Thermal Instability of Choline Chloride-Based Deep Eutectic Solvents and Its Influence on Their Toxicity─Important Limitations of DESs as Sustainable Materials

Abstract

Deep eutectic solvents (DESs) have become a hot topic in many branches of science due to their remarkable properties. They have been studied in a wide variety of applications. In particular, choline chloride (ChCl)-based DESs are one of the most commonly used representatives of these fluids. Nevertheless, in order to apply DESs in some fields, it is essential to guarantee their stability, reusability, and biocompatibility. In this context, the long-term stability of three ChCl-based DESs formed using glucose, malonic acid, and urea as hydrogen bond donors was investigated. Furthermore, the possible formation of toxic byproducts during long-term heating was evaluated for the first time, and toxicological studies using three bacterial strains (Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus) were performed. ChCl:urea DES revealed a high long-term thermal stability and was also found to be less toxic to the bacteria and thus can be considered as green solvent. ChCl:glucose DES started to decompose as a result of possible caramelization at 100 °C, and decomposition was further promoted at more elevated temperatures. Degradation of this DES did not affect greatly the toxicity toward bacteria, and low antibacterial properties were observed. The applicability of ChCl:malonic DES is not recommended as this DES was shown to be thermally unstable due to esterification and decomposition of malonic acid into acetic acid and carbon dioxide. Moreover, high toxicity of this DES in comparison to other DESs assayed in this study was reported.

Biodegradable Methane Sulfonic Acid-Based Nonaqueous Dissolution, Estimation, and Recovery: Toward Development of a Simplified Scheme for Plutonium-Bearing Fuel Matrices

“…In the quest to develop sustainable alternative to hydrometallurgical routes for actinide dissolution, RTIL, DES, and other green solvent-based routes are being explored fervently. ,− Nevertheless, the extent of “greenness” of various RTILs remains an unanswered question. Also, low thermal stability and high viscosity of DESs pose a challenge for routine operations. MSA has been proven to be a highly efficient dissolving media even for industrial operations owing to the high metal salt solubility and stabilization offered to metal ions against aerial oxidation .…”

“…18,24−28 Nevertheless, the extent of "greenness" of various RTILs remains an unanswered question. Also, low thermal stability 29 and high viscosity of DESs pose a challenge for routine operations. MSA has been proven to be a highly efficient dissolving media even for industrial operations owing to the high metal salt solubility and stabilization offered to metal ions against aerial oxidation.…”

Enhanced Dissolution of Metal Oxides in Hydroxylated Solvents – Towards Application in Lithium‐Ion Battery Leaching

“…Additionally, by‐products of ChCl degradation such as trimethylamine act as a basic pH buffer and could complicate metal recovery and DES reuse through the formation of metal‐amine complexes [10c,14] . These stability issues are further compounded by the partial esterification of ChCl under common leaching temperatures, [15] making the recycling of the DES phase over multiple cycles a non‐trivial problem as well as affecting its toxicity [15b] . In fact, it was recently suggested that the ChCl : EG DES in the absence of an additional reducing agent will be less efficient for LIB leaching as metal oxide dissolution primarily occurs through the degradation of the DES components at high temperatures [14,16] .…”

Elucidating the Structure, Dynamics, and Interaction of a Choline Chloride and Citric Acid Based Eutectic System by Spectroscopic and Molecular Modeling Investigations

“…Various classes of DESs have been successfully synthesized thus far. ,, Choline chloride (ChCl)-based DESs, conversely, have demonstrated to be particularly effective because of their low cost, biodegradability, and minimal toxicity. , Electrochemistry, biochemistry, materials science, and petrochemical engineering are just a few of the many fields that have found widespread use for these DESs as solvents or reaction media. − Although there has been substantial progress in the study of DESs, the combined experimental and theoretical investigations carried out to obtain the structure and interaction of DESs are limited. These features are widely recognized as being crucial to future DES research and application .…”