Deep eutectic systems – bio-based solvents designed with natural molecules

Introduction

Following the 12 principles of green chemistry, listed by Warner and Anastas (1), Ionic liquids (ILs) were one of the first advances in the development of alternative solvents. However, despite the number of scientific publications in the literature over the last two decades, the industrial application of ionic liquids has been hampered by the fact that their biodegradability and biocompatibility are compromised and therefore their sustainability is also called into question. There have been great efforts to design more harmless ionic liquids, but another major disadvantage in using these systems is their production method. The synthesis of these solvents is extremely expensive, given that their production process requires several steps, namely purification steps subsequent to the production process, and often the production yield is also relatively low. Deep eutectic solvents (DESs) are an alternative solvent system that were discovered almost a quarter of a century ago by Andrew Abbott’s group and have been used in many applications over this period. DESs are eutectic mixtures of two or more pure components with a eutectic point temperature significantly lower than that of an ideal liquid mixture. A good example is when choline chloride (ChCl) is mixed with urea in a ratio of 1:2, this is called reline.
There are 5 types of DESs; Type I (organic and metal salts), Type II (Organic and metal salt hydrate), Type III (Organic salt and H-bond donor), Type IV (Metal salt and H-bond donor), and Type V (non-ionic DESs – Hydrogen bond acceptor and Hydrogen bond donor).