126 



PROTOPLASM 



cipitated or coagulated at the oil-water interface and thus forms 

 a film which is not readily, if at all, influenced by electrolytes. 



The oil phase plays a part in determining the kind, stability, 

 and reversibility of an emulsion only in so far as it helps deter- 

 mine the type of stabilizing membrane. Kerosene and Nujol 

 oil give opposite types of emulsions with the same stabilizer. 



The role of the oil in determining the type of an emulsion is 

 evident in the interesting behavior of hydrocarbon oils of differ- 

 ent densities. If such oils are emulsified in water, with casein 

 as the stabilizing agent, the light oils (hexane and gasoline) all 

 form stable oil-in-water emulsions. Slightly heavier oils form 

 emulsions of the same type, but they are unstable. The medium 

 (light lubricating) oils either do not emulsify at all or form mix- 

 tures of both types; they lie in the zone of reversal. Slightly 

 heavier oils form emulsions of the reverse type — water in oil — 

 but they are unstable. The heaviest (heavy lubricating) oils 

 form stable water-in-oil emulsions. This information is tabu- 

 lated in the following table: 



It has not been possible to give an explanation of this behavior of 

 hydrocarbon emulsions on the basis of existing theories of 

 emulsification. 



The process of changing over, when observed under the micro- 

 scope, reveals some of the details of the mechanism of phase 

 reversal. At the onset of the collapse of an oil-in-water emulsion, 

 the dispersed oil globules become irregular in shape and grow in 

 size by the coalescence of several smaller globules (Fig. 83). 

 The now larger oil globules become attenuated (pear-shaped) 

 owing to reduced surface tension at the one, pointed end (e. 



