Question

Cleansing action of soap is based on which principle?

Answer 1

Hint: Soaps are sodium or potassium salts of fatty acids that are made by the process called saponification. When soap comes in contact with water, it starts foaming and forms small clusters called micelles.

Complete step by step answer:
First let us understand soaps.
Soaps sodium or potassium salts of fatty acids. These slats can be easily prepared through saponification in which an alkali e.g. Sodium or potassium hydroxide is reacted with oils or fats at 80°–100 °C.

Given below is the chemical reaction for the formation of soap:
$ \begin{matrix} { R }OCO-C{ H }_{ 2 }-CH(OCOR)-C{ H }_{ 2 }-OCO-{ R } \\ fat \end{matrix}+\begin{matrix} 3NaOH(or\quad KOH) \\ alkali \end{matrix}\xrightarrow { 80-10{ 0 }^{ o }C } \begin{matrix} C{ H }_{ 2 }-OH-CH-OH-C{ H }_{ 2 }OH \\ Glycerol \end{matrix}+\begin{matrix} 3R-C{ O }_{ 2 }-Na \\ soap \end{matrix}$
From the above reaction, it is quite clear that the structure of soap consists of a long non-polar hydrophobic hydrocarbon chain which is also called the tail and a polar hydrophilic –COONa group which is also called the head.

When grease or oil (non-polar hydrocarbons) is mixed with a soap- water solution, the soap molecules function as emulsifiers since they have both polar non-polar parts in their chemical structure. They form micelles and the fats or oils get trapped within the micelle. Since the micelles are soluble in water, they are easily rinsed off.

Micelles are formed when the soap molecules arrange themselves into tiny aggregates or clusters. In these clusters, the hydrophilic part of the soap molecules is directed outwards while the hydrophobic part accumulates together on the inside such that they don't come in contact with the water molecules. These clusters form colloidal particles and when they come in contact with oil or dirt, the hydrophobic part of the micelles gets attached to the oil or dirt particles such that the oil or dirt particles get trapped into the micelles.

Thus the cleansing action of soaps is based on micelle action.

Note: Micelles are formed only in soft water and not in hard water. Soaps form scum with hard water. Also for micelle formation to take place, the concentration of the surfactant must be greater than the critical micelle concentration (CMC) along with the temperature which should be above Krafft temperature.