Decanedioic acid (C10H18O4)/dodecanedioic acid (C12H22O4) system:: Polymorphism of the components and experimental phase diagram

The experimental temperature/composition phase diagram of the binary system decanectioic acid (C10H18O4)/dodecanedioic acid (C12H22O4) was established by combining X-ray powder diffraction (XRD), differential-scanning calorimetry (DSC), infrared spectroscopy (IR), scanning electron microscopy (SEM), and thermo-optical microscopy (TOM). Both compounds crystallize in the same ordered form, C (P2(1)/c), which is the phase that melts in both cases. The C form melts in C12H22O4 earlier than in C10H18O4, in contrast to other unbranched-chain compounds (alkanes, alkanols, and alkanoic acids) in which the melting temperatures increase as the C-atom number rises. Contrary to what might be expected, total solid-state miscibility is not observed. The C10H18O4/C12H22O4 binary system shows a complex phase diagram. At tow temperatures, a new monoclinic form, C-i (P2(1)/c), stabilizes as a result of the disorder of composition in the mixed samples; two [C + C-i] domains appear. Upon heating, four solid-solid and seven solid-liquid domains appear related by eutectic and peritectic invariants. All the crystallographic forms observed are isostructural.