Soap and Other Surfactants

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Every day, more than 6.5 lbs of these substances are produced per inhabitant of the Earth. No, we're not talking about food products. We mean compounds that reduce the surface tension of liquids.


Soap can arguably be considered the most popular surfactant. Sodium or potassium salts of fatty acids make up the core of the cleansing agent, and they have a complex structure. The molecule of such acids is “bilateral”: one half repels water (hydrophobic), and the other is attracted to it (hydrophilic). When you soap up your hands, the hydrophobic parts of the molecules come into operation, moistening the grease. When we put our hands in water, the hydrophilic side comes into play — it attaches itself to the water and carries away the whole molecule, along with harmful substances and bacteria, into the sewer system. If you simply rinse your hands with water, the fatty particles and the contaminants they have accumulated will remain on your skin.


Soap's process of removing contaminants
Soap's process of removing contaminants

The hydrophobic part of the molecule is active in all non-polar solvents, such as gasoline or oil, so soap is well-suited for the removal of such contaminants. It is interesting that surfactants can work only at the interface between two states: liquid-gas, liquid-solid, or two immiscible liquids. They look for boundaries between states, just like spies, and this is why substances that reduce surface tension are called surfactants, or “surface- active agents”. The surfactants have mastered an entire procedure for transit from the liquid solution to the state interface — adsorption, the essence of which lies in the accumulation of substances in the surface layer. Surfactant molecules are able to independently move to the interface of two substances.

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