278 PHENOMENA, ATOMS, AND MOLECULES 



flat in a water surface was reached by the writer in 1917 ^^ from a study 

 of Taube's and Szyszkowski's data on the surface tensions of dilute aqueous 

 solutions of fatty acids and other aliphatic compounds. The energy change 

 involved in the transfer of a — CH2 group from the interior of the solution 

 to the surface was found to be constant and equal to 625 calories per gram 

 molecule or 4.3 X 10"^^ erg per molecule. 



By Table III the difference between A for palmitic and for butyric acids, 

 due to the 12 CH2 groups, is 63 for horizontal molecules on water, while 

 for molecules in solution it is 162 for cylindrical and 99 for spherical mole- 

 cules. Thus the energy of transfer from the interior to the surface is 

 8.3 X lO"^"* for each CHo if the molecule in water is cylindrical and 

 2.8 X 10"^^ if it is spherical. The value 4.3 X 10"^^ found from the 1917 

 work lies between these values and is thus consistent with the results given 

 in Table III. 



In discussing the spreading of adsorbed films of the higher fatty acids 

 on water and in comparing these films with those of the lower fatty acids, 

 it was pointed out in the 19 17 paper that the films produced by small con- 

 centrations of the lower acids behaved like two-dimensional gases obeying 

 the gas law 



Fa = kT. (6) 



Here F is the spreading force (dynes per cm.) and a is the area (in cm.^) 

 per adsorbed molecule. It was found that 



"in the case of the higher fatty acids the films do not spread upon the 

 surface in the way required by this equation. Thus with palmitic acid 

 the force F falls to 0.2 dyne per cm. when a = 23 A^ whereas by the 

 equation the force should be 17.5 dynes per cm. for this value of a. 



"Any solid or liquid film must have a certain tendency to spread on 

 the surface by giving off separate molecules which follow the gas laws. 

 This tendency may be measured as a 'surface vapor pressure.' With 

 palmitic acid and higher fatty acids this pressure is less than o.i dyne 

 per cm. The smallness of this pressure for the higher fatty acids must 

 be due to attractive forces between the molecules powerful enough to 

 prevent their separation. These same forces tend to prevent the film 

 from evaporating from the surface into the vapor phase and from going 

 into solution in the water. There are thus intimate relationships between 

 the lowering of surface tension produced by fatty acids and the vapor 

 pressures and solubilities of these substances. These relationships will 

 be discussed in more detail in another part of this paper." 



It was intended to publish the work here referred to as Part III of the 

 paper on the Constitution and Fundamental Properties of Solids and 



