PHYSICAL PROPERTIES OF FATS AND OILS 255 



1 • Fig. 9. The double chain length structures or "tun- 



— ("t)Ti H J ing fork" type of configuration for simple triglycer- 



ides proposed b}' Malkin rt nl.^^- 



These trigh^cerides were found to crystallize in four different forms: 

 7, a, 13', and l3. The 7-polymorphs have the lowest melting points and the 

 /iJ-forms melt at the highest temperatures. The symmetrical mixed tri- 

 glycerides undergo transformation from one modification to another much 

 more readily than is the case with the simple triglycerides. Conse- 

 ciuently, data on melting points obtained by the capillary tube method may 

 be misatisfactory. Therefore, heating and cooling curves are employed 

 by preference.^*' The symmetrical mixed triglycerides exhibit spherulite 

 formation, but their appearance differs from that of the simple triglycerides 

 in showing a striking rippled effect which can be seen microscopically. 



A close relationship in x-ray diffraction pattern has been shown to exist 

 between the several members of the groups when they are arranged ac- 

 cording to differences in chain length of the component acids. The follow- 

 ing groups may be listed which are based upon the difference in length of 

 the single component acid compared with the 2-component acid (expressed 

 in number of carbons) : 1,2 less; 2, 2 more; 3, 4 less; 4, 4 more; 5, 6 less; 

 6, 6 more; 7, 8 less; 8, 8 more. 



When the chain lengths of the two acids vary by only two carbons 

 (Groups 1 and 2), the length of the unit cell is twice that of the longer fatty 

 acid molecule. The symmetrical mixed triglyceride has the tuning fork 

 arrangement of the simple triglycerides. Adjacent molecules are packed 

 side-by-side in reversed positions, which accounts for the length being twice 

 that of the longer chain acid. The stable /S-modification in Group 1 has 

 the chain tilted at an angle of 69°33', while in Group 2 the inclination 

 amounts to only 65°30'. Carbon-to-carbon distances also vary, being 

 1.18 and 1.147 A., respectively, for the two groups. The greater value for 

 the long spacings in the a- and jS'-form of Group 1 is attributed to the fact 

 that they possess vertical chains. In the case of Group 2, the a-polymorph 

 likewise possesses vertical chains, while in the /3' type they are inclined at 

 an angle of 70° 55'. All members in Group 1 except caprodilaurin have 

 been shown to solidify as the 7-form, while the mixed triglycerides in 

 Group 2 (except stearodipalmitin) crystallize in the a-modification. The 

 /3-type of polymorphs may be obtained directly by solvent crystallization. 

 The corresponding members of Groups 1 and 2, i.e., caprodilaurin and 

 laurodicaprin, possess essentially similar melting points in each of their 

 polymorphic forms. 



In the remaining six groups, in which the chain lengths of the component 

 acids differ from each other by 4, 6, or 8 carbon atoms, the triglycerides 



