82 



PHENOMENA, ATOMS, AND MOLECULES 



chloride (fl=i, c — o) and the fact that ethyl acetate and iodide both 

 contain hydrocarbon (R) surfaces, Equation (17) becomes much simpli- 

 fied. By means of the values of cp for the 3 mixtures Nos. 3, 4 and 5 we 

 obtain 3 equations containing 5 unknown values of y. From a study of the 

 surface tensions and other properties of alkyl chlorides and iodides we may 

 conclude that chlorine has properties intermediate between alkyl and 

 iodine. Thus it is reasonable to assume that y(C1-I) = y(R"C1). We have 

 seen in (9) that y(R-OH)= 34 and there are several indications that the 

 relation of -COO- to R is not materially different from that of OH to R. 

 Thus we place y(R-C) = 34, using C as an abbreviation of the -COO- 

 group in esters. We then have five equations for our five unknowns and 

 thus find the values given in Table II. 



From these values it should be possible to calculate the vapor pressures 

 of any binary mixtures of compounds such as RCl, RI, RCOOR where 

 each R represents any alkyl group. There is also no difficulty in including 

 compounds containing two or more CI, I or -COO- groups. 



It should also be possible from the principles that have been outlined 

 to develop equations for ternary mixtures and for those containing com- 

 pounds with molecules having three or more kinds of surface. Thus from 

 the values of y in Table II we should be able to calculate the partial vapor 

 pressures of a ternary mixture consisting of say propyl-di-chlor acetate, 

 ethylene chloride and amyl iodide, from the vapor pressures of the separate 

 components and the ordinary data of molecular volumes. 



TABLE II 



Values of Surface Enekgy y Calculated from Experimental Data on 



Vapor Pressures 



Unfortunately there have been very few experimental studies of the 

 vapor pressures of a series of related compounds which can serve at present 

 as a test of this theory. 



Measurements are needed of the vapor pressures of binary mixtures of 

 hexane with different simple alkyl compounds such as ethanol, propionic 

 acid, ethyl chloride, etc. From each such mixture studied, a separate value 

 of Y can be obtained and it would then be easy to calculate the vapor pres- 



