PHYSICAL PROPERTIES OF FATTY ACIDS 



75 



condensed films which occur when the molecules are in close contact at low 

 temperatures. The results of Adam^^* are given in Figure 12. 



Adam and Jessop-^'' have made a study of dibasic acids having 18, 22, 

 2G, and 34 carbons and found that these form films in which only one car- 

 boxyl group is oriented into the water. Diethyl esters of dicarboxylic 

 acids having 12, 13, 18, 22, and 34 carbon atoms also were shown to produce 

 monomolecular films, as was the monoethyl ester, HOOC(CH2)i6COOC2H5. 

 All of the films were rather unstable, and readily collapsed when pressure 

 was apphed. 



The formation of such surface films is influenced also by unsaturation of 

 the fatty acid. Such acids produce films of the expanded type. When the 

 double bond is in the middle of the molecule, the film expands more readily 



30 40 



TEMPERATURE, °C. 



Fig. 12. Effect of temperature on the compressibility of a monomolecu- 

 lar film of palmitic acid on 0.01 N hydrochloric acid under a constant force 

 of 1.4 dynes per centimeter. ^^^ 



than in the case of the corresponding saturated fatty acid. When the un- 

 saturated linkage is near the carboxyl end, as for example in 2-octadecenoic 

 acid, the expansion of the film is considerably restricted as compared with 

 that observed when the double bond is near the center of the molecule. 

 Extensive studies on film formation of polyethenoid acids, stearolic acid, 

 and triricinolein'* have been made by Rideal and his co-workers^"~^^^ and 

 by Adam.^®" 



25« N. K. Adam and G. Jessop, Proc. Roy. Soc. London, A112, 362-375, 376-380 

 (1926). 



2*7 A. H. Hughes and E. K. Rideal, Proc. Roy. Soc. London, AI40, 253-269 (1933). 

 «» G. Gee and E. K. Rideal, Proc. Roy. Soc. London, A15S, 116-128, 129-141 (1935). 

 25» G. Gee, Trans. Faraday Soc, 32, 187-195 (1936). 

 260 N. K. Adam, Proc. Roy. Soc. London, AI40, 223-226 (1933). 



