15.4.2 Polymorphism
Imagine the following experiment: a fat is melted, e.g., at 40 8 C, and a little
of the molten material is sucked into a thin capillary, which is then put in ice
water. It is observed that the contents turn turbid within a minute; this is
due to crystallization of part of the fat. The capillary is now brought to a
higher temperature, e.g., 20 or 25 8 C, and it is observed that the contents
become clear (implying that the crystals have melted); but they soon become
turbid again. The crystals now formed melt at a higher temperature, e.g.,
358 C. This seems to indicate that the material shows double crystallization
and melting. It should be realized, however, that it concerns a natural fat,
containing several triglycerides, and some of these may have a low clear
point while others have a high one.
Therefore the result has to be checked by experiments on pure
triglycerides, which indeed behave in about the same manner. They may
even show three times crystallizing and melting. It has long ago been
realized that this means that a triglyceride can crystallize in threemonotropic
modifications. These are currently designated thea-,b^0 - (pronounced beta
prime) andb-form, in order of increasing melting point and stability.
Polymorphism is of great practical significance, and it has been
extensively studied. Although the old division into three (main) forms is still
useful, it has been shown that the crystallization behaviour is more complex.
Crystal structure. All substances with long paraffinic chains (e.g.,
fatty acids, alcohols, and mono, di, and triglycerides) show about the same
geometries of chain packing in crystals. These are characterized by asubcell,
i.e., a part of the unit cell containing two C-atoms of each chain in it. The
most common subcells are illustrated in Figure 15.19a. In thehexagonalone
(typical for a crystals), each chain is surrounded by six others at equal
distances. Here the chains have some freedom to move, both rotationally
and by ‘‘wiggling’’; there is thus partial disorder. Theorthorhombicsubcell
(common inb^0 crystals) shows denser and more perfect packing. Notice that
the (vertical) planes through the zigzag of the chains are perpendicular to
each other. In thetriclinicsubcell (always found inbcrystals) the packing is
densest. The zigzag planes are now in the same direction, but the direction of
the C 22 C bonds at the same horizontal level alternates. Actually, other
subcells can occasionally be observed; the total seems to be nine. The
subcells are characterized by the repeat distances of the chains, and these
can be derived from x-ray diffraction (also called x-ray scattering). It
concerns the so-called short spacings, determined by wide-angle x-ray
diffraction (WAX).