THE COLLOID-CHEMISTRY OF SOAP MANUFACTURE 187 



in the instance of soaps designed to meet special purposes, the 

 stock sodium soap contains a long list of different fatty acids 

 and as such serves to meet technologic needs under the largest 

 imml>er of ordinary circumstances. Usually, in the manufacture 

 of such soap some liquid oil or fat (like cocoanut oil) has had 

 added to it smaller fractions of the more solid fats (like stearin, 

 tallow or Japan wax). The result is a set of some eight to fifteen 

 different soaps of widely varying " solubility " in water and sen- 

 sitiveness to salt action, and possessed of a chain of fatty acids of 

 progressively different melting points. 



When such a mixed soap is completely salted out from its 

 " solvent " as a curd soap it is a relatively dry affair, the yield 

 from 100 parts of the original fat being only about 150 parts of 

 finished soap. When neutral and carefully handled this is the 

 basis of most of the common toilet soaps. The advantages of 

 such a material for all ordinary purposes are obvious. The soap is 

 free from any excess of alkali or sodium chlorid and the series of 

 soaps present yields a satisfactory (liquid) hydrated colloid with 

 water at any of the ordinary temperatures at which it may be used. 

 Since, moreover, in the salting-out process, the higher acetic 

 series soaps come out first and the lower ones with the oleate come 

 out later, the arrangement of the different soaps within the solid 

 bar is also such that the most readily " soluble " soaps become 

 hydrated and dissolve first, thus favoring disintegration of the. 

 bar for the production of the hydrated liquid colloid required 

 fni washing. 



It is well to discuss here the composition and nature of the 

 so-called hot and cold water soaps and the marine soaps. A 

 cold water soap is one which will in cold water yield the (liquid) 

 hydrated colloid necessary for washing. Obviously only soaps 

 rirh in the lower fatty acids of the acetic scries will do this, and 

 hence the use of pure cocoanut oil for the production of such a 

 soap, and the omission from the soap kettle of fats rich in palmitic, 

 stearic and arachidic acids; for the same reason fats containing 

 in essence only oleic, linolic and similarly constituted acids 

 are used for such soaps. On the other hand a hot water soap 

 must contain the higher fatty acids, for at higher temperatures 

 ill' 1 lower SOapfi " diolve " MIX! pass through the liquid hvdro- 

 philic colloid state into true solution too rapidly. The marine 

 .soaps are such as will maintain their hydrophilic colloid | 



