Soaps for cleaning are obtained by treating vegetable or animal oils and fats with a strong base, such as sodium hydroxide orpotassium hydroxide in an aqueous solution. Fats and oils are composed of triglycerides; three molecules of fatty acids attach to a single molecule of glycerol. The alkaline solution, which is often called lye (although the term "lye soap" refers almost exclusively to soaps made with sodium hydroxide), induces saponification.
In this reaction, the triglyceride fats firsthydrolyze into free fatty acids, and then the latter combine with the alkali to form crude soap: an amalgam of various soap salts, excess fat or alkali, water, and liberatedglycerol (glycerin). The glycerin, a useful byproduct, can remain in the soap product as a softening agent, or be isolated for other uses.
Soaps are key components of most lubricating greases, which are usually emulsions of calcium soap or lithium soapand mineral oil. Many other metallic soaps are also useful, including those of aluminium, sodium, and mixtures of them. 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.
Action of soap
When used for cleaning, soap allows insoluble particles to become soluble in water, so they can then be rinsed away. For example: oil/fat is insoluble in water, but when a couple of drops of dish soap are added to the mixture, the oil/fat dissolves in the water. The insoluble oil/fat molecules become 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 it soluble. Anything that is soluble will be washed away with the water.
Effect of the alkali
The type of alkali metal used determines the kind of soap product. Sodium soaps, prepared from sodium hydroxide, are firm, whereas potassium soaps, derived frompotassium hydroxide, 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—these are used exclusively in greases.
Effects of fats
Soaps are derivatives of fatty acids. Traditionally they have been made fromtriglycerides (oils and fats). Triglyceride is the chemical name for the triesters of fatty acids and glycerin. Tallow,rendered beef fat, is the most available triglyceride from animals. Its saponified product is called sodium tallowate. Typical vegetable oils used in soap making are palm oil, coconut oil, olive oil, and laurel oil. Each species offers quite different fatty acid content and hence, results in soaps of distinct feel. The seed oils give softer but milder soaps. Soap made from pure olive oilis sometimes called Castile soap orMarseille soap, and is reputed for being extra mild. The term "Castile" is also sometimes applied to soaps from a mixture of oils, but a high percentage of olive oil.
Soap making processes
The industrial production of soap involves continuous processes, such as continuous addition of fat and removal of product. Smaller-scale production involves the traditional batch processes. The three variations are: the 'cold process', wherein the reaction takes place substantially at room temperature, the 'semi-boiled' or 'hot process', wherein the reaction takes place near the boiling point, and the 'fully boiled process', wherein the reactants are boiled at least once and the glycerol is recovered. There are several types of 'semi-boiled' hot process methods, the most common being DBHP (Double Boiler Hot Process) and CPHP (Crock Pot Hot Process). Most soapmakers, however, continue to prefer the cold process method. The cold process and hot process (semi-boiled) are the simplest and typically used by small artisans and hobbyists producing handmade decorative soaps. The glycerol remains in the soap and the reaction continues for many days after the soap is poured intomolds. The glycerol is left during the hot-process method, but at the high temperature employed, the reaction is practically completed in the kettle, before the soap is poured into molds. This simple and quick process is employed in small factories all over the world.
Handmade soap from the cold process also differs from industrially made soap in that an excess of fat is used, beyond that needed to consume the alkali (in a cold-pour process, this excess fat is called "superfatting"), and the glycerol left in acts as a moisturizing agent. However, the glycerine also makes the soap softer and less resistant to becoming "mushy" if left wet. Since it is better to add too much oil and have left-over fat, than to add too much lye and have left-over lye, soap produced from the hot process also contains left-over glycerol and its concomitant pros and cons. Further addition of glycerol and processing of this soap produces glycerin soap. Superfatted soap is more skin-friendly than one without extra fat. However, if too much fat is added, it can leave a "greasy" feel to the skin. Sometimes, an emollient additive, such asjojoba oil or shea butter, is added "at trace" ( the point at which the saponificationprocess is sufficiently advanced that the soap has begun to thicken in the cold process method) in the belief that nearly all the lye will be spent and it will escape saponification and remain intact. In the case of hot-process soap, an emollient may be added after the initial oils have saponified so they remain unreacted in the finished soap. Superfatting can also be accomplished through a process known as "lye discount" in which the soap maker uses less alkali than required instead of adding extra fats.
