94 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



jacent carbon moleculeg in the individual chain. The effect of chain length 

 on the diffraction patterns of the /^-aliphatic acids is illustrated in the photo- 

 graphs in Figure 20. Figure 21 illustrates the variations in the pattern of a 

 C22 acid caused by con\^ersion to the methyl or ethyl ester or by a reduction 

 to the corresponding alcohol. 



(8) Polymorphism^ 



a. Introduction. Fatty acids crystallize in more than one solid or poly- 

 morphic form. Such polymorphic forms are solid phases which have the 

 same chemical composition but differ from each other in crystalline struc- 

 ture, free energy, melting point, as well as in other physical and chemical 

 properties. However, upon fusion or evaporation, they yield identical 

 liquid or gaseous phases. This property differentiates the polymorphic 

 forms from isomeric forms, which do not lose their identity on melting or on 

 vaporization. 



There are two types of polymorphism. These are known as enantiotrop- 

 ism and monotropism. In the case of enantiotropic polymorphs, the two 

 forms have different vapor pressure curves which approach each other with 

 a change in temperature; the vayor pressure curves intersect at the so- 

 called transition point. At this temperature, both solid phases exist in 

 equilibrium as stable compounds. As heat is added to the system, a trans- 

 formation may take place to the form stable at the higher temperature, 

 without increase in temperature (isothermal transition). When heat is re- 

 moved, the reverse change may obtain at a constant temperature, namely, 

 the form stable at the lower temperature will result. This form of poly- 

 morphism is completely reversible without changing the solid state of the 

 compound. 



In the case of monotropism, the vapor pressures of the two components 

 do not meet at any temperature, and there is consequently no transition 

 point. If both polymorphs are present, the one with the higher vapor 

 pressure is always the metastable form, while the one possessing the lower 

 vapor pressure is the stable modification. On distillation or by some simi- 

 lar process, the metastable type can be changed to the stable type. No 

 reversibility exists between such polymorphic forms in the solid state. 



Polymorphism occurs in the case of practically all long-chain aliphatic 

 compounds, including not only the saturated fatty acids but also the un- 

 saturated acids, their esters, triglycerides, alcohols, and hydrocarbons. 

 The phenomenon is of very great importance in the fat industry, since the 

 consistency of samples of lard, butter, and hydrogenated fats, including 

 oleomargarines, is dependent upon the crystalline form. The fatty acids 

 exhibit only monotropism (irreversible polymorphism), although both 



