RECENT ADVANCES IN SCIENCE 561 



arrange themselves so that the methyl groups at the ends of 

 the hydrocarbon 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 the 

 hydrocarbons from hexane to liquid paraffin have nearly the 

 same surface energy, viz. 46 to 48 ergs per cm 2 , although the 

 molecular weights are so different. In the case of the alcohols, 

 it is found that the surface energies are almost identical with 

 those of the hydrocarbons. This would be expected if the 

 surface layer consisted of the alkyl groups, the hydroxyl 

 groups being all directed inward. Oxygen in virtue of its 

 tetravalency confers great activity upon any group into 

 which it enters. 



In benzene, the molecules arrange themselves so that the 

 benzene rings lie flat on the surface, the surface energy of benzene 

 being about 65 ergs per cm 2 . If an active group, such as OH, 

 replaces one of the H atoms, as in phenol, this group is drawn 

 inwards, with the result that the benzene ring is tilted, thereby 

 raising the surface energy to 75 ergs per cm 2 . Thus any active 

 group strong enough to tilt the ring raises the surface energy 

 to about 75 ergs per cm 2 . 



Evidence for this view of capillarity is also obtained by 

 examining the behaviour of oil films upon water. The experi- 

 ments of Devaux have shown fairly clearly that films which 

 spread out on water do so until the film is just one molecule 

 thick. It is natural to assume that the force which causes the 

 spreading of an oil on water is due to an attraction between 

 the water molecules and a certain part of the oil molecules. It 

 cannot be the entire oil molecule which is involved, as this 

 would lead to solubility and not merely to spreading. There 

 must be some portion of the oil molecule which has an affinity 

 for the water molecules. Let us take the case of oleic acid. 

 There is no doubt but that the carboxyl group has a strong 

 affinity for water, as is shown by the solubility of organic 

 acids in water, and the lack of solubility of the corresponding 

 hydrocarbons. Hence when oleic acid is placed upon water 

 it is probable that the carboxyl groups dissolve, i.e. combine 

 with the water. The long hydrocarbon chains have too much 

 attraction for one another, and too little for water to be drawn 

 into solution. By spreading out so as to form a film just one 

 molecule thick all the carboxyl groups can combine with water 



