CHEMISTRY: W. D. HARKINS 
563 
the amount of work involved in the process, which is equal to the 
decrease of free energy during the approach : 
where 71 and 72 give the free energy of the two unlike surfaces before 
their approach, and 71,2 is the free energy of the interface. 
It has been shown by Hardy, ^ and by Harkins, Brown, and Davies,^ 
that the adhesional work between organic liquids and water is repre- 
sented by values which are characteristic for each class of organic com- 
pounds. Our own work indicated also that at the interface between 
water and an organic liquid, any groups in the latter which contain 
oxygen, nitrogen, or either triple or double bonds, are oriented toward 
the aqueous phase, while the hydrocarbon radical turns toward the 
organic liquid. This orientation is of great importance in connection 
with either the adhesional work or the adhesional energy, since its 
result is to increase the adhesion between such an organic liquid and 
water. Further facts which are in accord with this theory of orienta- 
tion, also developed by Langmuir,^ are presented later in this paper, 
in the section on negative surface energy. 
Work of surface cohesion. — Since 7 is zero when the two liquids become 
identical, the Dupre equation reduces to 
where Wsc represents the work of the surface cohesion. The free 
surface energy (7) is usually measured in ergs per square centimeter. 
It is evident that Wsc is the work necessary to break a bar of liquid 
or solid with a cross section of 1 sq. cm., in such a way as to give two 
plane surfaces of 1 sq. cm. each, that is the break must occur perpen- 
dicular to the long axis of the bar. This amount of work is clearly that 
which would be done in the determination of the tensile strength of a 
steel or other metal bar, if the test could be carried out under ideal 
conditions, which are that the bar during the test must remain of uni- 
form cross section, and the break must occur in such a way as to give 
two surfaces which are as closely plane as the surface of a liquid, though 
in the actual tests this condition is not met, since the bar is much dis- 
torted. The work of surface cohesion may therefore be called the 
tensile work (Wt) • This is equal to the tensile force integrated through 
the distance necessary to pull the two surfaces completely apart, or 
Wa = — A7 = 71 + 72 — T1.2 
(1) 
Wsc = -Ay = 27 
(2) 
