EMULSIONS 37 



experience fall naturally into two groups generally known as oil-in-water 

 emulsions, and water-in-oil emulsions. In the former type an oil or some 

 other liquid insoluble in water, or practically so, is dispersed throughout a 

 water dispersion medium. In the latter type the converse is true, the oil or 

 other liquid immiscible with water constitutes the dispersion medium, whde 

 small aggregations of water molecules are dispersed through it. The propor- 

 tions of the components of most emulsions can be varied between wide limits. 

 Emulsions are not generally considered to be true colloidal systems, but, like 

 suspensions, approach them in properties. Emulsions occur commonly in the 

 cells of plants and animals, and are generally believed to be essential com- 

 ponents of the protoplasmic matrix. Both water-in-oil and oil-in-water emul- 

 sions are known to exist in living cells, but the latter type is commoner. When 

 examined under high magnification with a microscope protoplasm in its grosser 

 aspects often presents the appearance of an emulsion of fats and fat-like sub- 

 stances dispersed through the body of the protoplasm. 



Some common examples of oil-in-water emulsions are milk, cream, emul- 

 sions of olive oil in water, and the latex of the rubber tree, milkweed, etc. 

 Perhaps the only generally familiar water-in-oil emulsion is butter. Alany 

 other similar systems can be prepared, however. Water in olive oil is one 

 example of such a system. Water in kerosene is another. The word "oil" 

 as employed in this discussion is not restricted to those substances usually 

 classified chemically as oils, but is here somewhat generalized to include other 

 liquids which are immiscible with water. Kerosene, for example, is not 

 chemically an oil. 



Emulsions, with the exception of some very dilute ones, lack stability 

 unless there is also present in the sj'stem an emulsifier. In the absence of an 

 emulsifier, the two components of an emulsion rapidly separate, the oil, being 

 the component of lower specific gravity, rising to the top. The group of sub- 

 stances classified as emulsifiers is chemically a ver}^ heterogeneous one. Some 

 of the best known emulsifiers are the soaps, saponins, various substances which 

 form hydrophilic colloids when dispersed in water (gums, gelatin, etc.), and 

 fine suspensions of certain rather inert materials such as sulfur, carbon, silica, 

 and resin. The last group is probably of little importance in living organisms. 

 Oil-in-water emulsions may be stabilized by such substances as soaps of 

 the alkali metals (Xa, K, Li, etc.), gum acacia, and proteins; while soaps of 

 the alkaline earth metals (Ca, Ba, etc.), and gum dammar are examples of 

 stabilizers of water-in-oil emulsions. In general, emulsifying agents which 

 are soluble in water or form hydrophilic sols in water, stabilize oil-in-water 

 emulsions while those which are insoluble in water but soluble in oil stabilize 



