74 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



further concentration. The compressing force increased hnearly until the 

 film collapsed. Since the area occupied by each fatty acid molecule/ ir- 

 respective of chain length, is 21 X 10 ~^^ sq. cm., it is apparent that they 

 are in all cases arranged perpendicularly to the surface. The limiting 

 area is that occupied by a CH2 group, which obviously is the diameter of the 

 short axis. The thickness of any such monomolecular film will vary with 

 the different fatty acids according to the length of the fatty acid chain. 

 Since the surface tension of water is not decreased until the whole surface is 

 covered with a monomolecular film, the area per molecule can be cal- 

 culated by determining the number of molecules w hich must be added to a 

 given area before an abmpt change in surface tension can take place. The 

 force-area curve of saturated acids is given in Figure 10^^^ on page 73. 



The fact that the carboxyl groups are projected into the water would 

 seem also to be indicated by the effect of an acid solution on the com- 

 pressibility of the film. The force-area curves obtained with fatty acid 

 films in contact with a solution of hydrochloric acid as dilute as 0.01 N 

 are entirely different from those observed when pure water is employed. 

 It is not possible to compress such films to an area of 21 X 10 ~^^ sq. cm. 

 per molecule before resistance obtains, but only to an area of 25 X 10 ~^^, 

 in the presence of hydrochloric acid. This alteration in compressibility is 

 evident in Figure 1 1 as compared with Figure 10. 



The explanation for the increase in the area (from 21 X 10~^^ sq. cm. to 

 25 X 10~^^ sq. cm.) occupied per molecule of fatty acid when dilute acid is 

 used in place of distilled water is the fact that the attraction for the car- 

 boxyl group is decreased at the lower pH, and the fatty acid is forced to 

 lie flat on the surface rather than being projected into the aqueous medium. 

 The larger area occupied by each molecule of fatty acid in contact with a 

 dilute solution of acid is therefore controlled by the diameter of the head of 

 the molecule, i.e., the carboxyl group, rather than by the hydrocarbon 

 chain, in which case the smaller limiting diameter is that of the methylene 

 group. 



Temperature, also, has been shown by Adam^^-""^ to play an important 

 lole in relation to film compressibility. When a constant pressure of 1.4 

 dynes was applied to such an oil film in contact with 0.01 N HCl solution, 

 and the temperature was gradually raised from 0°C., no change in area oc- 

 cupied occurred until a temperature of 28°C. was reached. Between this 

 temperature and 35 °C., a marked increase in area was noted to about 

 42 X 10"'* sq. cm. As the temperature was further increased, a slow 

 progressive expansion in area occurred. This behavior is explained by 

 the fact that, at the elevated temperature, the thermal agitation of the 

 film becomes so violent that the lateral attraction of the molecules is over- 

 come. Such films are known as expanded films, in contradistinction to the 



