EFFECTS OF MOLECULAR DISSYMMETRY 279 



Liquids, but war work which began at that time prevented the carrying out 

 of this plan. The present paper, however, deals with this subject in the 

 manner that was then contemplated. 



It was also shown in the 191 7 paper that a film of palmitic acid on water 

 at 16° C. resists compression with a force greater than i dyne per cm. only 

 when a, the area per molecule, is less than 21 . A^. But if the water is heated 

 to 45° the film resists compression when a is 33. A- and a similar value of 

 a is found at 16° if a very small amount of acid is added to the water. Thus 

 adding acid or raising the temperature causes the film to expand if the 

 applied compressional force is not over 10 or 20 dynes per cm. Adam 

 has called such films "expanded films." He believes, as we have seen, that 

 the molecules in these films are in the condition of a two-dimensional gas 

 and that the molecules are oriented vertically. Let us see what light is 

 thrown on these questions by the calculations of Table IIL 



A palmitic acid molecule at the edge of a film of vertically close-packed 

 molecules has an energy A, = — 4 if the molecule is horizontal (Case 13) 

 and A = — 17 if the molecule is vertical (Case 14) like those in the rest 

 of the film. Thus the vertical orientation is the more probable. An isolated 

 molecule lying flat in the surface (Case 12) gives A = 64. The difference 

 81 is so great that the conclusion may be drawn that practically no molecules 

 will exist isolated in the surface layer but that they will all exist in clusters 

 in which the molecules have their hydrocarbon chains in contact with each 

 other. These values of 1 thus explain the observed fact that the two- 

 dimensional gas pressure is negligible for palmitic acid films. 



With a lower fatty acid such as butyric acid, however, the difference in 

 energy between an isolated molecule in the surface and one located at the 

 edge of a film is only 18 units so that it is not surprising that with these 

 lower fatty acids the molecules, at least in dilute solutions, do follow the 

 laws of two-dimensional gases. 



That isolated molecules of palmitic acid in a water surface cannot exist 

 in appreciable numbers is thus proved by the data of Table III and by the 

 experimental observation that for values of a greater than say 40A2, the 

 spreading force is immeasurably small. Thus Adam's view that the ex- 

 panded films consist of molecules which are largely isolated cannot be 

 correct. What is then the proper explanation of the expanded films ? 



In the 1917 paper in discussing the arrangement of molecules in 

 adsorbed films on the surfaces of fatty acid solutions it was concluded 

 (page 1888) 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 as- 



