Note What the investigators should do first is to determine by DSC the
needed parameters and check by x-ray diffraction which of the polymorphs
is involved.15.4.3 Compound Crystals
As mentioned, triglycerides that are closely similar in molecular structure
may form compound (mixed) crystals, i.e., more than one type of molecule
occurs in the same crystal lattice.
Binary Mixtures. Compound crystals have been best studied for
binary mixtures in the most stable polymorph, generallyb, occasionallyb^0.
Examples ofphase diagramsare given in Figure 15.23. The most common
situation is aeutecticmixture, where no compound crystals are formed, as
depicted in (a). The situation is the same as was shown in and discussed in
relation to Figure 15.14. If the two kinds of molecules are very similar, as in
Figure 15.23b, asolid solutioncan be formed, i.e., compound crystals in a
range of compositional ratios, which range may be from 0 to 1. This
situation will be further described below. A third situation is illustrated in
Figure 15.23c. Here the two components behave as a eutectic mixture,
except for molar ratios very close to unity, where compound crystals in the
b-form can form. For completeness, in cases like (a) and (c), compound
FIGURE15.23 Approximate examples of phase diagrams of binary mixtures of
triglycerides, indicated in each frame. The vertical axes give temperature ( 8 C), the
horizontal axes the mole fraction of the second triglyceride indicated. L is liquid,
crystals are indicated byborb^0 , and subscript C signifies compound crystals.