PRINCIPLES OF STRUCTURE 



45 



about 1/4- lo^*^ dynes/cm^, which result corresponds to the order of 

 magnitude mentioned above. 



The product of surface tension and area has the dimension of 

 energy : cm^ • force/cm = force • cm = energy. Instead of surface ten- 

 sion, the notion oi surface energy is therefore often used. If much work 

 has to be done to increase the surface, as for instance in water or 

 other hquids with many OH-groups in contact with air, the surface 

 energy is large (see Table V). 



TABLE V 

 SURFACE TENSION AGAINST AIR AT I 5 



(hober, 1922, p. 167) 



C 



o.zs molar solutions 



o 

 dyne cm 



Relative o 

 (a H^O = i) 



Water 



Cane sugar 



Urea 



Glycerol 



Acetic acid 



Ethyl alcohol . . . . 

 Ethyl ether (satur.sol.) 

 Ethyl acetate . . . . 



i-Valeric acid 



i-Amyl alcohol . . . . 



71.6 

 72.1 

 71.6 



71-5 

 66.8 



66.0 



53-1 

 41-^ 



34-9 

 29.9 



1.000 

 1.007 

 1. 000 

 0.999 

 0.932 

 0.922 

 0.742 

 0.578 

 0.487 

 0.417 



As it is impossible to disperse water in ethyl alcohol or other liquids 

 with which it is miscible, in the form of drops, obviously the water 

 molecules can be transferred to the surrounding dispersing medium 

 without doing any work. Thus the surface tension between two 

 mixing phases is 2ero and, therefore, no phase boundary is formed. By 

 analogy, a hydrated solid colloid particle cannot be supposed to pos- 

 sess surface energy if the water dipoles in the outer shell of the 

 hydration layer have the same mobility as those in the bulk of the 

 water. In that case we are dealing with the situation illustrated in Fig. 

 20a (p. 20), i.e., the particle loses its surface and is in stable solution 

 in the dispersing medium. 



The examples given show that it is not enough to speak merely of 

 the surface energy of a Hquid without specifying the medium in con- 



