Soap – a special molecule

Soap is a very special molecule because it can mix with both water and fats even though water and fats generally don’t mix. The fat on a frying pan can not be cleaned in pure water, since fats cannot be dissolved in water alone. We will have to use a soap water mixture to clean the pan. The explanation for why water cannot dissolve fats is that water molecules preferentially attract so-called polar molecules, while fats are non-polar molecules.

Soap molecules are polar and non-polar at the same time: Half of the molecule has an electric pole and attracts water. The other half is non-polar and binds to fats.

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The soap will preferentially be positioned at the surface of a mixture of water and soap. The reason for this is that the non-polar side of the soap will not mix with water and will therefore be pushed out of the water. For this reason the non-polar part of the soap molecules will erect out and away from the surface. Because the soap molecules are situated at the surface, they will displace some water molecules from the surface and down into the bulk liquid. There will be fewer surface water molecules in a soap water mixture than in pure water, resulting in a relatively lower surface tension for the soap water solution. The fewer water molecules in the surface per given surface area of a liquid, the lower will the surface tension be. Therefore, the surface tension will decrease with increasing soap concentration in the liquid. When a certain soap concentration is reached, the concentration of soap molecules at the surface cannot be further increased, and the soap will start to form small clusters in the bulk of the liquid, called micelles.

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 Right: In a soap bubble mixture the soap molecules will preferentially be situated in the surface of the liquid. Left: In soap water mixtures with a high concentration of soap the soap concentration at the liquid surface will be saturated and the soap molecules will form micelles, which are spherical clusters where the water-attractive part of the soap molecules are placed at the outer surface of the sphere.