From eutectics to ionic cocrystals: Thermodynamics of N-alkylquinolinium bromide + α,ω-diol systems

Solid–liquid equilibria of N-alkylquinolinium bromides ([Quin-Cn][Br], n = 4–12) with α,ω-alkanediols were investigated to elucidate the molecular mechanisms governing eutectic behaviour and cocrystal formation. Phase diagrams were established using differential scanning calorimetry and dynamic methods and subsequently correlated with the NRTL model to obtain excess thermodynamic functions and enthalpies of intermolecular compound formation. In parallel, a COSMO-RS-based molecular analysis was employed to derive surface descriptors and compatibility metrics from σ-profiles. The combined results demonstrate that, while strong charge-assisted hydrogen bonding dominates liquid-phase non-ideality, the decisive factor controlling solid-state organization and cocrystal formation is the geometric and amphiphilic compatibility between the alkyl chain of the quinolinium cation and the diol spacer. The study provides a concise molecular framework for rationalizing and screening eutectic mixtures and stable intermolecular compounds in ionic-liquid-based systems