90 



II. 



CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



diffraction of single stearic acid crystals, it was found that a unit cell of 

 stearic acid is an elongated monoclinic prism which contains four mole- 

 cules. Three characteristic spacings were found in the x-ray diffraction 

 patterns which Miiller and co-workers,'^*'^^^ in earlier work, had designated 

 the di axis (long spacing or c axis), ^2, and ds axes (b and a, respectively). 

 The axial dimensions of such an elementary parallelopiped are 5.546 A. 

 (a axis), 7.381 A. (6 axis), and 48.84 A. (c axis). The 6 axis is perpendicu- 

 lar to the ac plane, but the c axis is inclined to produce an angle (jS) of 

 63°38' between the a and c axes.* These relationships are pictured in the 

 diagrammatic representation of a stearic acid unit crystal given in Figure 

 18. 



Fig. 18. A diagrammatic representation of a unit cell 

 of a stearic acid crystal.'' 



The fatty acids all form monoclinic prisms and, in all cases, there are 

 four molecules in a unit. In the case of hydrocarbons, although the unit 

 cell contains four molecules, the crystal is orthorhombic. In the dicar- 

 boxylic acids having an even number of carbons, only two molecules exist 

 in each unit, although when they are composed of an odd number of car- 

 bons the unit cell contains four molecules. 



Numerous variations exist in the measurements of the different axes in 

 the various fatty acid crystals. The c axis is directly proportional to the 

 length of the carbon chain. Variations also occur in the a and b axes, al- 



