The complex world of eutectic solvents: Guidelines for a correct characterization and use in sample preparation

Abstract

Mixtures with low-temperature phase transitions have been known since the late 19th century, when the term “eutaxia” was first introduced. A major turning point came in 2003, when Abbott and colleagues coined the term “deep eutectic solvent” (DES) to describe mixtures whose melting points are significantly lower than those of their individual components. Since then, these systems have attracted growing interest and have widely been applied across various areas of Chemistry. However, the term DES has often been misused due to the lack of a strict and clear definition for many years. This has led to confusion among DESs (non-ideal systems), eutectic solvents (ESs, thermodynamically ideal systems), and low transition temperature mixtures (LTTMs, metastable systems). This ambiguity is especially pronounced in Analytical Chemistry, where the term DES is frequently used as a catch-all label for not-well characterized mixtures. In many studies, practical applications have been prioritized over a thorough understanding of the physicochemical nature of these solvents. As a result, essential thermodynamic data are often lacking, which hinders the rational design of new systems and the selection of optimal solvents for specific purposes, particularly in extraction processes.

This review aims to clarify the terminology and classification of ESs, DESs, and LTTMs through representative examples and practical applications. It traces the historical development of these unique solvents and highlights their strengths, limitations, and versatility in sample preparation. Special emphasis is placed on the role of water in tuning extraction efficiency and on the use of computational tools to design task-specific DESs. Helpful guidelines are proposed for a proper characterization leading to an precise classification and effective application.