Design of Deep Eutectic Systems: A Simple Approach for Preselecting Eutectic Mixture Constituents

Abstract

Eutectic systems offer a wide range of new (green) designer solvents for diverse applications. However, due to the large pool of possible compounds, selecting compounds that form eutectic systems is not straightforward. In this study, a simple approach for preselecting possible candidates from a pool of substances sharing the same chemical functionality was presented. First, the melting entropy of single compounds was correlated with their molecular structure to calculate their melting enthalpy. Subsequently, the eutectic temperature of the screened binary systems was qualitatively predicted, and the systems were ordered according to the depth of the eutectic temperature. The approach was demonstrated for six hydrophobic eutectic systems composed of L-menthol and monocarboxylic acids with linear and cyclic structures. It was found that the melting entropy of compounds sharing the same functionality could be well correlated with their molecular structures. As a result, when the two acids had a similar melting temperature, the melting enthalpy of a rigid acid was found to be lower than that of a flexible acid. It was demonstrated that compounds with more rigid molecular structures could form deeper eutectics. The proposed approach could decrease the experimental efforts required to design deep eutectic solvents, particularly when the melting enthalpy of pure components is not available.

Recent advances in green solvents for lignocellulosic biomass pretreatment: Potential of choline chloride (ChCl) based solvents

“…The potential constituents of ILs are limited to bulky or asymmetric ions, by which the fixed molar ratio of the ions in ILs is defined by the electroneutrality of the solution (Mainberger et al, 2017). In contrast, the complexity in designing DESs/ LTTMs is ascribed to the wide array of possibilities in formulating the molar ratio of the components in DESs/LTTMs that could be of any value (Alhadid et al, 2020). The time and cost for the synthesis and testing of such an impossibly large number of DESs/LTTMs poses a scientific challenge, given that the area of research is relatively new and lacks a comprehensive framework for the optimal design of DESs/LTTMs.…”

“…Most of the published works to date adopts the trial and error methodology in designing the DESs/LTTMs, where the preselected components are mixed at several molar ratios, typically 1:1 or 1:2, and mixtures that remain liquid at room temperature are documented for further studies (Alhadid et al, 2020). However, making selections based on the trial-and-error method can be very limiting and have a high chance to overlook good candidates (Rodriguez-Donis et al, 2018).…”

General Design Methodology for Organic Eutectic Electrolytes toward High‐Energy‐Density Redox Flow Batteries

“…[ 15b ] For instance, the interaction between urea and choline chloride (hydrogen bonding) in the choline chloride–urea eutectic system was found to change the density distribution of cations and anions around the given cations significantly, disrupting the long‐range ordered structure of choline chloride (increases in entropy). [ 15b,16 ] The enthalpy of mixing would be exothermic when the newly generated intermolecular interactions between different components are stronger than the original types and therefore the OEE formation would be a spontaneous process. On the other hand, even the new‐generated intermolecular interactions are not strong enough, OEE would be still thermodynamically favorable if the contribution of entropy of mixing is much higher than enthalpy of mixing.…”

“…[ 15 ] In thermodynamics the entropy of mixing is always positive due to the increase in the freedom of molecules. [ 15b ] For instance, the interaction between urea and choline chloride (hydrogen bonding) in the choline chloride–urea eutectic system was found to change the density distribution of cations and anions around the given cations significantly, disrupting the long‐range ordered structure of choline chloride (increases in entropy). [ 15b,16 ] The enthalpy of mixing would be exothermic when the newly generated intermolecular interactions between different components are stronger than the original types and therefore the OEE formation would be a spontaneous process.…”

“…[8c] The entropic term is the change in the disorder and randomness of the system and depends on the molecular symmetry and conformational degrees of freedom of molecules. [15] In thermodynamics the entropy of mixing is always positive due to the increase in the freedom of molecules. [15b] For instance, the interaction between urea and choline chloride (hydrogen bonding) in the choline chloride-urea eutectic system was found to change the density distribution of cations and anions around By virtue of strong molecular interactions, eutectic electrolytes provide highly concentrated redox-active materials without other auxiliary solvents, hence achieving high volumetric capacities and energy density for redox flow batteries (RFBs).…”