ADSORPTION 



27 



daries of this sort are usually referred to as interfaces, and the tensions de- 

 veloped at interfaces are called interfacial tensions. The term surface tension 

 is usually restricted to tensions developed at the surface of a liquid when in 

 contact with a gas, i.e. at the "interface" between a liquid and a gas. Sur- 

 face tension is therefore merely one variety of interfacial tension. The inter- 

 facial tensions of a number of pure liquids as measured against a water surface 

 are given in Table J. 



TABLE 7 INTERFACIAL TENSIONS OF LIQUIDS AGAINST WATER AT 30° C. 



The magnitude of interfacial tensions is influenced by the factors of tem- 

 perature and solutes in the same general way as surface tensions. 



Adsorption. — The molecules of most organic compounds, as we have al- 

 ready seen, have a lesser attraction for water molecules than the molecules of 

 water have for each other. This relatively greater attraction between water 

 molecules than between the molecules of water and those of such "non-polar" 

 solutes results in the displacement into the surface or interfacial layers of a 

 disproportionately large number of the solute molecules. Hence the concentra- 

 tion of most organic solutes is greater in the surface or interfacial layers than 

 in the body of the liquid. The concentrating of solute molecules in the sur- 

 face or interface of a liquid is called positive adsorption. 



The molecules of inorganic salts, on the other hand, are more strongly 

 attracted to water molecules than the water molecules are attracted to each 

 other. Because of this stronger attraction of the water molecules for such 

 "polar" solutes relatively more solute molecules are pulled from the surface 

 films into the body of the liquid than water molecules. Molecules of "polar" 

 solutes, therefore, generally occur in greater concentration in the body of the 

 liquid than in the surface layers. This phenomenon is called negative adsorp- 

 tion. When negative adsorption occurs the solvent molecules become rela- 

 tively more concentrated in the surface layer than in the body of the liquid. 

 For reasons which will become clear later, negative adsorption rarely occurs 

 at interfaces. 



