270 PHENOMENA, ATOMS, AND MOLECULES 



of the surface of the molecules is exposed, while the heat of evaporation 

 corresponds to the whole surface exposed. It is probable that the difference 

 is caused by the more unsymmetrical state of the molecule when only part 

 of its surface is exposed and by the resulting increase in the surface energy 

 or stray field.* 



From the fact that "k for the hydrocarbon molecule in water or in the 

 vapor state is so large we may conclude from the Boltzmann equation that 

 the vapor state is so large we may conclude from the Boltzmann equation 

 that the vapor pressure and the solubility are very small in the case of a 

 hydrocarbon with a molecule as large as we are considering (i6 carbon 

 atoms). With lower molecular weights the surface area of the molecules 

 becomes less and the volatility and solubility thus increase. The relatively 

 small solubility of even the lower hydrocarbons may be explained in this 

 way. 



The energy of transfer of a hydrocarbon molecule to an adsorbed film 

 on water will depend on the configuration and the orientation of the 

 molecule in the surface. Let us consider three possible cases : 



a. A spherical molecule half immersed in the water. The energy X is 

 thus the sum of the energies of the two hemispherical surfaces minus the 

 energy of the water surface which is destroyed by the presence of the 

 hydrocarbon molecule. The cross-section of the molecule through the center 

 is 76. A^ while each hemisphere has a surface of 152.A2. Taking the total 

 surface energy of water to be 117 ergs per cm.^ we have 



^ = (59 X 152 + 50 X 152 - T17 X 76) lo-i^ 

 = 77 -X, 10"^^ ergs. 



b. A cylindrical molecule, half immersed in the water, oriented with its 

 axis parellel to the water surface. The area of the section parallel to the axis 

 is 128. A^ and half the molecular surface is 210 A- so that 



^ = (59X210 + 50X210- 117 X 128) lo-^^ 

 = 79 . X io~i-* erg. 



c. A cylindrical molecule, half immersed in the water, oriented with its 

 axis perpendicular to the water surface. The cross-section of the molecule 

 parellel to the water surface is 20. A^. Thus we have 



^ = (59 X 210 + 50 X 210 — 117 X 20) io~^^ 

 = 2o6.Xio'i^ 



* In a paper to be published in the Journal of Physical Chemistry this theory of 

 evaporation is developed and discussed in more detail and gives quantitative agree- 

 ment with the heats of evaporation not only of hydrocarbons, but of alcohols and 

 other substances. 



