84 PHENOMENA, ATOMS, AND MOLECULES 



It is interesting to note that the value of y(R-OH) = 33.7 obtained 

 from these data on the vapor pressures of mixtures of bromides and 

 alcohols agrees with the value 34 given in (9) for y^^ which corresponds 

 to the energy of the same kind of interface, but determined in an entirely 

 different way, viz., from the heats of evaporation of alcohols. 



Perhaps the best test of Equation (17) available at present is that 

 based on the published data on the vapor pressures of mixtures of alcohols 

 and water and mixtures of fatty acids and water, although the polar 

 character of these substances undoubtedly causes some mutual orientation 

 and segregation of molecules in these mixtures, so that the assumptions 

 underlying this equation are not completely fulfilled. 



Table IV contains the results of calculations based on Wrewsky's data 

 (M. Wrewsky, Zeit. phys. Chem. 81, i (1913) and 82, 551 (1914)) on 

 the partial vapor pressures of aqueous mixtures of methyl, ethyl and propyl 

 alcohols at temperatures of 30° and 50°. 



TABLE IV 

 The Mixture Energies qp of Alcohol-Water Mixtures 



(pc«i is based upon 

 y(R-OH) =Z2.7 

 yCR-HzO) =37.4 and 

 yCOH-H^O) = — 12.6 



The value of y(R-OH) was taken to be the same as found from the 

 data of Table III, and the other two y's were chosen to make cpou and qpcoi 

 agree as well as possible. The fact that the two constants could be chosen 

 so as to give such good agreement with the three values of qposs is evidence 

 for the applicability of Equation (17) from' which cfcai was derived. 



In these calculations the water molecule was assumed to have a single 

 kind of surface. The value y = 37-4 is in reasonable agreement with the 

 value 59 obtained by Harkins for the total surface energy of a hexane- 

 water interface by direct measurement. 



The negative surface energy y = — 12.6 for the interface between 

 water and the hydroxyl group is not contrary to known facts and is an 

 expression of the very strong effect of this radical in increasing the solu- 

 bility of organic substances in water. 



Table V gives a summary of calculations based upon Konowalow's 

 data {Ann Phys., 14, 34 (1881) ) on the total vapor pressures of mixtures 

 of water with various fatty acids. The values of y found from these data 



