Soap is one of the oldest known manufactured chemical substances and was first produced thousands of years ago through the reaction of animal fats with the ashes from plants. Early soaps were used primarily for the cleansing of clothing, not for personal hygiene, because of cultural as well as practical reasons. For instance, these animal fat soaps possessed almost unbearable odors and probably contained excessive amounts of unreacted caustics. Today soaps are produced using a variety of processes, including kettle boiling, continuous saponification, and hydrolysis/neutralization, as well as different fats and oils feedstocks, yielding finished materials that possess specifically desired properties for application as personal cleansing products.
Soap is one example of a broader class of materials known as surface‐active agents, or surfactants. Surfactant molecules contain both a hydrophilic or water‐liking portion and a separate hydrophobic or water‐repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aliphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air–water interfaces. It is this special chemical structure that leads to the ability of surfactants to clean dirt and oil from surfaces and produce lather.
Although soaps have many physical properties in common with the broader class of surfactants, they also have several distinguishing factors. First, soaps are most often derived directly from natural sources of fats and oils. Fats and oils are triglycerides, i.e., molecules comprising a glycerol backbone and three ester‐linked fatty oils. Other synthetic surfactants may use fats and oils or petrochemicals as initial building blocks, but generally they require additional chemical manipulations such as sulfonation, esterification, sulfation, and amidation.
Second, soaps form insoluble complexes, commonly referred to as curd, in the presence of calcium and magnesium ions in solution. Calcium and magnesium ions are the principal metal ions found in water and their level is commonly referred to as the hardness of the water; hard water has high levels of both of these ions, soft water has very low levels. This curd reduces the effectiveness of soap as a surfactant and gives rise to other undesirable properties during use, e.g., precipitation on surfaces. Many synthetic surfactants are considerably less susceptible to water hardness. This water hardness insensitivity has led to the replacement of soap by synthetic surfactants in a variety of applications, such as dish and laundry detergents and shampoos. Although soap is still the predominant material used in personal cleansing products, e.g., facial, body, and hand cleansing, soap‐based personal cleansing products are being rapidly replaced by products that contain increasing amounts of synthetic surfactants to meet changing consumer needs, such as rinsing and lather in hard water and improved mildness to the skin.
