272 PHENOMENA, ATOMS, AND MOLECULES 



than the 152 for the vapor or 179 for the water phase. We can thus see 

 that the presence of molecules in the interface is very much more probable 

 than in either the water phase or the vapor. This conclusion is strikingly in 

 accord with the facts. 



Hexane vapor coming into contact with water lowers its surface tension 

 very considerably. According to Gibbs' Law this proves that there is an 

 adsorbed film of hexane molecules in the surface having a much higher 

 concentration than that of either the vapor phase or the water phase. With 

 hexane the energy of transfer to the adsorbed film is roughly about %6ths 

 of that for hexadecane but the ratio of this energy to that in the vapor or 

 water phase is about the same for the heavier molecule. As the length of 

 the chain increases the vapor pressure, solubility and amount of substance 

 adsorbed decrease, but the ratio of the amount adsorbed to that in either 

 phase must increase. 



We shall see that this is the fundamental cause of the formation of 

 monomolecular films of organic substances on water. The ratio between the 

 concentration of the substance in the film and in the water is not greatly 

 altered by the presence of an active group (such as carboxyl) in the 

 molecule. The principal effect of the active group is to increase the solubility 

 of the substance in water and therefore cause a corresponding increase in 

 the amount adsorbed. 



The Effect of an Active Group. 



When we replace a hydrogen atom in a hydrocarbon by a hydroxyl, 

 carboxyl or similar radical we change the surface energy of a part of the 

 molecule very greatly without having very much effect on the remainder of 

 the molecular surface. From this viewpoint we have at once an explanation 

 of the fact that the introduction of such a radical or active group lowers the 

 vapor pressure and increases the solubility. 



Let us now take up again the consideration of the palmitic acid molecule 

 as an example of a molecule having an active group. We shall adopt Adam's 

 terminology and refer to the hydrocarbon portion of the molecule as the 

 tail and to the active group — COOH as the head. Representing the mole- 

 cule by the usual symbol RX we shall thus let R denote the tail and X the 

 head. For the calculations of the surface energies we shall adopt the follow- 

 ing values for the surface energies (ergs per cm.^). 



Yb = 50- yRw = 59- 



yx =144- ynx ^ 20. 



yw= 117. yxw = — 30- 



The subscript w refers to water. The surface energies of hydrocarbons 

 and of water, yn and yw respectively are the values we have already used 



