Due to the presence in the fatty acids of the carboxyl group (-COOH), they behave as moderately strong acids, which allows them to carry out chemical reactions of great biological importance:
In the esterification reaction, a fatty acid joins an alcohol through a covalent bond, forming an ester and releasing a water molecule . By hydrolysis , the ester dissociates and gives rise to the fatty acid and alcohol again.
An ester is the union of a fatty acid and an alcohol through a covalent bond called an ester bond . Most lipids are esters.
By Toby Phillips (Own work) [Public domain], via Wikimedia Commons
The saponification reaction is a typical reaction of fatty acids, in which they react with a strong base (NaOH or KOH) and give rise to a fatty acid salt, called soap.
Soap molecules have amphipathic behavior, with a lipophilic or hydrophobic zone, which avoids contact with water, and a hydrophilic or polar zone, which tends to bind to water.
For example, a soap such as sodium palmitate (CH3 - (CH2 )14 -COONa), has a lipophilic zone, the hydrocarbon chain, which establishes Van der Waals bonds with lipophilic molecules. The hydrophilic zone (-COONa) is ionized, leaving the carboxyl group with a negative electric charge (-COO-), establishing electric-type attractions with water molecules and other polar groups . For this reason, they do not form true solutions, but rather they constitute colloidal dispersions, forming micelles., which can be monolayers, or bilayers if they include water inside.
By Ttzavaras at Greek Wikipedia (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons
Soaps have amphipathic behavior, that is, they have a hydrophilic or water-soluble end, and another that is hydrophobic, which rejects water and tends to bind to fat, which detach and remain floating in the aqueous medium, forming micelles. Micelles can contain fats inside them, so soap has a cleansing power.
If the water is hot or we rub the clothes, the action of the soap is greater, since the hydrophilic end acquires more power to remove the dirt. In hard water, with a large amount of calcium and magnesium, these salts react with the soap to form an insoluble precipitate and it is necessary to use a softener to achieve a more pleasant touch.
The esterification is the binding of glycerin with fatty acids to form an ester more water.
The saponification is the formation of soaps from unsaponifiable lipids by chemical reaction between a triglyceride and a base (NaOH or KOH). From a triglyceride, three molecules of the sodium or potassium salts corresponding to the fatty acids are formed and glycerin is released. The salts formed are called soaps.
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