THE CONSTITUTION OF LIQUIDS 69 



themselves so that the methyl groups (CH3) at the ends of the hydro- 

 carbon chains form the surface layer. The surface layer is thus the same, 

 no matter how long the hydrocarbon chain may be. As a matter of fact, 

 the surface energy of all these many different substances from hexaine to 

 molten paraffin, have substantially the same surface energy — namely, 46 

 to 48 ergs per square centimeter, although the molecular weights differ 

 very greatly. 



If, now, we consider the alcohols such as CH3OH, C2H5OH, etc., we 

 find that their surface energies are practically identical zvith those of the 

 hydrocarbons. The reason for this is that the surface layer in both cases 

 consists of CH3 groups. 



With such substances as CH3NO2, CH3I, we find that the surface 

 energy is much greater than that of the hydrocarbons. This is due to the 

 fact that the volume of the I or the NO2 is so great that the surface cannot 

 be completely covered by the CH3 radicals. The forcing apart of these 

 groups increases the surface energy. 



Particularly interesting relations are found with benzol derivatives. 



In benzol itself, the group molecules arrange themselves so that the 

 benzol rings lie flat on the surface, since the flat sides of these rings are 

 the less active portions of the molecules. The surface energy of benzol is 

 about 65 ergs per square centimeter. 



If, now, an active group, such as OH, is substituted for one of the 

 hydrogens in the benzol (forming phenol or carbolic acid), this group is 

 drawn into the body of the liquid, tilting the benzol ring up on edge and 

 raising the surface energy to about 75 ergs per square centimeter, which 

 corresponds to the activity of the perimeter of the benzol ring. Thus any 

 active group strong enough to tilt the ring up on edge raises the surface 

 energ}^ to about 75. Two active groups side by side (ortho position) have 

 no greater effect than one. But two active groups opposite one another 

 (para position) cannot both go wholly below the surface, so that the sur- 

 face energy then becomes abnormally large (about 85 in case of para 

 nitrophenol). The substitution of methyl or ethyl groups in the benzol 

 ring lowers the surface energy except where an active group in an adjacent 

 position draws these groups below the surface. 



Some of the best evidence in support of the new theory is derived from 

 experiments on thin films of oil on water or mercury. Oleic acid on water 

 forms a film one molecule deep, in whicH the hydrocarbon chains stand 

 vertically on the water surface with the COOH groups in contact with the 

 water. 



Acetic acid is readily soluble in water because the COOH group has a 

 strong secondary valence by which it combines with water. Oleic acid is 

 not soluble because the affinity of the hydrocarbon chains for water is less 



