PHYSICAL propp:rtip:s of fatty acids 89 



pared the solid elaidinized form, elaidolinolenic acid, which melts at 29- 

 30°C. This is probably the al\-trans isomer, but there is some question as 

 to whether all the cis linkages have been changed to tJ-ans forms in the latter 

 acid, or whether the change is only a partial one. 



(7) Crystalline Structure 



Although it is generally considered that solid matter exists in onl}^ two 

 states, crystalline and amorphous, it is well known that the same molecular 

 structures are able to arrange themselves in different patterns, with the re- 

 sult that they may occur in several crystalline forms. Because of the close 

 relationship of such structures to the melting point, it is of especial impor- 

 tance to consider at some length the nature of the variations. Other physi- 

 cal properties related to crystalline form are refractive index, compressi- 

 bility, coefficient of thermal expansion, electrical conductivity, and heat 

 conductivity. While some idea of crystal structure may be obtained by 

 utilizing the laws of symmetry and by observing the external forms of nat- 

 ural crystals, such methods of approach have not proved fruitful in the 

 case of the fatty acids. 



a. X-Ray Diffraction Patterns and Crystal Structure. Although the 

 application of x-ray technic to the determination of crystalline structure 

 was first applied to such inorganic materials as calcite and sodium chloride, 

 it was soon found that it could equally well be employed with organic com- 

 pounds such as long-chain hydrocarbons, fatty acids, esters, alcohols, 

 glycerides, ketones, and many other compounds. 



Two methods are available for the application of this technic. It can be 

 emploj^ed by making use of a single crystal sufficiently large to allow the x- 

 rays to be focussed through it. This technic is exceedingly difficult and 

 cumbersome, but the maximum information can be obtained as to the 

 structure of the single crystal which is being studied. The commoner and 

 simpler procedure involves the use of an oriented layer or film of the sub- 

 stance under investigation. This is especially satisfactory for determining 

 the purity of a fatty acid, an ester, or other compound. 



^\^lile a space lattice occurs in the case of the organic as well as of the 

 inorganic crystals, the unit cells differ. In the case of the inorganic com- 

 pound, the individual points of the lattice are occupied by atoms; in the 

 fatty acids, the units represent a group of molecules which contain a repre- 

 sentation of the lattice; each of these unit cells is the smallest parallelo- 

 piped which, on repetition, eventually forms a microscopic crystal. 



Through the pioneer work of Muller,^^°~^22 ^^y^q determined the x-ray 



. 320 4 Miiller, Proc. Roy. Soc. London, AlU, 542-561 (1927). 



321 A. MiUler, Proc. Roy. Soc. London, A120, 437-459 (1928). 



322 A. Miiller, Trans. Faraday Soc, 25, 347-348 (1929). 



