Cleaning Action of Soap: A soap molecule consists of two dissimilar parts:
(i) A short ionic part comprising the carboxylate salt, -COO- Na*. This is the polar end. This is water soluble (i.e., hydrophilic or water attracting) and, therefore, remains attached to water.
(ii) A long hydrocarbon chain which is the non-polar end. This end hydrophobic (i.e., water repelling) and is soluble in oil and grease.
When soap is dissolved in water, it forms a colloidal suspension. In this colloidal suspension, the soap molecules cluster together to form micelles and remain radially suspended in water with the hydrocarbon end towards the centre and the ionic end directed outward. This is shown in the figure.
The dirt particles always adhere to the oily or greasy layer present on the skin or clothes. When a dirty cloth is dipped into a soap solution, its non-polar hydrocarbon end of micelles attach to the grease or oil present in dirt and polar end remains in water layer. The mechanical action of rubbing subsequently dislodges the oily layer from the dirty surface shaping it into small globules. A stable emulsion of oil in water is formed. The emulsified oil or grease globules bearing the dirt can now be readily washed with water.
