Physicochemical Characterization and Simulation of the Solid–Liquid Equilibrium Phase Diagram of Terpene-Based Eutectic Solvent Systems

Abstract

The characterization of terpene-based eutectic solvent systems is performed to describe their solid–liquid phase transitions. Physical properties are measured experimentally and compared to computed correlations for deep eutectic solvents (DES) and the percentage relative error er for the density, surface tension, and refractive index is obtained. The thermodynamic parameters, including the degradation, glass transition and crystallization temperatures, are measured using DSC and TGA. Based on these data, the solid–liquid equilibrium phase diagrams are calculated for the ideal case and predictions are made using the semi-predictive UNIFAC and the predictive COSMO RS models, the latter with two different parametrization levels. For each system, the ideal, experimental, and predicted eutectic points are obtained. The deviation from ideality is observed experimentally and using the thermodynamic models for Thymol:Borneol and Thymol:Camphor. In contrast, a negative deviation is observed only experimentally for Menthol:Borneol and Menthol:Camphor. Moreover, the chemical interactions are analyzed using FTIR and 1H-NMR to study the intermolecular hydrogen bonding in the systems.

Experimental Investigation and Modeling of Cocrystal Formation in L-Menthol/Thymol Eutectic System

“…The mixture melting temperature can be measured by visual methods or by differential scanning calorimetry (DSC). The main advantage of DSC is the possibility to identify different phase transitions, such as the solid–solid transition and the glass transition, which is not possible with visual methods. − To generate the phase diagram over the full composition range, the obtained SLE data are modeled using the equation where x i L and γ i L are the mole fraction and activity coefficient of component i in the liquid solution, respectively, Δ h m ,i and T m ,i are the melting enthalpy and melting temperature of pure component i , respectively, R is the universal gas constant; T is the liquidus temperature, and Δ c p ,i is the difference between the liquid and solid state heat capacities of pure component i at constant pressure. Several approaches can be found in the literature considering the treatment of the heat capacity term .…”

Lactic Acid-Based Natural Deep Eutectic Solvents to Extract Bioactives from Marine By-Products

“…For Lac:Fr, a chemical shift of the –OH protons occurred from 6.11 to 5.91 ppm when increasing the temperature, as shown in the circled peak of Figure 3 a. Hence, as the temperature increases, the H-bonded protons undergo an upfield shift [ 44 , 45 , 46 , 47 ]. A similar behavior was observed for Lac:Ur, in which a broad peak was observed at 5.38 ppm and it shifted to a lower wavelength while becoming less broad with the temperature increase until it reached 5.15 ppm at 50 °C, as shown in the circled peak of Figure 3 b.…”

“…The viscosities of these lactic acid-based NaDES were shown to be comparable to those of choline chloride:urea (1:2) system, which were shown to be 748.09 and 119.81 mPa/s at 25 and 50 °C, respectively [ 49 ]. In contrast, the lactic acid-based NaDES were shown to be much more viscous than terpene-based NaDES, as the values were 19.23 and 45.63 mPa/s for menthol:camphor (3:2) and thymol:borneol (7:3) at 25 °C, respectively [ 47 ]. Hence, this proved that this property highly differs in the chemical composition and structure in the systems.…”

Influence of the Molecular Structure of Constituents and Liquid Phase Non-Ideality on the Viscosity of Deep Eutectic Solvents

“…The high viscosity and the glass formation can result from strong intermolecular interactions, the low melting temperature of the mixture, and the molecular structure of the constituents, i.e., cyclohexyl ring. The latter explains why aqueous sugar solutions, i.e., sugars containing saturated rings [ 35 , 36 , 37 ], and borneol and camphor based eutectic systems [ 38 ] are glass-forming mixtures.…”