136 SOAPS AND PROTEINS 



XI 



THE FOAMING, EMULSIFYING AND WASHING PROPERTIES 



OF SOAPS 



There is still much debate as to what is the property of soaps 

 (and similarly acting compounds) which when added to water 

 favors the production and maintenance of foams or emulsions. 

 Without first entering upon a discussion of the theories which 

 have been proposed, what do the colloid-chemical facts outlined 

 in the preceding pages contribute toward a possible solution 

 of the problem? In the several series of soaps described we have 

 thus far correlated their chemical composition and that of the 

 various " solvents " used with them, with various physico- 

 chemical properties of the resulting systems. What is the relation- 

 ship between such a property of a soap as its hydration capacity 

 and its ability to yield a foam; or what is the relationship between 

 this hydration value and the production and maintenance of an 

 emulsion? A proper answer to these questions may prove of 

 help for the solution of that secondary technological problem 

 which concerns the washing properties of soap, which, as now 

 held by various authors, are intimately associated with its foaming 

 and emulsifying qualities. The following paragraphs show that 

 the foaming, emulsifying and washing properties of soaps are a 

 function, in the main, of their hydrophilic colloid character. Only 

 those soaps foam or emulsify which under the conditions of their use 

 yield liquid and hydrated colloids. 



1. The Foaming Properties of Soaps 



We need to begin these paragraphs by a definition of what 

 constitutes a foam. As ordinarily understood, it is a subdivision 

 of a gas in a liquid. There exist also, however, what may be 

 termed solid foams, namely, subdivisions of a gas in a solid, as 

 in ordinary pumice, but since such solid foams were invariably 

 produced when the now solid phase was liquid, these are really 

 only a subclass of the liquid foams. When not otherwise specified 

 we refer in these pages only to the liquid foams and the conditions 

 surrounding their production and maintenance: 



