Co-crystals formation specified from binary phase diagram studies; their thermochemical, spectroscopic and optical properties

Abstract A thermodynamic investigation on co-crystals formed by the complexation in binary systems involving 2-amino-5-chloropyridine (ACP) as electron donor with electron acceptors; 3-nitrobenzoic acid (NBA) and 2-chloro-4-nitrobenzoic acid (CNBA) have been studied separately by establishing the phase diagram. The solvent-free synthesis method has been adopted to prepare various compositions of binary mixtures. A co-crystal is formed at 1:1 mole ratio and two eutectics are formed, one on either side of co-crystal, in each system. The thermal properties of co-crystals, eutectics and parent components have been studied by DSC method. The various thermodynamic parameters of co-crystal and eutectics are also studied based on heat of fusion values obtained from DSC. The co-crystals have been characterized by FT-IR, powder XRD and UV-vis. absorption techniques. The FT-IR study revealed the ionic interaction followed by hydrogen bonding (N-H...O) in both co-crystals. Powder XRD study ascertained the particle sizes and crystalline nature of co-crystals. Besides phase diagram, DSC and PXRD studies, the UV-vis. absorption study also has confirmed the complexation between parent molecules to form the co-crystals.