SURFACE CHEMISTRY 41 



forces is given by the 40-fold increase in the value of "k from 1.67 to 67. 

 The surface energy 70 has been increased 24-fold. 



The data for argon illustrate the properties of an atom having a com- 

 plete electron shell or octet. The fact that in spite of the much larger 

 electron configuration (with a kinetic theory diameter of 2.9) the volume 

 per molecule in the liquid is the same as for hydrogen, shows that the field 

 of force around the molecule is far greater than for hydrogen and helium. 

 The heat of evaporation per unit area l/S is a very convenient measure of 

 the intensity of this field. With Ho, He and A it is about one-half of the 

 surface energy Yq. 



The data for methane indicate that this molecule has a value of X/S, 

 about the same as that for argon. This molecule has a completed octet 

 which shares pairs of electrons with hydrogen atoms. Evidently the pres- 

 ence of the hydrogen atoms does not appreciably alter the field of force ; 

 the main effect is to raise the molecular volume, thus raising X and the 

 boiling point Tb almost in proportion to S. 



The values for A/S for the saturated aliphatic hydrocarbons beyond 

 propane are practically constant and equal to 28. The same constancy is 

 shown in the surface energy Yo- The data for octane shown in the table are 

 typical. The surfaces of the molecules of the higher saturated hydrocarbons 

 are thus very much like those of argon atoms, the effect of the chain forma- 

 tion manifesting itself principally in the increase in X/S from 18 to 28. 



The constancy of X/S is also an indication that the molecules of the 

 vapors of the higher hydrocarbons are approximately spherical in form. 

 The known flexibility of the chain and the surface energy reduction result- 

 ing from the more compact form afford ample explanation of this spherical 

 form. 



The effect of substituting a hydrogen atom in octane by a hydroxyl 

 radical is illustrated in the last line of the table by the data for n-Octyl 

 alcohol. There is a small decrease in V paralleling that observed in H2O, 

 except that now, since each OH group in the liquid cannot be close to many 

 other OH groups, the decrease in volume is considerably less. The large 

 field of force around the OH group is best illustrated by the increase of 26 

 units in the value of X. The fact that this increase is less than half of that 

 observed in the change from H2 to H2O shows that the OH group in the 

 molecule of the vapor of octyl alcohol is able to bury itself, at least in part, 

 in the approximately spherical molecule. 



In spite of the large increase in l and l/S in octyl alcohol, the surface 

 energy Yo is the same as for octane. This can be explained by the orienta- 

 tion of the molecules in the surface layer of octyl alcohol. The great attrac- 

 tive forces exerted by the hydroxyl groups causes them to be drawn down 



