PHYSICAL PROPERTIES OF FATTY ACIDS 



73 



Such oriented films have several characteristic properties. Most import- 

 ant of these is their response to compression, which in turn varies with 

 pH, temperature, and with the particular fatty acid. The extent of such 

 compression of an oxidized mineral oil can be qualitatively determined by 

 the color which it produces. As the pressure is increased, the color pro- 

 gressively changes through yellow, gold, red-purple, blue, and green, and 

 this may be repeated until the film again becomes colorless. Quantitative 

 measurements may be made by a film balance^ or by a device introduced 

 by N. K. Adam. 251 



(b) Factors Altering Monomolecular Films. The type of monomolecular 

 film was found to be similar for all fatty acids from C12 to Coe, including the 

 even- and odd-chain acids. Adam-"-"-^^ observed that when the several 

 fatty acids were spread on distilled water at a uniform temperature of 20° C, 

 all acids failed to display measurable resistance to compressibility until 



21 23 



AREA, cm.^ x 10'^^ 

 Fig. 10. Force-area curve for 

 monomolecular films of satu- 

 rated fatty acids on distilled 

 water at 20 °C. 255 



25 



50 



40 



- \30 



5:2 20 



10 



1 I ^ I I I 



19 



21 



25 



.2 . l/^-16 



27 



29 



23 

 AREA, cm." x 10 

 Fig. 11. Force-area curve for monomolecu- 

 lar films of saturated fatty acids on 0.01 N 

 hydrochloric acid solution. ^55 



the area reached a value of 21 X 10 ~'^ sq. cm. per molecule. When they 

 were compressed beyond this point, there was a sudden opposition to 



251 N. K. Adam, Proc. Roy. Soc. London, AlOl, 452-472 (1922). 



252 N. K. Adam, Proc. Roy. Soc. London, A99, 336-351 (1921). 



253 N. K. Adam, Proc. Roy. Soc. London, AlOl, 516-531 (1922). 



25* N. K. Adam, Proc. Roy. Soc. London, A103, 676-687, 687-695 (1923). 

 255 Data from H. S. Taylor, ed., A Treatise on Physical Chemistry, Vol. I, Van Nostrand, 

 New York, 1925, p. 137, Fig. 6: p. 139, Fig. 7. 



