PHYSICAL PROPERTIES OF FATTY ACIDS 99 



kno^\Ti about the nature of this polymorphic form other than tliat the 

 length of the long axis is the shortest in any of the four modifications of the 

 fatty acid crystals. 



The long-spacing values for the fatty acids are summarized in Table 32. 

 AVlien the spacings of the B (j3) or C (a) types are plotted against the 

 length of the chain, a straight-line curve is obtained for the even- and the 

 odd-chain acids (Fig. 22). 



c. Polymorphism in Unsaturated Fatty Acids. Although the evidence 

 for polymorphism has not been demonstrated for many of the poly- 

 ethenoid acids which are liquids under most conditions, evidence that 

 oleic acid can exist in two forms was advanced over 30 years ago by Kirsch- 

 ner.338 This proof was largely based upon the fact that tAvo distinct melt- 

 ing points exist for oleic acid in different preparations, one at about 13 °C. 

 and the second at about 16 °C. Although Smith^^^ and Stewart and 

 Wheeler^^" were able to confirm Kirschner's results unequivocally, it has 

 been demonstrated only recently, by Lutton,^^^ that the polymorphic 

 forms have a different crystalline structure as shown by x-ray diffraction 

 patterns. The lower melting sample (13.3°C.) had a main short spacing 

 at 4.19 A. and its long spacing at 40.5 A., which indicated similarities to the 

 B and C forms of stearic acid. On the other hand, the higher melting 

 polymorph (16.2°C.) had main short spacings at 4.65 A. and 3.67 A. and a 

 long spacing at 84.4 A. (or possibly 42.2 A.). Lutton^" suggests that it is 

 probably unique in crystal structure among the long-chain monocarboxylic 

 acids. 



(9) Physical Properties of Fatty Acids in Liquid State 



A number of the physical properties of fatty acids can be determined only 

 in the liquid state. These properties are not only of theoretical impor- 

 tance but they are also of considerable practical value. They include 

 density, molar volume, refractive index, surface tension, viscosity, boiling 

 point, specific heat, specific conductance, and dielectric constant. These 

 can be only briefly considered here, but are more extensively discussed in 

 the monographs of Markley* and Ralston.^ 



a. Density. The density of the fatty acids shows the greatest regular- 

 ity when determined on the compounds in the liquid state. Density is 

 ordinarily considered as the weight in grams per cubic centimeter and is 

 frequently called the absolute density. This value varies only slightly 

 from that of the specific gravity, which is the ratio of the weight of the 

 substance compared with the weight of an equal volume of water at 4°C. 



338 A. ICirschner, Z. physik. Chem., 79, 759-761 (1912). 



339 J. C. Smith, /. Chem. Soc, 1939, 974-980. 



3W H. W. Stewart and D. H. Wheeler, Oil & Soap, 18, 69-71 (1941). 

 »" E. S. Lutton, Oil & Soap, 23, 265-266 (1946). 



