Solid-liquid phase diagrams of binary mixtures Acetylsalicylic acid(1) + E(2) (E = salicylic acid, polyethylene glycol 4000, D-mannitol)

This study reports the investigation of three binary mixtures represented by acetylsalicylic acid (ASA) with its most important degradation product, salicylic acid (SA), and two of the most commonly used excipients (polyethylene glycol 4000 (PEG-4000) and D-mannitol (MA)). The liquidus and solidus equilibrium temperatures determined by DSC for pure components and solid binary mixtures at a fixed composition (mass fraction of ASA, w) were used to construct the corresponding solid-liquid phase diagrams. On the basis of the DSC results, the binary mixtures ASA/SA and ASA/PEG exhibit eutectic behavior (T(eu) = 155.0 +/- 0.5 degrees C, w(eu) = 0.55 +/- 0.02 and T(eu) = 53.3 +/- 0.5 degrees C, w(eu) = 0.327 +/- 0.011), respectively), while the binary mixture ASA/MA revealed the presence of a monotectic with a mean melting temperature of 162.2 degrees C in the range 0.2 < w(1) < 0.8. The composition of the two eutectics formed was confirmed by the related Tamman triangles. Finally, the liquidus curves of ASA/SA and ASA/PEG mixtures were also successfully predicted providing suitable polynomial (second-order) fitting equations.