254 
PHYSICS: I. LANGMUIR 
creases about in proportion to the number of carbon atoms in the chain. 
This is a direct proof that the molecules are oriented on the surface 
with the COOH groups in contact with the water, and the hydrocarbon 
groups placed vertically above them. Dividing the value of r by the 
number of carbon atoms in these molecules, we obtain an average of 
1.19 X 10~s cm. It is not likely that the carbon atoms in a hydro- 
carbon molecule are closer to each other than in a diamond, so that 
we must conclude that the carbon atoms in the chain are not arranged 
along a straight line, but in a zigzag fashion. The cross-section a is 
ample for such an arrangement. 
The data for tristearin show that the three —COO— groups are all 
located on the surface of the water and that each occupies the same 
area as it does in the stearic acid molecule. The three chains are placed 
side by side. Cetyl palmitate occupies actually less space than pal- 
mitic acid. The two chains lie side by side, each having less than one- 
half the cross-section, but twice the length, that it has in palmitic acid 
or cetyl alcohol. 
These results prove that the areas are determined by the space on 
the water required by the active groups —COO— or —OH and not by 
the cross-section of the hydrocarbon chain itself. 
The unsaturated acids and esters occupy much larger areas than the 
saturated ones, and have correspondingly shorter lengths. This is 
clearly due to the double bond (and the —OH in ricinoleic acid) itself 
occupying a space on the water. MarceHn^ has shown that when a 
globule of oleic acid is placed on a water surface, the film obtained is 
twice as thick as one which just alters the surface tension of water. 
MarceKn concludes that such a film is two molecules thick. The above 
data show that when the thin film of oleic acid is doubled in thickness, 
each molecule then occupies the same area as does one of stearic acid. 
Clearly, when an excess of oleic acid is added, the —COOH groups 
displace the double bonds and the chains assume an erect position on 
the surface. 
These facts prove that the hydrocarbon chain is extremely flexible 
and has no definite shape of its own. The term ^chain' thus describes 
its properties admirably. 
The above method makes it possible to determine the lengths and 
cross-section of molecules of non-volatile, insoluble substances. De- 
vaux has shown that oils spread on clean mercury surfaces as well as on 
water, so that it may be possible to study many substances soluble in 
water by this method. 
