S8 



PHENOMENA, ATOMS, AND MOLECULES 



and the partial pressures of water from solutions containing less than 

 0.5 mol fraction of water also agree with this value reasonably well, the 

 values of cp for the higher concentrations of water decrease rapidly, so 

 that with a mixture containing 0.8 mol fraction of water cp = — 20. With 

 ethanol and propanol similar effects exist but are much less marked. 



In order to form a more concrete conception of the principles which 

 govern the orientation of organic molecules within a liquid, let us consider 

 the following model. The inner circle in Fig. 4 represents a molecule AC, 

 the sector a being the A-surface. The annular space between the two 

 circles represents diagrammatically the surfaces of the molecules which 

 surround the given molecule AC. In choosing the symbols a, h, c, d to 

 represent the surface fractions we may, without loss of generaUty, so 

 assign them among the 4 kinds of surface that & < a < c <c?. When the 

 molecule AC is oriented as shown, with the whole of the B-surface in 

 contact with A-surface, all of the C-surf ace will be in contact with D-surface 

 and the surface fraction of A-surface in contact with D is a — Z?. Thus the 

 total surface energy is 



l^^S[hya^-\- {a — h)yai-\-nci\ (30) 



Let the molecule AC now be turned so that the A-surface does not 

 come into contact with B but only touches D. Then the total surface 

 energy is 



Xo = S[ayad + h^hc + {c— b)ycd] 



(31) 



Fig. 4. — Diagram illustrating the orientation of a molecule AC in a liquid BD. 



