Structural Biochemistry/Lipids/Soap

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Soap is a form of lipid which is a mixture of sodium salts of various naturally occurring fatty acids. When air bubbles are added to a molten soap, the density of the soap decreases thus making it float on water. A softer soap lather results when the fatty acid salt contains potassium rather than sodium. Soap is the product of a saponification or basic hydrolysis reaction of a fat or oil. Sodium carbonate or sodium hydroxide is currently used to neutralize the fatty acid and convert it to the salt.

The basic hydrolysis reaction is as follows:

Fat + NaOH → sodium salt of fatty acid + glycerol

This reaction occurs in two steps and the net effect is the broken ester bonds. The glycerol first turns back into an alcohol, then the fatty acid portion is turned into a salt due to the presence of the basic NaOH solution. The oxygen of the carboxyl group now has a negative charge which attracts the positive sodium ion.


Micelles are formed when a certain molecule are added with water. The molecule is fatty acids, phospholipids, or a salt of a fatty acid (soap). The molecule has a strong polar head and non-polar hydrocarbon tail. When this molecule is added to water, the non-polar tails associate each other in the center like a ball because their hydrophobic tails or “water hating” are not soluble in water. The polar heads of the molecule form a shell outside of the ball and interact with the water molecules.

Types of Soap:

The unique properties of different soaps are based on the type of fatty acid and length of the carbon chain of the molecule. Tallow or animal fats give primarily sodium stearate (18 carbons)- a very hard, insoluble soap. Fatty acids with longer chains are even more insoluble. As a matter of fact, zinc stearate is used in talcum powders because it is water repellent.

Coconut oil is a source of lauric acid (12 carbons) which can be made into sodium laurate. This soap, on the other hand, is very soluble and will lather easily even in sea water.

Fatty acids with only 10 or fewer carbons are not used in soaps because they irritate the skin and have objectionable odors.

Cleansing Action of Soap:

The cleansing action of soap is determined by its amphipathic properties- polar and non-polar structures, as well as its solubility characteristics. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water); and the "salt" end of the soap molecule is ionic and hydrophilic (water soluble).

Monolayer: As soap is combined with water, the ionic-salt end of the soap molecule is attracted to the water molecules and consequently dissolves in it. The non-polar hydrocarbon end of the soap molecule is repelled by the water molecules. A drop or two of soap in water forms a monolayer on the water surface. The soap molecules "stand up" on the surface as the polar carboxyl salt end is attracted to the polar water. The non-polar hydrocarbon tails are repelled by the water, which makes them appear to stand up.

Soap vs. Oil vs. Water:

Due to their opposite polarity, water by itself cannot penetrate grease or oil. However, when grease or oil (non-polar hydrocarbons) are mixed with a soap-water solution, the soap molecules work as a "bridge" between polar water molecules and non-polar oil molecules. Soap molecules are amphipathic and thus have both properties of non-polar and polar at opposite ends of the molecule.

The oil is a pure hydrocarbon so it is non-polar. The non-polar hydrocarbon tail of the soap dissolves into the oil. That leaves the polar carboxylate ion of the soap molecules are sticking out of the oil droplets, the surface of each oil droplet is negatively charged. As a result, the oil droplets repel each other and remain suspended in solution (this is called an emulsion) to be washed away by a stream of water. The outside of the droplet is also coated with a layer of water molecules. This is also similar to a micelle which works with the same principles- the center of the micelle would contain the oil.

Effect of Hard Water:

When soap is used in "hard" water, it will be precipitated as a "bath-tub ring" by calcium or magnesium ions present in the "hard" water. The effects of "hard" water calcium or magnesium ions decrease with the addition of "builders". The most common "builder" is sodium trimetaphosphate. The phosphates react with the calcium or magnesium ions thus keeping them in solution but away from the soap molecule. The soap molecule can then do its job without interference from calcium or magnesium ions. Other "builders" include sodium carbonate, borax, and sodium silicate, which are currently in detergents.


Water is the primary ingredient in shampoo, making up 60-70% of its content. The ingredients in shampoo are most often biodegradable. Detergents are the next most abundant ingredients in shampoo. Detergents are surfactants that react with the surface in order to remove oil and dirt particles from the hair follicle. The insoluble portion aligns with the hair’s oil particles while the water soluble portion aligns on the outside, creating a micelle. Common detergents include ammonium laurel sulfate, sodium laurel sulfate, and sodium laurel ether sulfate. Sodium laurel ether sulphates are a popular ingredient in shampoos because of their biodegradability and ability to give a “foaming” effect. The sulphates are made from either natural or synthetic linear C12-C15 alcohols. Thickeners are then added in order to increase viscosity. Popular thickeners include sodium chloride and methylcellulose. Shampoo strips the hair of its sebum, which is required to protect the hair shaft. Therefore, shampoos must also contain an ingredient to replace the lost sebum. Silicones, polymers, and quaternary agents coat the hair and replace the lost sebum. Because shampoos consist of mostly water and organic compounds, preservatives such as parabens are added to prevent the growth of bacteria and maintain freshness.


Ophardt, Charles E. Elmhurst College, Virtual Chembook. 2003