WILLIAM D. HARKINS 159 



liquid are oriented in a characteristic fashion is of comparatively recent origin. It 

 seems peculiar that the birth of so obvious a conception should have been so long de- 

 layed, but it is probable that this is due to the general habit of 

 considering molecules as spherical, even when their formulas are 

 highly elongated. In such cases the former conception was that 

 it would roll itself up into a sphere. It is obvious that even in a 

 dissymmetrical field of force, such as may be assumed to exist at 

 the surface of a liquid, a molecule which is a perfectly sym- 

 metrical sphere could exhibit no orientation. However, even in a 

 uniform gravitational field a perfect sphere may orient itself Fig. 10.— Illustrates 

 provided its mass is not uniformly distributed, as will be seen if the orientation of a 

 a sphere is weighted on one side, so that even if all molecules weighted sphere under 

 were spherical, molecular orientation would not be at all impos- theintiuence o 

 sible (Fig. 10). ^""'^- 



SOLUBILITY AND MOLECULAR ORIENTATION IN SURFACES 



The cohesion in liquid ethane, 



H H 



1 I 

 H— C— C— H , 



I I 

 H H 



is extremely low, so low that liquid ethane cannot exist at ordinary temperatures. 

 The introduction of one oxygen atom gives rise to ethyl alcohol in which the cohesion 

 is equivalent to a pressure of 3,000 atm. The hydrocarbon molecule (ethane) is non- 

 polar, but the hydroxyl group ( — OH) of the alcohol is polar. The attraction between 

 two such polar groups is very much greater than that between two non-polar groups 

 or that between a polar and a non-polar group. 



The solubility of an organic acid, an alcohol, or an amine in water is due to its 

 polar group which is greatly attracted by the water. The solubility of such a substance 

 in hexane is, on the other hand, due to the presence of the hydrocarbon groups. Thus, 

 all saturated hydrocarbons are practically insoluble in water. The old rule is: Similia 

 similibus solvimtur ("Like dissolves like"). 



If, now, we have a two-phase system consisting of water below and hexane above, 



and add butyric acid, 



H H H 



I I I 

 H— C— C— C— C— 0— H , 



I I I II 

 H H H O 



the hydrocarbon end, which we may designate by CZZl , is soluble in the hexane, but 

 not in the water, and the carboxyl group O is soluble in water but not in the hexane. 

 However, the carboxyl group will drag some molecules of the butyric acid into the 

 water, while the hydrocarbon group will drag others into the oil. 



At the interface between the water and the oil, however, the hydrocarbon of the 



