cleansing agent?

          Soap is made in a process called saponification. The three main ingredients in soap-making are triglyceride, lye and water. Triglyceride is the main constituent of vegetable oils and animal oils. It is formed by the ester linkage of three fatty acids to a glycerol. Fatty acids consist of long hydrocarbon chain with carboxyl group (-COOH) at one end. They can be either saturated or unsaturated. Saturated fatty acid such as stearic acid does not have double bonds between the carbon atoms. On the other hand, unsaturated fatty acid such as oleic acid has one or more double bonds between the carbon atoms. Glycerol or propane-1, 2, 3-triol is an alcohol with three carbon atoms and three hydroxyl (-OH) groups.

          Lye is a strong alkali used in saponification. Lyes can either be prepared as sodium hydroxide (caustic soda) or potassium hydroxide (caustic potash). Traditionally, lye was prepared by pouring water to wood ashes. This is because the ashes contain potassium carbonate, the solution that results is basic. Lye is very corrosive chemical that can causes serious injury and damage. Thus, it must be handled in a safe and proper way. When triglyceride is hydrolyzed with the presence of strong alkali, the ester linkage is broken down. As a result of this reaction, a glycerol molecule and fatty acid salt are produced. The fatty acid salt that formed is called soap. This is also the industrial method to prepare glycerol.

saponification of triglyceride by using sodium hydroxide as lye

          Soap can be categorized into two kinds: soft soap and hard soap. Soft soap is more readily dissolve in water compared to hard soap. Soft soap is made by using caustic potash whereas hard soap is made by using caustic soda. In addition, soft soap can be prepared by either using cold process or hot process, but hard soap can only be prepared by using hot process. The cold process soap-making requires several days until the soap is completely saponified. Unlike cold processed soap, the hot process soap-making takes only a few hours. Hot process soap-making is more preferable than cold process soap-making because in hot process, exact measurement of the amount of lye and fats/oils is not required. 

This video shows the demonstration of making soap (Adapted from website http://www.youtube.com/watch?v=LFpZYfwbR-E)

          A soap molecule is made up of both polar and non-polar structures. One end is the long hydrocarbon chain which is non-polar and hydrophobic while another end is hydrophilic head consists of sodium or potassium ion. The non-polar end tends to attract non-polar molecules such as grease and oil and the polar end attracts water.  When soap is dissolved in water, the sodium or potassium ions will move freely in the solution.The long chain carboxylate ions do not exist as individual ions in aqueous solution. Instead, they will arrange themselves in a spherical clusters called micelles. A soap micelle is formed with hydrophilic heads point outwards, forming outer surface of the spherical structure and the non-polar tails are buried in the center of micelle in order to minimize their contact with water. 

          Water alone is not a very effective cleaner because dirt is carried by non-polar molecules. Soap acts as an emulsifier by trapping the fats/oils within the micelles. Since the surface of micelles is charged, therefore individual micelles repel each other instead of clustering together. The soap micelles are easily washed away in soft water. However, the concentrations of calcium and magnesium are high in hard water, micelles will aggregates to form precipitate that we called soap scum.

The cleansing action of soap ( Adapted from website  http://www.youtube.com/watch?v=NjZDTiV2s_w&feature=fvwrel)

          Soap making is actually creative and fun. It is not really difficult to make your own soap. You can get more information and learn more about soap making here <http://www.soap-making-resource.com/>.