Hydrophilic End
A soap molecule is a hydrocarbon with a carboxyl group (COO-) on one end. The carboxyl group consists of a carbon atom attached to an oxygen atom with a double bond. The carbon atom is attached to the other oxygen atom with a single bond. This oxygen molecule also forms a weaker ionic bond with a sodium atom. The end of the soap molecule with the carboxyl group is known as the “head” of a soap molecule. The negative charge provides the head of a soap molecule with hydrophilic properties, meaning that it attracts water.
Lipophilic End
The other part of the soap molecule consists of a chain of carbon atoms that have two hydrogen atoms each. This hydrocarbon chain ends in a methyl group (CH3) that consists of three hydrogen atoms bonded to a carbon atom. This end of the soap molecule is called the “tail” of the molecule. It has lipophilic properties, meaning that it attracts oil.
Action of Oil and Water
Electrostatic charges cause oily substances to stick to your hands. Oils tend to be negatively-charged, while the surface of your hands tends to be positively charged. When you place your hands in water without soap, the water molecules move between the oil particles and your hand. This usually causes the oil particles to have similar electrical charges, which cause the particles to repel each other.
Action of Soap
The soap molecules disperse uniformly when the soap is mixed with water. The tails are attracted to each other, causing the soap molecules to form small, spherically-shaped structures called micelles. The heads of the soap molecules in the micelle therefore face outward. When oil is added to this mixture, it’s repelled by the heads of the soap molecules and attracted by the tails. This causes the oil to be broken up into small particles, which are taken inside the micelle. This allows the oil to be rinsed away.
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