Binary phase diagrams of pyridinecarboxamide isomers

Pyridinecarboxamide binary phase diagrams have been established using differential scanning calorimetry, infrared spectroscopy and X-ray powder diffraction. Simulated diagrams obtained using with the Schröder-van Laar and Prigogine–Defay equations are in very good agreement with experimental data. The binary systems nicotinamide (Form I)–picolinamide (Form II) and isonicotinamide (Form II)—picolinamide (Form I and Form II) occur as eutectic mixtures. Nicotinamide (Form I)—isonicotinamide (Forms II and Form I) solid–liquid phase diagram confirms formation of the cocrystal with molar ratio of 1:1, whose crystal structure was already solved by Báthori et al. (Cryst Growth Des 11:75–87, 12 ) with X-ray diffraction on single crystals obtained by crystallization from common solution of nicotinamide and isonicotinamide. The corresponding eutectic composition and temperatures, beyond polymorphism of isonicotinamide and picolinamide under these molecular environments are discussed.