Tension and Complex Molecules. 



391 



one constituent can considerable differences be expected pro- 

 vided the mutual solubility is not very high. But in systems 

 with low solubility there arises another difficulty which is 

 not so easy to overcome. E. g., for the pairs of liquids like 

 water-benzene and water-chloroform, both a 2 an d a 12 can be 

 determined pretty accurately, whereas a x cannot be estimated 

 by ordinary methods, e. g. the capillary method cannot be 

 used at all. To explain this, general properties of systems 

 with limited mutual solubility have to be considered. 



As a rule, the surface tension as a function of concentra- 

 tion varies in the following way, as can be seen on fig. 2 and 

 fig. 3, where the concentrations are plotted along the abscissae 



Fig-. 3. 



* A 



■> Concentration. 



> Concentration. 



and the surface tensions along the ordinates. The dotted 

 curves represent the solubility curves inside which there is a 

 region of two layers. If the mutual solubility is considerable, 

 the solubility curve intersects almost horizontal parts of 

 the surface-tension curve as in fig. 2. Whereas when the 

 mutual solubilities are small, the solubility curve intersects 

 a nearly vertical part of the surface-tension curve on the 

 side where the liquid with the higher surface tension is in 



excess, i. e. near B on fig. 3. In such a case ~ is large, 



OjG 



where a is the surface tension and c the concentration, 

 i. e. the slightest change in the concentration provokes a 



I 



