Soap is a salt of a fatty acid (sometimes other carboxylic acids) used for cleaning and lubricating products as well as other applications. In a domestic setting, soaps, specifically "toilet soaps", are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are used as thickeners, components of some lubricants, emulsifiers, and catalysts. Soaps are often produced by mixing fats and oils with a base. Humans have used soap for millennia; evidence exists for the production of soap-like materials in ancient Babylon around 2800 BC.
Types
Toilet soaps
In a domestic setting, "soap" usually refers to what is technically called a toilet soap, used for household and personal cleaning. Toilet soaps are salts of fatty acids with the general formula (RCO2−)M+, where M is Na (sodium) or K (potassium). When used for cleaning, soap solubilizes particles and grime, which can then be separated from the article being cleaned. The insoluble oil/fat "dirt" becomes associated inside micelles, tiny spheres formed from soap molecules with polar hydrophilic (water-attracting) groups on the outside and encasing a lipophilic (fat-attracting) pocket, which shields the oil/fat molecules from the water, making them soluble. Anything that is soluble will be washed away with the water. In hand washing, as a surfactant, when lathered with a little water, soap kills microorganisms by disorganizing their membrane lipid bilayer and denaturing their proteins. It also emulsifies oils, enabling them to be carried away by running water. When used in hard water, soap does not lather well but forms soap scum (related to metallic soaps, see below).
Non-toilet soaps So-called metallic soaps are key components of most lubricating greases and thickeners. A commercially important example is lithium stearate. Greases are usually emulsions of calcium soap or lithium soap and mineral oil. Many other metallic soaps are also useful, including those of aluminium, sodium, and mixtures thereof. Such soaps are also used as thickeners to increase the viscosity of oils. In ancient times, lubricating greases were made by the addition of lime to olive oil, which would produce calcium soaps. Metal soaps are also included in modern artists' oil paints formulations as a rheology modifier. Metal soaps can be prepared by neutralizing fatty acids with metal oxides:
2 RCO2H + CaO → (RCO2)2Ca + H2O A cation from an organic base such as ammonium can be used instead of a metal; ammonium nonanoate is an ammonium-based soap that is used as an herbicide. Another class of non-toilet soaps are resin soaps, which are produced in the paper industry by the action of tree rosin with alkaline reagents used to separate cellulose from raw wood. A major component of such soaps is the sodium salt of abietic acid. Resin soaps are used as emulsifiers.
Soapmaking
The production of toilet soaps usually entails saponification of triglycerides, which are vegetable or animal oils and fats. An alkaline solution (often lye) induces saponification whereby the triglyceride fats first hydrolyze into salts of fatty acids. Glycerol (glycerin) is liberated. The glycerin is sometimes left in the soap product as a softening agent, although it is sometimes separated. The glycerine makes the soap softer. The addition of glycerol and processing of this soap produces glycerin soap.
Additives Sand or pumice may be added to produce a scouring soap. The scouring agents serve to remove dead cells from the skin surface being cleaned. This process is called exfoliation. To make antibacterial soap, compounds such as triclosan or triclocarban can be added. There is some concern that use of antibacterial soaps and other products might encourage antimicrobial resistance in microorganisms. The type of alkali metal used determines the kind of soap product. Sodium soaps, prepared from sodium hydroxide (soda lye), are firm, whereas potassium soaps, derived from potassium hydroxide (potash lye), are softer or often liquid. Historically, potassium hydroxide was extracted from the ashes of bracken or other plants. Lithium soaps also tend to be hard and are used almost exclusively in greases. For making toilet soaps, triglycerides (oils and fats) are derived from coconut, olive, or palm oils, as well as tallow. Triglyceride is the chemical name for the triesters of fatty acids and glycerin. Tallow, rendered fat, is the most available triglyceride from animals. Hence, the fat compound in many soaps is known as sodium tallowate. Each species offers quite different fatty acid content, resulting in soaps of distinct feel. The seed oils give softer but milder soaps. Soap made from pure olive oil, sometimes called Castile soap or Marseille soap, is reputed for its particular mildness. The term "Castile" is also sometimes applied to soaps from a mixture of oils with a high percentage of olive oil. Most soaps for body washing are "refatted" or "superfatted". Superfatting agents can achieve multiple goals. First, superfatting ensures the absence of lye in the soap. For cake soaps, superfatting confers plasticity, which prevents cracking. Finally, superfats replenish (refat) the skin oils that are washed away during the cleansing process. Typical superfatting agents are lecithins, lanolin, and various alkanolamides. Handmade soap can be refatted using fat or coconut oil beyond that needed to consume the alkali used (in a cold-pour process, this excess fat is called "superfatting"), and the glycerol left in acts as a moisturizing agent. Superfatted soap is more skin-friendly than one without extra fat, although it can leave a "greasy" feel. Sometimes, an emollient is added, such as jojoba oil or shea butter.
Gallery
History
Ancient world
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