Saponification

Potassium or sodium salts of fatty acids are the main ingredients of soap. The process by which they are formed is called saponification. Saponification is simply the hydrolysis of fats and oil with caustic alkali to form propane-1,2,3-triol (glycerol) and the corresponding salt or soap.

Fat or Oil + caustic alkali → soaps + propane-1,2,3-triol

There is a need to clarify the difference between soap and detergents. While detergent is any substance that has the ability to clean an object, soap is an example of detergent. This includes soap powders, liquids soap and water. But there are two types of detergents called, soapless detergent and soapy detergents. What is generally known as soap is simply soapy detergent.

Soap is produced commercially from vegetable oils or animal fats like mutton. Palm kernel oil, red palm oil, olive oil and coconut oil are examples of vegetable oils used in making soap in commercial quantities. The saponification of fats and oils involves heating the oil with steam and alkali (sodium hydroxide or potassium hydroxide). Then, concentrated sodium chloride (common salt) is added to initiate a process salting out where salt helps to decrease the solubility of the soap so that it separates out as hard cake on the surface when cooled. Soap obtained by this means is further purified with water and steam, then other ingredients like perfumes, disinfectants are added before the soap is shaped into tablets or bars.

Soap produced from potassium hydroxide.KOH, is different from that obtained from sodium hydroxide, NaOH. The nature of the soap is also affected by the type of lipid used. Hard soaps are mostly produced from saturated fatty acids and sodium hydroxide, while soft soaps are mainly composed of potassium salts of unsaturated acids. Hard soaps are soaps mainly used in laundering while soft soaps are those that are categorised as toilet soaps.

Soap molecule is made up of a long hydrocarbon chain, R, attached to an ionic head that could be COO^-Na⁺ or COOK⁺. The hydrocarbon chain is called hydrophobic tail as it is soluble in organic solvents and not soluble in water. While the ionic head is the hydrophilic head as it is soluble in water and not organic solvents. This dual nature of soap enables it to dissolve in both organic and inorganic solvents or a mixture like oil and water.

The structure of soap molecule allows it to form a spherical cluster in water called micelles. A single micelle is an aggregate of hundreds of soap molecules with the ionic heads pointing outwards and the hydrophobic tails pointing inwards. The structure is the consequence of ionic similarity between the water molecules and the hydrophilic heads, as the hydrophopic tails are locked in the core of the micelle.

But this is different when there is an interphase of aqueous-oil layer. Here, the hydrophilic head will be attached to the aqueous layer (water or inorganic medium) while the hydrophobic tail is attached to the oil or organic medium.

When oil is shaken vigorously with water it forms tiny droplets within the water called an oil-oil water emulsion. But the emulsion formed is unstable and will soon coalesce to form the initial oil-water layer. Here, soap can act as an emulsifying agent where the soap molecules crowd around the oil droplets so that the hydrophilic heads are attached to the surrounding water as the hydrophobic tails dissolves in the oil. This form a stable emulsion of oil in water, as the soap act as an emulsifying agent. The ability of the hydrophobic tail to dissolve in organic materials and the hydrophilic head in water is basis of the cleaning action of soap.

There is no dispute about the use of soap as a cleaning agent. But there are some limitations that are better catered for by the use of detergents - soap-less detergents. Soap reacts with excess hydrogen ions in acidic solution to unionised fatty acids thus reducing the cleansing action of soap. The solubility of soap is also reduced in hard water where it forms scum. But soap is biodegradable, as such it's not an environmental pollutant.