SURFACES OF MOLECULES 231 



This result justifies us in attributing definite properties to difi^erent 

 parts of the surface of the molecules of aliphatic compounds. For examnle, 

 the field of force around a carboxyl group in a fatty acid should he in- 

 dependent of the length of the hydrocarbon chain, provided that this is 

 more than about two carbon atoms long. That is, the forces near the 

 surface of the carboxyl group should be practically constant for all the 

 acids higher than propionic acid and would be roughly the same in the 

 case of acetic acid, but might be considerably difi^erent in the case of 

 formic acid. Similarly, we should not be able with any reasonable ac- 

 curacy to regard the field of force around the carboxyl group as due merel\- 

 to the super-position of the efifects of the hydroxyl group and the carbonyl 

 group, for these two groups are too close together in the molecule to be 

 without important efifect on one another. 



PRINCIPLE OF INDEPENDENT SURFACE ACTION 



From the foregoing analysis of the forces acting between molecules 

 of various types, we conclude that there is a large class of substances for 

 which we are justified in regarding the forces between two molecules in 

 contact as being dependent mainly on the nature of the surfaces of the 

 molecules which are in contact. This principle of independent surface action 



(12) will always be only an approximation to the truth, but there are many 

 cases where it applies with sufficient accuracy and so greatly simplifies the 

 problems of the interactions between molecules that useful results are 

 obtained in problems so complex that no solution is otherwise possible. Let 

 us consider some of these problems. 



The theory of adsorption in monomolecular films on solids and liquids 



(13) is an example of the application of this principle. According to this 

 theory, the force which holds an adsorbed molecule or atom on a surface 

 depends on the character of the surface of contact between the molecule 

 and the solid. If a second layer of molecules should form, the forces hold- 

 ing the molecule to the second layer are thus entirely different from those 

 holding the molecules of the first layer. The rates of evaporation of the 

 molecules from the first layer or from the second layer will differ greatly, 

 especially since they depend upon the magnitude of the forces according 

 to an exponential relation of the Boltzmann type. Two general cases must 

 be considered. If the forces holding the molecules in the first layer are 

 greater than those holding those of the second, there will be a wide range of 

 pressures of the adsorbed gas for which the film will never exceed one 

 molecule in thickness. We thus arrive at the conception that a certain frac- 

 tion, 6, of the surface is covered by adsorbed molecules, and that the 

 properties of this adsorbed film depend primarily on 6. If the forces 



