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

Abstract

Hydrophobic deep eutectic solvents (DES) have recently been used as green alternatives to conventional solvents in several applications. In addition to their tunable melting temperature, the viscosity of DES can be optimized by selecting the constituents and molar ratio. This study examined the viscosity of 14 eutectic systems formed by natural substances over a wide range of temperatures and compositions. The eutectic systems in this study were classified as ideal or non-ideal based on their solid–liquid equilibria (SLE) data found in the literature. The eutectic systems containing constituents with cyclohexyl rings were considerably more viscous than those containing linear or phenyl constituents. Moreover, the viscosity of non-ideal eutectic systems was higher than that of ideal eutectic systems because of the strong intermolecular interactions in the liquid solution. At temperatures considerably lower than the melting temperature of the pure constituents, non-ideal and ideal eutectic systems with cyclohexyl constituents exhibited considerably high viscosity, justifying the kinetic limitations in crystallization observed in these systems. Overall, understanding the correlation between the molecular structure of constituents, SLE, and the viscosity of the eutectic systems will help in designing new, low-viscosity DES.

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

“…It is worth mentioning that a glass transition at 217 K was observed for samples with x thymol = 0.75 and 0.70 prepared by slow crystallization (the DSC curves are shown in Figure S2 in the Supporting Information). This can be attributed to incomplete crystallization in the samples with a lower thymol content prepared by the slow crystallization method due to their high viscosity …”

“…In the following, the second part of the phase diagram in the l -menthol-rich region, i.e., x thymol ≤ 0.67, is discussed. As already mentioned, it was impossible to crystallize the l -menthol-rich samples by slow crystallization due to the high viscosity of the mixture close to its melting temperature . Therefore, only the second preparation method, i.e., rapid crystallization, was considered.…”

“…The l -menthol/thymol eutectic system has attracted much attention in the literature because of its very low eutectic temperature, low viscosity at room temperature, and poor miscibility with water. − The SLE data for this system are available in the literature. , In a previous work, it was shown that the peaks observed in the DSC curves of the l -menthol/thymol eutectic system, which were interpreted as eutectic peaks, were broad, indicating a possibility of several thermal events. The present study investigates the SLE of the l -menthol/thymol eutectic system in detail to comprehend these thermal events and characterize the formed solid phases.…”

Carbon Dioxide Solubility in Nonionic Deep Eutectic Solvents Containing Phenolic Alcohols

“…Thus, it is desirable to have L-menthol as a DES constituent. First, the activity coefficients of CO 2 at infinite dilution were calculated using COSMO-RS in pure constituents at 293.15 K because the physicochemical properties of pure constituents influence the physicochemical properties of DES ( Alhadid et al, 2021b ), and the results are shown in Figure 1 . As COSMO-RS is based on quantum mechanical calculations, the calculated activity coefficients implicitly include the atomistic rationalization and quantify the intermolecular interactions between CO 2 and the constituents in the liquid phase.…”

“…Recently, hydrophobic DES based on natural and inexpensive nonionic constituents has attracted much attention ( van Osch et al, 2019 ). Hydrophobic DES containing L-menthol possess outstanding properties, such as low viscosity and eutectic temperatures, especially when L-menthol is mixed with phenolic alcohols, such as thymol or carvacrol ( Alhadid et al, 2020a ; Alhadid et al, 2020b ; Alhadid et al, 2021a ; Alhadid et al, 2021b ). Phenolic IL are good solvents for CO 2 capture applications ( Vafaeezadeh et al, 2015 ).…”