The Chemistry of Soap
(Contains technical chemistry data - not for the faint of heart!)
While we usually think of "soap" as a cleaning agent that bubbles and lathers, there are actually a number of different substances that are technically soap - it just depends what they are made of. In this article we will mainly deal with the chemistry of the bubbling, cleaning kind of soap.
Soap, Technical Definition
Soap, technically, is defined as: the alkali salt of a fatty acid. It is the product that results from the reaction of a fatty acid and a strong base (alkali). In cleansing soaps, the fatty acids come from oils and fats; the strong alkali base is sodium hydroxide, also known as "lye", (for hard soaps) or potassium hydroxide, also known as "potash", (for soft soaps).
The Components of Soap
In order to understand the chemistry of soap, it is first necessary to take a look at the chemistry of the component parts.
Fatty Acids
A fatty acid is made up of a long chain of hydrogen and carbon atoms, with an extra hydrogen atom at one end and a special group of atoms called a "carboxyl group" (made up of two oxygen, one hydrogen and one carbon atom) on the other end.
A fatty acid can be "saturated" or "unsaturated". In a saturated fatty acid, the carbon atoms are bonded with single bonds; they share one set of electrons. As a result, saturated fatty acids have two hydrogen atoms for each carbon atom. Palmitic Acid, pictured above, is a saturated fatty acid.
In an unsaturated fatty acid, there is at least one double bond where one set of carbon atoms is bonded by sharing two sets of electrons, instead of each being connected to a hydrogen atom. Oleic Acid, below, is an unsaturated fatty acid.
Fatty acids are found in fats and oils.
Oils and Fats
There is no sharp distinction between a fat and an oil. "Oil" commonly means a liquid which at ordinary temperature will flow as a slippery, lubricating, fairly thick fluid. "Fat" normally implies a greasy, solid substance slippery to the touch. It is necessary to differentiate the oils and fats used in the manufacture of soap.
Hydrocarbon (petroleum-based) oils or paraffins, while included in the general term "oil," do not contain fatty acids and cannot be used to make traditional soap. Animal- and vegetable-based oils and fats do contain the necessary fatty acids, in the form of triglycerides.
Triglycerides
When the carboxyl group ends of three fatty acid molecules combine with one molecule of glycerol it produces a triglyceride.
This is what we usually think of as "oil" or "fat". The actual physical characteristics of the oil depend upon which fatty acids have attached to the glycerol and whether they are connected to the top, middle or bottom of the glycerol molecule. If primarily unsaturated fatty acids are contained in the triglyceride, then the oil is considered to be an "unsaturated fat". The type of fatty acids also determines whether the triglyceride is solid or liquid at room temperature, how thick it is, the nutritional value and - for soapmakers - the qualities the oil will impart to the soap and its lather.
