Cocrystal Formation in Choline Chloride Deep Eutectic Solvents

Deep eutectic solvents (DESs) are eutectic mixtures representing a new generation of tunable solvents. The majority of the DESs studied in the literature contains choline chloride (ChCl). The knowledge of solid–liquid equilibria (SLE) in the DESs is crucial for the identification of the composition range in which the mixture is liquid below process-relevant temperatures. ChCl-based DES phase diagrams are usually assumed to be simple eutectics with no cocrystal formation. However, the simple eutectic assumption is questionable without a detailed characterization of the crystallized solid phases. This study investigates the formation of cocrystals in ChCl-based DESs. The SLE in ChCl/catechol and ChCl/hydroquinone eutectic mixtures were studied using powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The research demonstrated the formation of two cocrystals in the ChCl/catechol system and one cocrystal in the ChCl/hydroquinone system; the crystal structures were obtained using single-crystal XRD. The solid–solid transition of ChCl in the mixture was observed by DSC and variable-temperature XRD. The SLE data were correlated using the nonrandom two-liquid (NRTL) model and the melting properties of the pure components and cocrystals. Modeling the SLE data enabled one to evaluate the melting properties of ChCl estimated in the literature.