Soap is strictly defined as an alkaline salt of a fatty acid, although this definition may not be helpful to most readers. It is first important to distinguish soaps from other cleaning substances such as solvents and detergents. Soap is formed when a strong alkaline base is reacted with a fatty acid such as those found in tallow, palm oil, or coconut oil. The base used is almost always sodium hydroxide or potassium hydroxide. This process of reacting a fat or oil with a strong base to create soap is known as saponification.
Oils and fats are not composed of any one single molecule. Sodium Tallowate, when seen on a list of ingredients, is a convenient simplification. Rather than produce a list of all the individual fatty acids contained within tallow
, manufacturers may list a simplier version, though many oils and/or fats do not have commonly used simple equivalents -- one does not see Sodium Lardate or Potassium Butterate, even though lard and butter may be used to make soap. Common fatty acids are stearic acid, palmitic acid, and oleic acid. These acids are differentiated based on the number of carbon atoms and the location/type of molecular bonds between hydrocarbon groups. Some fatty acids are modified natural products.
Saponification means soap-making process. During saponification, an alkaline molecule (usually a hydroxide / lye) breaks off a long hydrocarbon chain from the fatty acid molecule and combines with it, resulting in a soap molecule. More technically, saponification is ester hydrolysis to form a carboxylate and an alcohol. As a byproduct of this reaction, glycerin is produced from the breaking of the fatty acid to form soap molecules. The relative proportion of glycerin depends on the type of fat or oil used. Furthermore, it is worth noting that not all fats and oils are saponifiable, only fatty acids.
Soap can also be manufactured by pre-breaking (via hydrolysis) the fatty acid, usually a triglyceride, into its glycerine and smaller fatty acid components. The pre-split fatty acid reacts very quickly with the alkaline (hydroxide / lye). If this process is used, the soap reaction is a simple neutralization between an acid -- the fatty acid -- and a base -- the hydroxide. No glycerine is produced in this reaction because it was removed to produce the fatty acids.
Closely related to pre-broken fatty acids are other modified natural oils (sometimes listed as coconut derived surfactants) used to create synthetic surfactants such as sodium cocoyl isethionate
and sodium laureth sulfate
Detergents perform a similar function to soap, as they are both surfactants and can bind to water on one end and oil on the other end of the molecule, but are sometimes made from petroleum or an alcohol instead of natural lipids (all are long carbon chain molecules). Detergents do not leave soap scum or a bathtub ring, normally caused by a reaction with iron, calcium, magnesium, and other minerals found in hard water
. Some body wash bars are not soap but rather a detergent, so they also do not leave soap scum or a bathtub ring.