EFFECTS OF MOLECULAR DISSYMMETRY 273 



and are those given by Harkins for the liquids in bulk. The same is true for 

 the interfacial energy Jrw- The value of yx has been obtained from a study 

 of heats of evaporation of hydrocarbons and fatty acids while yax and 

 YxTT were found from a study of the vapor pressures of binary mixtures 

 by a method which we shall soon describe briefly. The negative value for 

 the surface energy between carboxyl and water which results from these 

 studies is in accord with the very marked increase in the solubility in water 

 caused by the presence of this group. 



We have already estimated the surface area of the palmitic acid mole- 

 cule, but we now need to know the surface of the carboxyl group. From a 

 comparison of the volumes of the molecules of the lower fatty acids we may 

 estimate the surface to be 45. A^ which is the value that was assumed in 

 obtaining the values of yz, Jbx and yxw- 



Table II gives the dimensions of palmitic and butyric acid molecules 



TABLE II 



Dimensions of Palmitic and Butyric Acid Molecules 



Palmitic Arir! 

 C,r,Hr,,COOH 



Molecular weight 256. 



Density 0.85 



Volume per molecule 497. A' 



Sphere 



Diameter » 9.83 A 



Total surface 304. A' 



Surface of head 45. A 



Surface of tail 259. A 



Section of tail 76. A 



Cylinder with Hemispherical Ends 



Diameter 5.04 A 



Length of cylindrical part 21.50 



Total surface 420. A 



Surface of head 45. A 



Surface of tail 375. A 



Transverse section of tail 20. A 



Longitudinal section of tail 108. A 



2 



Hexagonal Prism 



Side of hexagon 2.64 A 



Total surface 420. A' 



Transverse section 20. A^ 



Longitudinal section 108. A 



2 



calculated from the molecular volumes assuming three different shapes. 

 The hydrocarbon tail is taken as spherical, cylindrical or of the form of a 

 hexagonal prism, this latter shape being useful when considering molecules 

 packed in surface films. 



