38 COLLOIDAL SYSTEMS 



water-in-oil emulsions. Many pharmaceutical preparations are stabilized by 

 means of gum acacia or gum dammar. 



Emulsions found in nature are usually stabilized by proteins. This is true 

 of milk and cream, in which the droplets of fat are dispersed throughout a 

 water medium. The latex of plants, one of the best examples of a naturally 

 occurring emulsion, is also stabilized by proteins. ^Mayonnaise dressing is 

 essentially an emulsion of olive oil in water, stabilized by the proteins of the 

 egg which is the third essential component of the system. Protoplasmic emul- 

 sions are probably also stabilized by proteins, although soaps may also act as 

 emulsifiers in plant cells. 



The mechanism of the stabilizing effect of emulsifiers upon emulsions is 

 not well understood, and it is probable that it varies with different tj'pes of 

 emulsifiers. It is well known, however, that any substance which greatly 

 lowers the interfacial tension between water and another liquid will usually 

 stabilize an emulsion composed of those two liquids. The usual interfacial 

 tension between water and benzene, to cite one example, is about 35 dynes 

 per centimeter. Addition of a little sodium oleate (a soap) to water will 

 lower this interfacial tension to about 2 dynes per centimeter. Sodium oleate 

 is an excellent stabilizing agent for an emulsion of benzene in water. 



Suggested for Collateral Reading 



Alexander, J. Colloid chemistry. Vols. I and II. Chemical Catalog Co. 

 New York. 1926 and 1928. 



Bancroft, W. D. Applied colloid chemistry. McGraw-Hill Book Co. New 

 York. 1926. 



Bayliss, W. M. Principles of general physiology. 4th Ed. Longmans, 

 Green and Co. London. 1924. 



Freundlich, H. Colloid and capillary chemistry. Translated by H. S. Hat- 

 field. E. P. Button and Co. New York. 1926. 



Gortner, R. A. Outlines of biochemistry. 2nd Ed. John Wiley and Sons. 

 New York. 1938. 



Kruyt, H. R. Colloids. Translated by H. S. Van Klooster. John Wiley 

 and Sons. New York. 1927. 



Seifriz, W. Protoplasm. McGraw-Hill Book Co. New York. 1936. 



