Enhancing insights into the phenomena of deep eutectic solvents

Environmental concerns have spurred a quest for more sustainable and safer solvents, aiming to replace aggressive and harmful chemical products in industrial processes. In response to this need, deep eutectic solvents (DES) have emerged as a progressive evolution from ionic liquids. These innovative solvents result from the synergistic combination of two or more chemical compounds, exhibiting a significant reduction in melting point when blended in specific molar fractions, ultimately achieving a liquid state at room temperature. In recent years, a natural variant known as Natural Deep Eutectic Solvents (NADES) has gained prominence. This environmentally friendly alternative is derived by skilfully combining compounds such as sugars, amino acids, or organic acids, presenting a promising avenue for sustainable and eco-friendly chemical processes. These "green" solvents go beyond applications in chemical or materials engineering, finding application in diverse fields such as biocatalysis, extraction processes, and carbon dioxide capture, among others. Despite their numerous advantages, including low cost, ease of preparation, tuneable properties, and biorenewability, the full potential of DES remains elusive due to insufficient understanding, hindering their seamless integration into industrial applications. While previous reviews have predominantly focused on defining and showcasing the applications of DES, they often overlook the crucial aspect of physicochemical characterization. Similar to other solvent classes, the physicochemical properties of DES such as polarity, viscosity, density, and conductivity play a pivotal role in determining their applicability. Recognizing this gap, the primary objective of this review is to provide a practical guide encompassing the preparation, characterization, and application of DES, thereby facilitating a comprehensive understanding of these solvents for both researchers and practitioners alike. Moreover, the manuscript will delve into the diverse types of DES, exploring their unique physicochemical properties and potential modifications tailored for various applications across different fields.