568 RICE 



ART. L 



pressure was discernible until the area per molecule was reduced 

 to 22 sq. A.* At 20.5 sq. A the pressure was very marked and 

 increased very rapidly for further decrease. It was a significant 

 fact that these figures were not altered by using different acids 

 provided the long-chain molecule contained a sufficient number 

 of groups. It was this fact which led to the introduction by 

 Langmuir of his well-known theory that such molecules are 

 oriented into vertical or nearly vertical positions in the surface, 

 suggesting that the sectional area of such a molecule is about 



o 



20 sq. A. ^ As the volume of a CH2 group is known to be about 

 29 cubic A, this gives 1.4 A as an approximate measure of the 

 distance of one carbon atom from the next in the chain, a 

 measure substantially in agreement with the results obtained 

 by X-ray analysis. This conception illuminates the whole 

 subject. At the end of the fatty acid or alcohol molecules 

 there is the group OH or COOH which is very soluble in water. 

 This group tends to get into the body of the water, and although 

 not able to drag the whole of a very long molecule in also,- it 

 succeeds in "anchoring" the molecule as it were in an almost 

 upright position. In this oriented state the molecules adhere 

 laterally, and this adhesion keeps them together as a ''coherent" 

 film showing no sign of surface pressure as soon as each mole- 

 cule has about 22 sq. A room for its cross section. Thus there 

 are "condensed" films close-packed and strongly adhering, and 

 "liquid-expanded" films in which adhesion and packing are less 

 marked. In addition Langmuir found that certain films such as 

 those of the short-chain fatty acids were quite different in 

 behavior; these appear to lie flat on the surface — the argument 

 has been given earlier in connection with statistical considera- 

 tions — and to move about independently, resembling a two- 

 dimensional gas. Such "gaseous films" appear to exert a 

 pressure, by reason of a bombardment on the barrier due to 

 thermal movement, entirely analogous to the three-dimensional 

 pressure of an ordinary gas. Just as there are no "ideal" gases 

 so there are no "ideal" gaseous films; nevertheless the laws 

 which have been discovered to hold between the surface pressure 



* 1 A (1 Angstrom unit of length) = 10 ~^ cm. 



