Soap is sodium or potassium salt of a higher fatty acid and maybe represented as RCOO–Na+(e.g., sodium stearate CH3(CH2)16COO–Na+,which is a major component of many bar soaps). When dissolved in water,it dissociates into RCOO–andNa+ions. The RCOO–ions, however, consistof two parts – a long hydrocarbon chain R (also called non-polar ‘tail’)which is hydrophobic (water repelling), and a polar group COO–(alsocalled polar-ionic ‘head’), which is hydrophilic (water loving).
The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the hydrocarbon chains R staying away from it andremain at the surface. But at critical micelle concentration, the anions arepulled into the bulkof the solution and aggregate to form a sphericalshape with their hydrocarbon chains pointing towards the centre of the sphere with COO– part remaining outward on the surface of the sphere. An aggregate thus formed is known as ‘ionic micelle’.
The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic partprojects out of the grease droplet like the bristles.(a) Grease on cloth
(b) Stearate ions (from soap) arranging around the grease droplets
(c) Micelle formed
Since the polar groups can interact with water, the oil droplet surrounded by stearateions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.