THE DISTRIBUTION OF MOLECULES 



77 



boiling points of hydrocarbons and alcohols shows that the variation of 

 \ with chemical composition and structure is in excellent agreement with 

 this equation, even in the case of mono-, di- and tribasic alcohols and the 

 compounds having branching chains. 



The surface energies y whicli give best agreement with these data 



are 



In 



Y.v 



Yaa 



32.7 for CH3 groups 

 38.2 for CH, " 

 190. for OH 

 34 for the interfacial energy of 



OH against R (hydrocarbon) 



(9) 



The results show unmistakably the tendency for the larger hydrocarbon 

 molecules of vapor to become spherical and for the hydroxyl group to 

 become partly submerged under hydrocarbon groups in the vapor molecule 

 especially when the hydrocarbon chain is long enough for its end to reach 

 back to the hydroxyl group without too sharp curvature. In molecules 

 containing more than one hydroxyl there is a particularly strong tendency 

 for the hydroxyls to come into contact with each other. The surface ener- 

 gies involved in these changes in configuration are sufficient, according to 

 the Boltzmann equation, to overcome the effects of thermal agitation and 

 probably of the constraints due to imperfect flexibility of the molecules. 



C 



Fig. 3. — Diagram of molecules A and B and of a molecule A in a mixture of A and B. 



It is believed that more accurate measurements of vapor pressures of 

 a variety of organic compounds can lead to the development of very definite 

 knowledge of the configurations of their molecules in the vapor phase. 



Vapor Pressures of Binary Mixtures. Let -us now calculate the energy 

 "k needed to transfer one molecule in a binary mixture to another mixture 

 of the same components having different concentrations. Let us designate 

 the two types of molecules corresponding to the two components by A and 

 B as indicated in Fig. 3. Each molecule is assumed to have a composite 

 surface. The fraction a of the surface of the molecule A is an A-surface, 



