94 PHENOMENA, ATOMS, AND MOLECULES 



ethanol molecule by surrounding molecules which are oriented in random 

 directions. Actually these molecules will tend to be oriented by the given 

 molecule and this in turn would increase the orienting effect on the original 

 molecule. It is probable that value of I — Ib given by (45) or (48) 

 should be approximately doubled to take into account this mutual orienta- 

 tion. 



In any case we may conclude that with mixtures of many common 

 organic compounds there will be no appreciable orienting effect and that 

 we are therefore justified in assuming a random orientation as a first 

 approximation. The magnitude of the orienting effect is sufficient, how- 

 ever, for it to become of considerable importance as-a cause of deviations 

 in the case of polar substances, and particularly of those whose molecules 

 act as dipoles. 



ORIENTATION OF MOLECULES IN INTERFACES 



The example of the orientation of alcohol molecules in a free surface 

 which was considered in the introduction has served to show how the 

 principle of independent surface action may be applied in such cases. 



When a sufficiently small amount of a substance such as palmitic acid 

 is placed upon a clean surface of water in a tray the surface tension of the 

 water is not appreciably lowered. But if the free surface of the water is 

 decreased, by sliding a strip of paper over its surface, the surface tension 

 begins to decrease quite suddenly when the free surface becomes about 

 20 A- per molecule of palmitic acid. By raising the temperature of the 

 water or by introducing traces of acid into the water, the point at which 

 the surface tension begins to decrease may occur at an area of 30 to 50 A-. 

 Such films may be called expanded films. 



In 1917 in a discussion'^ of the arrangement of molecules in adsorbed 

 films on the surfaces of fatty acid solutions it was concluded that in dilute 

 solutions "the hydrocarbon chain lies spread out flat on the surface of the 

 water, but as the concentration of the solution increases, the molecules 

 become more closely packed and when the surface becomes saturated, the 

 molecules are all arranged with their hydrocarbon chains placed vertically," 

 The transition between the two states was assumed to take place in the 

 manner that was suggested (on page 1865) for the expanded film of oleic 

 acid, viz., in the expanded film "the molecules are partly reclining on the 

 surface, while in the second case (compressed film) they are packed tightly 

 side by side and are more or less erect upon the surface." 



The area covered by a vertically placed palmitic acid molecule is about 

 20 A^ while the same molecule lying flat on the surface should cover an 



' Langmuir, Jour. Amcr. Chem. Soc, 39, 1848 (1917). 



