Makch 31, 1916] 



SCIENCE 



469 



peutic emulsions, fatty degeneration, the for- 

 mation of fatty secretions, etc. 



Of the long list of mutually immiscible 

 liquids that might have been chosen for a 

 study of emulsiflcation, we have worked 

 chiefly with water and oil. The mixture of 

 two such immiscible liquids may yield two 

 types of emulsions, as Walther Ostwald first 

 showed; one consisting of oil in water, a sec- 

 ond, of water in oil. "With much water and 

 little oil, the first type of emulsion is usually 

 obtained; with much oil and little water, the 

 second type. When medium amounts of the 

 two liquids are mixed with each other, either 

 type may be produced, depending upon the 

 methods of mixing. 



Oil placed in contact with water does not 

 lead spontaneously to the formation of an 

 emulsion. To produce such, the two must be 

 beaten together. The amount of oil that 

 may be emulsified in piu:e water is very 

 small, in no case exceeding one or two per 

 cent. These emulsions are, however, stabile. 

 The oil particles in such emulsions are rather 

 small, their dimensions lying within the realm 

 of the colloids. These low concentrations of 

 oil in water, therefore, really represent colloid 

 suspensions of oil in water and possess not 

 only the stability characteristics of such sys- 

 tems, but also their well-known " saturation 

 limit." 



The term " emulsion " is ordinarily used to 

 cover the subdivision of one fluid in a second 

 in amounts exceeding these low values. The 

 mixture must, moreover, show a fair degree of 

 stability; in other words, the two liquids con- 

 stituting the dispersoid must not separate in 

 the course of weeks, months or years. A tem- 

 porary subdivision of any quantity of oil in a 

 given volume of water, or the converse, can, 

 of course, be obtained by merely beating the 

 two together. 



The problem of emulsiflcation therefore re- 

 solves itself into the question of how, once 

 the division of oil in water has been accom- 

 plished, this can be, or is, stabilized. Con- 

 trary to the general belief of different workers 

 who have each tried to discover some one ele- 

 ment as responsible for this stabilization, a 



number of different factors evidently play a 

 role, the relative importance of which may not 

 only vary in different emulsions, but in the 

 same emulsion under different circumstances. 



It is generally held that the formation and 

 the maintenance of an emulsion depend upon 

 the slight surface tension of the dispersing 

 medium, and its high viscosity. While hozh 

 these factors undoubtedly play a part, their 

 inadequacy in explaining the stability of all 

 emulsions is generally admitted. N'ot only 

 does the stability of emulsions not universally 

 parallel the surface tension values of the 

 liquids making up a given dispersoid, but di- 

 lute soap solutions with low viscosity act as 

 better emulsifying agents than more viscid 

 glycerin solutions. Pickering has emphasized 

 the importance of a third factor in the main- 

 tenance of an emulsion, namely, the develop- 

 ment of an encircling film about the droplets 

 of the divided phase through the accimiulation 

 in the surface between oil and dispersion 

 medium, of finely divided particles of a third 

 substance. But this explanation, too, seems 

 adequate only for selected examples of emul- 

 sions. 



n 



In reviewing the empirical instructions 

 available for the preparation of emulsions, and 

 in our own attempts to formulate such as 

 would always yield permanent results, we were 

 struck with the fact that their production is 

 always associated with the discovery of a 

 method whereby the water (or other mediurn) 

 which is to act as the dispersing agent is all 

 used in the formation of a colloid hydration 

 (solvation) compound. In other words, when 

 it is said that the addition of soap favors the 

 formation and stabilization of a division of 

 oil in water, it really means that soap is a 

 hydrophilic colloid which, with water, forms 

 a colloid hydrate with certain physical char- 

 acteristics, and that the oil is divided in this. 

 The resulting mixture can not, therefore, he 

 looJced upon as a subdivision of oil in water, 

 hut rather as one of oil in a hydrated colloid^ 



The amount of colloid necessary for stabili- 

 zation, at least in the preparation of an emul- 

 sion, is rather great. It must be sufficient to 



