The Binary System Tetradecanedioic Acid–Hexadecanedioic Acid: Polymorphism of the Components and Experimental Phase Diagram

Abstract

Complementary techniques had to be applied to investigate the binary system tetradecanedioic acid (C14H26O4)–hexadecanedioic acid (C16H30O4), because all the forms observed have the same space group (P21/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 C14H26O4, and by a eutectic invariant for compositions rich in C16H30O4. At higher temperature, there appears a second peritectic invariant for compositions rich in C14H26O4, together with a metatectic invariant for compositions rich in C16H30O4. 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.