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The basic structure of all soaps stay relatively the same despite the differing ingredients. This consists of a hydrophobic hydrocarbon tail (detracts water) and a hydrophilic anionic head (attracts water). 

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The type of fat or oil can result in a different length of the hydrocarbon but all in all the hydrophobic tail remains long. The carboxylate head is usually balanced by either using potassium + or sodium + . When making soap, triglycerides in fats and oils are heated with a strong alkali base like that of sodium hydroxide (lye). This produces three molecules of soap for every molecule of glycerol. 

This process is called saponification (illustrated below)

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Soaps are surface active substances also referred to as surfactants. This means they make water clean surfaces better. Water has very high surface tension, which means when you pour water on a table, it sticks together. Surfactants make it easier for water molecules to wet surfaces and increase the ability of dissolving dirt, and oil stains. 

The general rule for how soap works is ‘like dissolves like’. The nonpolar hydrophobic tails of soap attract the grease and oils that help dirt and stains adhere to surfaces. The hydrophilic heads remain surrounded by the water molecules which they are attracted to. As more soap molecules embed into a stain they surround and isolate little particles of the stain called micelles. The micelle has tails of soap molecules facing toward the grease with the head facing outward into water, which results in an emulsion of soap mixed with grease in water. Soap molecules partially dissolve the stain to form the emulsion that is kept in the water until the soap is rinsed with more water. 

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Soaps don’t clean everything perfectly, like with water containing calcium or magnesium ions. Calcium and magnesium salts are insoluble which causes soaps to dirty surfaces because the soap binds to the calcium or magnesium. 

Soaps have since been replaced by synthetic detergents that have sulfonate instead of carboxylate head because sulfonates don’t precipitate with calcium or magnesium ions and have better solubility with water. But this doesn’t mean that synthetic detergents are necessarily better for the skin. Detergents are made for cleaning surfaces, clothes, dishes and other things that aren’t related to the skin. 

 

Bibliography

Brady, James E.; Russell, Joel W.; and Holum, John R. (2000). Chemistry: Matter and Its Changes, 3rd edition. New York: Wiley.