Cocrystal Formation in Choline Chloride Deep Eutectic Solvents

Abstract

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.

Assessment of COSMO-SAC Predictions for Solid–Liquid Equilibrium in Binary Eutectic Systems

“…Thus, the melting properties of thermally unstable substances were determined in the literature indirectly from experimental SLE data and assuming the ideal solution model (γ i = 1). However, as reported in the literature, ,,,− in salt-based ESs, a negative deviation from ideality is usually expected. Moreover, the melting properties of pure components and their activity coefficients in the liquid phase are interrelated .…”

Cocrystal Formation in l-Menthol/Phenol Eutectic System: Experimental Study and Thermodynamic Modeling

“…The experimental liquidus data and the information about the formed cocrystals acquired by DSC and XRD analyses were used to model the phase diagram of the l -menthol/phenol system. The correlative thermodynamic model NRTL was shown to be successful in describing the SLE in eutectic mixtures with cocrystal formation. , In this work, the predictive thermodynamic model COSMO-RS performance was evaluated in describing cocrystal formation in eutectic mixtures.…”

“…So far, the SLE data available for nonionic DESs show that they follow ideal solution behavior, − with an exception for DESs formed by mixing cyclohexyl and phenolic alcohols, such as l -menthol/thymol and l -menthol/carvacrol. , Due to substantial negative deviation from ideality, DESs formed by mixing cyclohexyl and phenolic alcohols are suspected of glass formation . Moreover, some phenolic alcohols are known coformers that have been shown to form cocrystals with several substances. ,− Glass and cocrystal formations complicate measuring the SLE phase diagram of eutectic mixtures containing cyclohexyl and phenolic alcohols, which limited the number of SLE studies available in the literature for this distinctive type of eutectic mixtures. ,, …”

“…Correlative models, such as regular solution theory or the nonrandom two-liquid (NRTL) model, have been successfully used in the literature to model SLE in nonideal eutectic systems. − However, predictive models are preferable to prevent the need for experimental data to model SLE. The conductor-like screening model for realistic solvation (COSMO-RS) was proposed to calculate the activity coefficients and thus predict the eutectic point and the phase diagram of ionic and nonionic DESs. ,, SLE calculations using COSMO-RS have been exclusively performed assuming that the constituents crystallize in pure form, i.e., the simple eutectic behavior. ,, However, cocrystal formation has been identified in some DESs. , Cocrystal, hydrate, and solvate formations in binary eutectic mixtures have been successfully modeled using the perturbed-chain statistical associating fluid theory (PC-SAFT). − To the best of our knowledge, no study has evaluated the performance of COSMO-RS in predicting the phase diagram of binary eutectic systems with cocrystal formation, as COSMO-RS has been only used in several studies to screen and select suitable coformers apriori. − Loschen and Klamt used COSMO-RS to predict the SLE phase diagram of several ternary systems containing an active ingredient, coformer, and solvent. However, due to the strong association in the liquid phase, an additional binary interaction parameter was introduced and fitted to experimental data, which withdrew COSMO-RS predictive ability.…”

Deep Eutectic Solvents or Eutectic Mixtures? Characterization of Tetrabutylammonium Bromide and Nonanoic Acid Mixtures

“…In this approach, ions are treated at the quantum chemical level separately, which allows studying the contribution of both the cation and anion to predict the interaction in the DES mixture. The plot of the phase diagram was prepared based on eq , in which the change in calorific capacity (Δ C p ) was neglected due to the fact that this parameter is not readily available in the literature and that it is difficult to measure. ,− where x i is the molar ratio of component i , γ i is the activity coefficient of component i , Δ H fus, i is the enthalpy of fusion of component i [J·mol –1 ], Δ H tr, i is the solid–solid transition enthalpy of component i [J·mol –1 ], R is the universal gas constant [8.314 J·mol –1 ·K –1 ], T is the temperature of the system [K], T fus, i is the melting point temperature of component i [K], and T tr, i is the solid–solid transition temperature of component i [K].…”