Stearic acid

Overview
Stearic acid (IUPAC systematic name: octadecanoic acid) is one of the useful types of saturated fatty acids that come from many animal and vegetable fats and oils. It is a waxy solid, and its chemical formula is CH3(CH2)16COOH. Its name comes from the Greek word stéar (genitive: stéatos), which means tallow. The term stearate is applied to the salts and esters of stearic acid.

Production
Stearic acid is prepared by treating animal fat with water at a high pressure and temperature, leading to the hydrolysis of triglycerides. It can also be obtained from the hydrogenation of some unsaturated vegetable oils. Common stearic acid is actually a mix of stearic acid and palmitic acid, although purified stearic acid is available separately.

Uses
Stearic acid is useful as an ingredient in making candles, soaps, plastics, oil pastels and cosmetics, and for softening rubber. Stearic acid is used to harden soaps, particularly those made with vegetable oil.

Stearic acid is also used as a parting compound when making plaster castings from a plaster piece mold or waste mold and when making the mold from a shellacked clay original. In this use, powdered stearic acid is dissolved in water and the solution is brushed onto the surface to be parted after casting.

Esters of stearic acid with ethylene glycol, glycol stearate and glycol distearate are used to produce a pearly effect in shampoos, soaps, and other cosmetic products. They are added to the product in molten form and allowed to crystalize under controlled conditions.

In fireworks, stearic acid is often used to coat metal powders such as aluminium and iron. This prevents oxidation allowing compositions to be stored for longer.

It is used along with simple sugar or corn syrup as a hardener in candies.

Reactions
Stearic acid undergoes the typical reactions of saturated carboxylic acids, notably reduction to stearyl alcohol, and esterification with a range of alcohols.

Metabolism
An isotope labeling study in humans concluded that the fraction of dietary stearic acid oxidatively desaturated to oleic acid was 2.4 times higher than the fraction of palmitic acid analogously converted to palmitoleic acid. Also, stearic acid was less likely to be incorporated into cholesterol esters. These findings may indicate that stearic acid is less unhealthy than other saturated fatty acids.