Thermal analysis of some diclofenac salts with alkyl and alkylhydroxy amines

A range of diclofenac salts was prepared with a variety of alkyl, hydroxyalkyl and alkyl hydroxyalkyl linear amines, and characterized by thermal analysis (DSC, TGA, and HSM). Another seven similar salts, previously described, were also prepared for a more careful analysis and comparison. The aim of this paper is a deeper knowledge of this class of compounds, previously poorly examined and that have, on the contrary, proved to offer complex situations in the solid state, as resulted by thermal analysis. The whole class of these salts presents a variety of behaviours, ranging from the formation of hydrates to polymorphs and hydrate polymorphs in the solid state. The salts with diethanolamine, triethanolamine, N-ethyl monoethanolamine and TRIS bases crystallize anhydrous. All the salts demonstrated thermal instability at temperature above the melting point, showing a dramatic loss of weight. In each case the TGA profile indicates that it corresponds to the base content inside the salt: this event is associated with a broad endotherm in the DSC thermogram that follows or overlaps with that of the melting endotherm. Prolonged heating in the oven of salts with very volatile amines causes decomposition, even at low temperatures, leaving the starting acidic diclofenac as residue. Different phases, in the case of polymorphs, were revealed, registering melting/re-crystallization by hot stage microscopy, as in the case of the salts with triethylamine and N-methyl monoethanolamine; while in the case of the salt with monoethanolamine evidence was obtained by means of DSC. The relationship between the structure of the starting bases and solid-state nature of these diclofenac salts studied is briefly discussed.