Complementary techniques had to be applied to investigate the binary system tetradecanedioic acid (C
14H
26O
4)–hexadecanedioic acid (C
16H
30O
4), because all the forms observed have the same space group (
P2
1/
c; Z = 2). We studied the polymorphism of the two single compounds and of their mixtures by X‐ray powder diffraction, differential‐scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo‐optical microscopy (TOM). The two diacids were found to be isopolymorphic. At low temperature, they crystallize in the same ordered
C‐form, and, on heating, adopt the ordered
Ch‐form, 1° below their melting point. In contrast to similar compounds (unbranched alkanes, alkanols, and fatty acids), the solid–solid and solid–liquid phase‐transition temperatures decrease with increasing chain length. At low temperature, a new monoclinic form,
Ci, appears as a result of the disorder of composition in the mixed samples. There are two [
C +
Ci]‐type solid–solid domains. On heating, the solid domains are related to solid–liquid domains by a peritectic invariant for compositions rich in C
14H
26O
4, and by a eutectic invariant for compositions rich in C
16H
30O
4. At higher temperature, there appears a second peritectic invariant for compositions rich in C
14H
26O
4, together with a metatectic invariant for compositions rich in C
16H
30O
4. All the solid forms observed in this binary system are isostructural. Nevertheless, the equilibrium between them is complex near the melting point, and their miscibility in the solid state is reduced.
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