The cleansing action of soap is due to the action of sodium salts of higher fatty acids. These sodium salts can be divided into two parts: one part is the polar hydrophilic group -COONa, which is water-soluble, while the other part is the non-polar hydrophobic chain hydrocarbon group -R, which is hydrophobic and lipophilic. When washing clothes, the hydrophobic chain part of the soap molecule dissolves into the oil stains, while the hydrophilic carboxyl group extends into the water surrounding the oil stains. The oil stains are surrounded by soap molecules, forming stable emulsions. Through mechanical agitation and rinsing with water, the oil stains and other dirt are detached, dispersed into smaller emulsion droplets, and removed by washing away with water.
When soap is dissolved in water, on the water surface, the hydrophilic carboxyl group of soap molecules tends to enter the water molecules, while the hydrophobic hydrocarbon chain group is repelled and oriented outside the water, forming a directed arrangement of soap molecules. The presence of this layer of sodium salts of higher fatty acids weakens the attractive forces between water molecules on the water surface, so soap can significantly reduce the surface tension of water, making it a surfactant.
When the concentration of soap in water is low, soap molecules exist in a monomeric form, with these molecules aggregating at the water surface - the hydrophilic groups entering the water, and the hydrophobic groups being repelled outside the water. As the soap concentration in water gradually increases, the number of soap molecules gathered on the water surface increases, forming a monomolecular layer. As the soap concentration continues to increase, since the water surface is already occupied, the hydrophobic hydrocarbon chains within the soap molecules in the aqueous solution begin to gather together due to van der Waals forces, while the hydrophilic carboxyl groups wrap around the outside, forming colloidal-sized aggregated particles known as micelles.
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