Stearic acid is a chemical compound consisting of an 18 carbon chain whose terminal carbon is connected to an oxygen atom with a double bond and a hydroxyl group (OH) by a single bond. It belongs to class of materials known as fatty acids, produced primarily from natural fats and oils. Stearic acid is an important component in soap and other cosmetic and industrial preparations.
Stearic acid is derived predominantly from fats and oils. These materials contain triglycerides which are glycerine molecules attached to long hydrocarbon chains. These hydrocarbons can be removed from the glycerine backbone through a variety of techniques. When a triglyceride molecule is split, it yields three molecules of fatty acid and one molecule of glycerine. The major fat used in the production of stearic acid is beef fat, also known as tallow. Stearic acid is also obtained in lesser amounts from herring and sardine. Plant oils such as cotton, coconut, palm kernel, castor beans, rapeseed, soybeans, and sunflowers are also natural sources. In the United States, almost all stearic acid is made from tallow and coconut oil, although lesser amounts are made from palm oil. The other plant sources are more commonly used in third world countries.
Historically, stearic acid has been made by a process known as hydrolysis, which involves heating the fat in an alkaline solution. This process is also known as saponification. The alkali that is traditionally used is sodium hydroxide, also known as caustic soda or lye. Hence the term “lye soap.” Other methods used to produce fatty acids include solvent crystallization, hydrogenation, and distillation.
Pure stearic acid is a white, waxy solid crystalline material that melts at 156°F (69°C). It is odorless and tasteless. However, because of its natural origin, pure stearic is hard to obtain. Instead, stearic acid usually includes minor amounts of other fatty acids with different carbon chain lengths, such as lauric and palmitic acids. These trace impurities can cause the acid to vary in molecular weight, solubility, melting point, color, odor, and other physical and chemical properties. In addition to the carbon chain distribution, the degree of neutralization, or the amount of free acid present, also determines the acid’s properties. These are a number of physical and chemical specifications used to ensure that the stearic acid is of a consistent quality. Specifications include the acid’s saponification value, iodine value, peroxide value, free fatty acids, unsaponifiables, moisture, and trace impurities.
When fatty acids are neutralized with an alkali, the resultant salt is known as a soap. No one knows for sure when soap was first discovered, but it was well known in the ancient world. Some legends attribute it to the Romans, others say the ancient Gauls accidentally discovered it as they tried to extract oil from animal fat. Soap making techniques were common in the Old World and the Phoenicians, Arabs, Turks, and the Celts were all regarded as master soap makers. It wasn’t until the dawn of the nineteenth century, however, that soap chemistry was understood. Two key events triggered the modern soap making industry. First, in 1790, Nicholas Leblanc discovered a process to make sodium hydroxide from sodium chloride; this established an inexpensive source of lye for soap making. Then, in 1823, Michel Chevreul identified the structure of fatty acids. Chevreul discovered the first fatty acid when he analyzed a potassium soap made from pig fat. After treating the soap with various chemicals, he found that it yielded a crystalline material with acid properties. Hence the first fatty acid was isolated. Over the next decade, Chevreul decomposed a variety of soaps made from different animal soaps. He identified and named many fatty acids, including stearic and butyric.
While soaps can be made using a variety of fatty acids, stearic acid is one of the most popular. Stearic acid salts made with sodium, potassium, calcium, strontium, barium, and magnesium are used in a variety of applications. Sodium stearate is the most common type of soap and has been used extensively in cleansing for personal care in bar soaps. Potassium stearate is softer and more water soluble and has been used in water solutions for hard surface cleaning. Despite their widespread use, stearic acid soaps do have certain drawbacks. First, it is difficult to prepare concentrated solutions of these soaps because they are only marginally soluble in water. Furthermore, they can react with minerals present in hard water and form insoluble salts such as calcium stearate. These insoluble salts are responsible for bathtub ring and can leave undesirable film on hair, skin, and clothing. In the 1940s, due to wartime shortages of certain natural materials, synthetic soaps, also known as detergents, became commercially available. These detergents had the cleansing properties of soap without its negative properties.
Beside soap making, stearic acid is used to form stable creams, lotions and ointments. It is used in products like deodorants and antiperspirants, foundation creams, hand lotions, hair straighteners, and shaving creams. It is also used as a softener in chewing gum base and for suppositories. It may be further reacted to form stearyl alcohol which is used in a variety of industrial and cosmetic products as a thickener and lubricant. It is also used in candles to modify the melting point of the waxes.