EFFECTS OF MOLECULAR DISSYMMETRY 277 



The most interesting applications of the data in Table III are in con- 

 nection with the arrangements of molecules in surface films. Comparing 

 Cases 2, 5, 7 and 8 we see that the decrease in energy corresponding to 

 Case 7 is so great that practically no molecules could exist in either a water 

 phase or a hydrocarbon phase until the interface becomes saturated with 

 molecules which are all oriented with their heads in the water phase. The 

 relative number of molecules having this orientation as compared to those 

 having the reverse orientation (Case 8) is found from the Boltzmann 

 equation to be as 10-^: i for palmitic acid and 10^: i for butyric acid. 



In a similar way we may estimate the probabilities of the various 

 possible orientations of molecules adsorbed at the free surface of water. 

 The high value of 1 in Case 1 1 in which the molecule is oriented vertically 

 with the carboxyl group uppermost shows that no molecules can be in this 

 position. 



Without such analysis as we are now making it might seem that single 

 molecules of palmitic acid adsorbed on the surface of water would take up 

 a verticle position as is illustrated in Case 10, the carboxyl group being 

 downwards. In fact N. K. Adam as one result of work described in a series 

 of valuable papers ^^ concludes that "expanded films are two-dimensional 

 gases" * in which there is considerable space vacant between the molecules 

 and that in these films "the molecules are oriented perpendicular to the 

 water surface." f An examination of Table III shows, however, that the 

 value of A. corresponding to this orientation has the high value of 181, while 

 for a molecule lying flat in the water surface (with the carboxyl group 

 turned downward into the water at one end of the molecule) X is only 64 

 (Case 12). A shghtly lower energy (X = 39) is calculated (Case 9) for 

 spherical molecules half immersed in the water surface. But the a priori 

 probability of the spherical form (particularly in case of butyric acid) is so 

 much less than that of the spread-out form of a molecule which lies flat, 

 that this latter form becomes the more probable. 



This conclusion that the hydrocarbon chains of adsorbed molecules lie 



* See paper by G. Friedel, Colloid Chemistry, No. 372, pp. 102, Aug. 1926. Some 

 of the structures he describes may be one or two dimensional solids. J. A. 



t In a footnote in his last paper Adam rejects the suggestion that I had made to 

 him that the molecules in expanded films tend to lie flat on the water surface becoming 

 overlapped if sufficiently crowded. He says "There is the more probable alternative, 

 however, that as soon as the molecules have separated, through expansion of the 

 films, they sink in among the water molecules so as to satisfy the attractive forces of 

 the chains. If this is so they will probably remain vertical. Nevertheless it must be 

 remembered that the pressure in two dimensions in the surface is only about one sixth 

 of that expected from the gas laws, and no satisfactory explanation of this fact is at 

 present available." The analysis in the present paper shows that a sinking of the 

 molecules does not satisfy the attractive forces, but that the hydrocarbon chains must 

 be in contact with one another whenever possible, otherwise they must lie flat in the 

 surface. 



