crystals can form during crystallization in a less stable polymorph, but they
will soon transform into the most stable polymorph and then give the phase
diagrams shown.
In Various Polymorphs. In practice, the examples just discussed
are not very interesting. Compound crystals form more readily and are
much more common in theb^0 -form and tend to be abundant in thea-form,
provided that the temperature is below the respective clear points. Table
15.3 gives the melting dilatations ofaandbcrystals of SSS, and it follows
that the density of the former crystals is significantly smaller than that of the
latter; b^0 crystals will have an intermediate density. This means that a
crystals can more readily accommodate molecules of somewhat different
form than b^0 crystals, and that this is generally impossible in the b
polymorph. Most likely, the ends of the three chains of the triglyceride
molecules in a crystals are not rigid but ‘‘liquid,’’ as it were, allowing various
chain lengths in one crystal.
Multicomponent Fats. Compound crystals will be particularly
abundant in fats of a wide compositional range. The following argument
may explain this. Assume that a fat contains 10 different, but closely similar,
species, that each has about the same mole fraction (say, 0.02), enthalpy of
fusion (say, 100 kJ?mol^1 ) and melting point (say, 320 K). Putting these
data into Eq. (15.8) yields a clear point of 290 K. For each of these
components, the supersaturation lnb¼0 at that temperature, but for the
joint components in a compound crystal (mole fraction 0.2) the
supersaturation will be lnb¼ln (0.2/0.02)¼2.3, which is a considerable
value. In other words, the driving force for crystallization is far greater for
the compound. The reasoning given is not fully correct, since the melting
point and the molar heat of fusion of the compound will be somewhat lower
than presumed, but its supersaturation will nevertheless be much larger than
the average values of its components.
In principle, it is even possible that a given mixture of triglycerides is at
a given temperature supersaturated as a compound in theb^0 -form, but not in
theb-form, since that does not allow compound crystal formation. In other
words, for this triglyceride mixture, theb^0 polymorph will appear to be the
most stable form. At a much lower temperature, however, the saturation
concentration of the single components will also be surpassed, and theb-
form will then become the most stable one.
Polymorphic Transitions. In many cases, a crystals form first
upon cooling of a liquid fat, because nucleation is easiest in thea-form. In
fats of relatively homogeneous composition, thea-form is short-lived; it