LIPIDES (fats and RELATED SUBSTANCES) 87 



is a typical soap, and the above equatit)n illustrates the commercial manu- 

 facture of soaps. 



Soaps 



A soap is defined chemically as a metallic salt of a fatty acid contain- 

 ing ten or more carbon atoms. All commercial soaps, however, are mix- 

 tures of several individual "soaps" because they are made from natural 

 fats which are mixtures of glycerides. The glycerides are all saponified 

 at once, and each fatty acid radical is converted into the corresponding 

 soap. Thus the product is a mixture corresponding in composition to 

 the fatty acid make-up of the original fat. 



Sodium and potassium soaps, being fairly soluble in water, are useful 

 washing agents. Soaps of other metals, although too insoluble in water 

 to form a lather, are very valuable for other purposes, as described below. 

 The consistency and washing qualities of soaps depend partly on the 

 metal and on the fatty acid radicals of which they are composed. Thus 

 from a given fat, sodium hydroxide will tend to produce the harder and 

 potassium hydroxide, the softer soap. On the other hand, if the same 

 alkali is used throughout, a liquid fat, containing unsaturated or low 

 molecular weight fatty acid radicals, will tend to produce a soft or liquid 

 soap, whereas a hard fat like tallow will make a hard soap. Other things 

 being equal, the soaps of capric, lauric, and myristic acids, that is the 

 saturated fatty acids containing 10, 12, and 14 carbon atoms, lather 

 better, and of these the lauric soaps are the best. It is for this reason 

 that fats such as palm and coconut oils, yielding a large amount of these 

 particular fatty acids on hydrolysis, are so valuable in soap making. 



Water-soluble soaps are classified as detergents, substances which, when 

 dissolved in water, lower the surface tension of the water and help to 

 loosen and wash away particles of grease and dirt. Other kinds of 

 detergents are also produced more or less directly from fats and have 

 become so popular that over 1.2 million lb. were produced in 1950. 

 These so-called "synthetic detergents," which should not be called soaps, 

 are of many types, but all of them consist of a water-soluble, salt-like 

 group attached to a long-chain, fat-like residue. A typical example is 

 sodium alkyl sulfate, ROSOsNa, where R represents alkyl groups cor- 

 responding to various fatty acids such as lauric, myristic, palmitic, and 

 stearic. These synthetic detergents differ mainly from ordinary soaps by 

 having a sulfate in place of the carboxyl group. Since they form soluble 

 calcium and magnesium compounds, which are not precipitated by the 

 minerals in hard water, they are as effective washing agents in hard water 

 as in soft water. Their aqueous solutions are also practically neutral, 

 whereas those of ordinary soaps are quite strongly alkaline and have a 

 pH of about 9. 



