a 
f 
dimensions of a unimolecular layer. 
from the results which he published in 1917 :— 
B.—CHEMISTRY. 
Working by means of the method of Devaux, Langmuir put these 
ideas to the test of experiment, and determined the internal molecular 
6 
5 
The following is an excerpt 
Molecular Molecular | Length per 
es Cross Section V8 Length C atom 
S 
(sq. cms.) (cms.) (cms.) 
Palmitic Acid 21x10 | 46x10 | 240x107 | 1°5x107 | 
Stearic Acid . 225e10n's ALIX MOR 525°0X 1078 |" 24x 107% 
Cerotic Acid . 25x 10746 5°0x 10% 31°0X10% | 1:°2x10°% | 
| 
It is at once evident that these results agree in a wonderful manner 
both with the idea of a unimolecular layer and with that of molecular 
orientation. The molecular cross section is practically constant, as we 
should expect, since it must represent the cross section either of a 
carboxyl or CH, group. Since the molecular length is determined from 
the thickness of the layer, and is found to be five or six times the value 
of s (molecular ‘ thickness ’), we perceive here the first actual experi- 
mental proof of the theory of molecular orientation. Another fact of great 
significance emerges from these results. If we calculate the average 
distance between two adjacent carbon atoms in the three acids, we 
obtain a value of 1.4x10-*cm. Now this distance must be of the order 
of magnitude of the distance between the centres of the carbon atoms 
in the crystal structure of a diamond. This latter distance is known to 
be 1.52x10-*cm. The agreement is striking. 
These regularly oriented and unimolecular surface films on water 
have been recently investigated in a very detailed and careful manner 
by N. K. Adam, who has improved the method employed by Devaux and 
Langmuir. From a closer analysis of the relationship between the 
force of surface compression and the surface concentration (expressed 
as area occupied per molecule) he has shown that a distinction must be 
‘made between the close packing of the polar or active end groups (head 
_ groups) of the molecules and the subsequent close packing of the 
hydrocarbon chains. The following table contains a few of Adam’s 
results for the higher aliphatic acids :— 
Cross Seation (ea. cms.) sop 
£ ‘OX. 
No. of ae Length 
C atoms == (ems.) 
8 
Chain Head telat 
4 
| Myristic Acid AN, as 14 21°0 2571 Q1°1 
| Pentadecylic Acid . : 15 21°0 25°1 22°4 
| Stearic Acid . 5 3 18 21°'0 251 26°2 
Behenic Acid = e 22 21°0 25°1 31°4 
Although these results must be considered as more accurate and 
detailed than those of Langmuir, they provide an ample confirmation 
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