SURFACES OF MOLECULES 233 



electrons or ions varies approximately linearly with 9, the fraction of the 

 surface covered by the adsorbed atoms, and thus, according- to the Boltz- 

 mann equation, the logarithm of the rate of evaporation of electrons or ions 

 varies linearly with B. In the case of the evaporation of neutral atoms — 

 for example, atoms of oxygen from adsorbed films on heated tungsten 

 filaments- — the rate of evaporation is much more nearly proportional to 6. 

 In studies of the surface tensions of organic Uquids (2) (17) and 

 of the properties of adsorbed films of organic substances on water (3) 

 (4) and of oil films on water, the principle of independent action has proved 

 itself particularly useful. It gives immediately a simple reason for believing 

 that these films should rarely be more than one molecule in thickness ; or 

 rather, that if they are more than one molecule in thickness, their properties 

 will lie markedly difl^erent from those of films in which the surface is not 

 completely covered with a single layer. The theory also gives immediately 

 reasons for believing that adsorbed films of many organic substances on 

 water will consist of orientated molecules. Thus the area on a water surface 

 occupied by molecules of the various fatty acids is dependent on an inter- 

 action between the carboxyl group and the water, and is independent of the 

 length of the hydrocarbon chain. This simple result, proved by experiment, 

 demonstrates that the molecules are orientated so that the carboxyl group is 

 in contact with the water, leaving the hydrocarbon tails to form a layer 

 above the carboxyl group which will have the properties characteristic of a 

 liquid hydrocarbon. 



The theory is equally applicable in pure organic liquids. The total 

 surface energies (surface tension extrapolated to the absolute zero) of 

 hexane and of hexyl alcohol are practically identical. This is readily ex- 

 plained by the orientation of the molecules which prevents the hydroxyl 

 groups from coming in contact with the surface, so that in both cases the 

 actual surface is that of a pure hydrocarbon. 



Studies of the surface tension of aqueous solutions of various aliphatic 

 compounds in water prove that in concentrated solutions the surface be- 

 comes covered with a closely packed monomolecular film consisting of 

 orientated molecules like those in oil films. With sufficiently dilute solutions, 

 h.owever, the amount of substance adsorbed in the surface is not enough 

 to cover the whole surface with closely packed molecules. Under these 

 conditions, the hydrocarbon chain lies flat in the water surface and the 

 energy needed to transfer any molecule from the surface to the interior 

 increases by a definite amount for each additional CH2 group in the hydro- 

 carbon chain. This is an excellent example of the application of the 

 principle of independent surface action, each CH2 group producing its 

 effect independently of the others. 



Similar views prove useful in studies of many other properties of 



