Solubility Data as a Response for a Challenge for Formulation Chemists: Imidazolium-Based Ionic Liquids and Antitubercular Antibiotic Medicines

The evolution of technology is directed to reduce industrial waste from chemical processes to zero. In the past decade interest in ionic liquids (ILs) has grown, and they have been recognized as potential substitutes for volatile organic solvents used in certain chemical processes, including those in the pharmaceutical industry. Following this increasing interest and because ILs can have a great potential for the pharmaceutical industry, we tested the solubility of two antitubercular medicines, pyrazinecarboxamide and isoniazid, in imidazolium ionic liquids, namely [C4mim]­[BF4], [C4mim]­[PF6], [C4mim]­[OTf], and [C8mim]­[OTf], in the range of 279.98 to 417.05 K. Both drugs exhibited an improved solubility in [C4mim]­[BF4] than in the other studied ILs. The solutes had an enhanced solubility in more hydrophilic ILs, such as [C4mim]­[BF4] and [C4mim]­[PF6]. The solubility decreased in ILs containing the hydrophobic anion trifluoromethanesulfonate and with the increasing alkyl chain length in the cation. The solid–liquid phase equilibria of all investigated systems were described using six different correlation equations. The satisfactory results which revealed a good description with an acceptable standard deviation temperature range were collected for isoniazid or pyrazinecarboxamide in [C4mim]­[PF6], [C4mim]­[OTf] and [C8mim]­[OTf]. Comparison to solubility in NTf2 and [C10mim]­[OTf] was provided.