CHAPTER VII 



EMULSIONS 



Viewed through the microscope, protoplasm presents the pic- 

 ture of an emulsion — a colloidal dispersion of fats and like 

 substances in an aqueous medium. Whether or not we regard 

 this visible structure of protoplasm as fundamental, it neverthe- 

 less plays a significant part in the life of the cell so that a knowl- 

 edge of it, and therefore of emulsions in general, becomes most 

 important for an understanding of protoplasmic behavior. 



An emulsion is a system in which one liquid is finely and perma- 

 nently dispersed in another with which it is not miscible. Very 

 dilute emulsions of oil and water alone are model suspension 

 (lyophobic) colloids; that is to say, they have the properties 

 typical of solid (metallic) colloidal suspensions. Concentrated 

 emulsions, on the other hand, have certain (electrical) properties 

 in common with colloidal suspensions but also certain (viscous) 

 properties in common with the gluelike or lyophilic colloids. 

 For reasons that will become obvious later on, the several lyo- 

 phobic characters of emulsions are more significant than is the 

 one lyophilic property; consequently, the emulsions are classed 

 as lyophobes. 



Emulsions, as ordinarily met with commercially and in nature, 

 are of oil and water. The oil may be dispersed in water, or the 

 water dispersed in the oil. Milk is an oil-in-water emulsion, 

 and cold cream a water-in-oil one. 



Pure emulsions of oil and water alone can be made only if one 

 of the phases (ingredients) is less than 1 part in 100 parts of the 

 other. Such emulsions are truly colloidal, in that the dispersed 

 particles are of ultramicroscopic size. The common emulsions 

 are more concentrated, having nearly equal proportions of oil 

 and water, and the dispersed particles are often relatively large. 

 All such emulsions must be stabilized by a third substance — the 

 stabilizer, or emulsifier — which is usually a lyophilic colloid, such 

 as gelatin or soap. This third phase plays its role as stabilizer 

 by forming a membrane around the dispersed globules. When 



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