CHEMISTRY: W, D. HARKINS 
157 
between them. It may be considered that each region (phase, surface, or 
interface) exerts a certain restraining force upon (has affinity for) the solute 
molecules. Since at equilibrium the thermodynamic potential of the solute 
is the same in all of the regions, it may be considered that the concentration 
of the solute (at equilibrium) in each phase, interface or surface, gives an 
index of the restraining force exerted by that region upon the solute molecules. 
Let us now assume that we have a number of exactly similar two phase sys- 
tems, each of which consists of equal volumes of a polar liquid, such as water, 
and a nonpolar liquid such as octane, with an interface of a definite area 
between them, and into each of these systems we put N molecules of the 
polar-nonpolar type o . The hypothesis indicates that with a given polar 
group the distribution of the N molecules will vary in such a way that with 
an increase in the length of the nonpolar part of the molecule, the number 
of molecules, and therefore the restraining force in the octane will increase, 
while in the water both of these will decrease. The reverse of this occurs 
when with a given nonpolar chain, there is an increase in the number of polar 
groups. The greatest restraining force would be exerted on such molecules 
when they are in the interface, where the nonpolar end of the molecules 
could turn toward the nonpolar liquid, and the polar end toward the polar 
liquid. Since the restraining force is greatest at the interface, the concen- 
tration in this region should also be the greatest, which agrees with the facts 
as found by experiment. 
In the preceding paragraph it was assumed that the water is completely 
covered by the octane. Let us now assume that the water has a surface, or a 
water-vapor interface as well, and that benzene is the nonpolar (slightly 
polar) liquid. At the water- vapor interface at ordinary temperatures, the 
drop in average intensity between the stray field in the water and that in the 
dilute vapor (in which the intensity is practically zero), is much greater than 
that between water and benzene, so the restraining force on molecules of the 
polar-nonpolar type should be much greater at the former interface. In 
complete agreement with this idea experimental tests indicate that the re- 
straining force is about three times greater at the water-vapor interface than at the 
water-benzene interface, when butyric acid is the solute, and somewhat similar 
results are obtained when the solute is acetic acid. The experimental work 
was carried out in 1913 by E. C. Humphrey, and was afterward repeated and 
made more precise by H. H. King and H. McLaughlin. The above results 
may be expressed in other words as follows: the thermodynamic potential of 
butyric acid is about three times as great in the water-benzene interface as it is 
in a water surface, when the concentration of the butyric acid in both of 
these interfaces is the same, or the drop in mechanical potential which a 
molecule of butyric acid undergoes in passing from water into the interface 
is much greater when the second phase is a dilute vapor than it is when the 
second phase is a nonpolar liquid. This is an illustration of a general case. 
