A soap molecule is made up of 2 parts - a long hydrocarbon part and a short ionic part containing - COONa+ group. The soap molecule is said to have a tadpole structure as shown in figure 1.
The hydrocarbon part of the soap molecule is insoluble in water but soluble in oil and grease. The ionic portion of the soap molecule is hydrophilic. So the ionic portion of the soap molecule is soluble in water but insoluble in oil and grease. Now the clothes which contain dirt substances are soaked in water.
When soap gets dissolved in water it forms a colloidal suspension in which the soap molecules cluster together to form a micelle. The micelles remain suspended in water because negative charges at the end of each soap molecule repel each other. In a micelle, the soap molecules are arranged in a radical manner with the hydrocarbon end directed towards the centre and the ionic end directed outwards.
When greasy, oily clothes are immersed in soap solution, the soap micelle entraps the dirt particles by attaching the hydrocarbon part of the soap molecules to the greasy or oily particles as shown in figure 2.
Since the ionic part of the soap molecules remain attached to the water molecules, the dirt particles get dispersed in water and the cloth gets cleaned.
Vatsala Santhanam, Chennai
ANSWER 2: Soap has been used as a detergent for more than 2000 years. Soap is made from oil or fat which are esters of fatty acids or glycerol. The fatty acids contain chains of 16 to 18 carbon atoms. When oil or fats are heated with a solution of sodium hydroxide, they breakdown to form sodium salt of the respective fatty acid and glycerol. The process of splitting the fat is called saponification. It produces soap which is separated from the solution by the addition of salt.
A molecule of soap can be considered to be made up of two components. One part is a hydrocarbon and the other belongs to the COONa group. Hydrocarbons are water repelling-hydrophobic and the other parts are water loving - hydrophillic. When soap is dissolved in water it forms micelles. In a micelle, the soap molecules are arranged radially, with the hydrocarbon end towards the centre and the water loving end outwards. Dirt and grease present on a piece of cloth attach themselves to the hydrocarbon component of the soap molecule. The other component which is attached to the water molecules pulls the dirt away from the surface thereby making the cloth clean.
Washing powder contains about 15-30 per cent detergents by weight. Sodium sulfate and sodium silicate are added to keep the washing powder dry. Sodium tripolyphosphate or sodium carbonate is added to maintain alkalinity which is helpful in removing dirt. Carboxyl methyl cellulose is added to keep the dirt suspended in water. A mild bleaching agent such as sodium perborate is also added to produce whiteness.