# Deep Eutectic Solvents as Extraction Media for
## 1 Introduction
### Deep Eutectic Solvents  
The extraction media of choice for green extractions could very well be Deep Eutectic Solvents (DES). DES are solvents that occur when a mixture of substances has a melting point that is much lower than that of the two constituents. In order to form a DES system, there needs to be a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), which when mixed at proper ratios create a new \"mesh\" of hydrogen-bond-interconnected molecules with interesting physicochemical properties. DES can be highly viscous, inhibiting their use in processes that require diffusion or flow, however, research into their chemical structure, as well as the use of additives (like water), largely alleviates this issue [4].  
The potential to use multiple molecules as constituents and create a fluid mixture that can be used as a solvent is the first key interesting point regarding DES. Adding to this notion, the potential for many naturally occurring molecules to form DES, thus providing a natural solvent system with low vapor pressure; low cost, even at larger industrial scales; and the potential to remove the need for solvent retrieval (Natural DES-NaDES could remain in a consumable end product) highlights their importance. The evolution of DES, and more importantly, NaDES, means that the design of the newer solvents can focus on a solvent that is a capable and biocompatible storage media, a readily available active ingredient, a very efficient catalyst, or a molecular or compound carrier [5; 6].  
Competing with ionic liquids (ILs), another category of green solvents so far favors DES, since they are largely cheaper to produce, less toxic, and offer great variety. Seemingly, NaDES is the natural step forward from conventional ionic liquids, since they can be formed from green sources, can be  
Figure 1: Flavonoid skeleton and general structures of the main flavonoid categories.  
polar or non-polar, hydrophilic or hydrophobic, and can be tailored for any situation. DES could become designer solvents, designed and synthesized for every extraction individually, with maximum efficiency in mind.  
DES can be synthesized via a number of methods. Considering the research listed in this review, predominant methods are often simple, such as stirring at room to high temperatures (80 \({}^{\circ}\)C), freeze-dry mixing, or ultrasound-assisted mixtures [7,8]. The synthesis method can be selected with cost in mind (lower temperatures prevail), speed or efficiency (favoring higher temperatures and/or ultrasound assistance) or limited by the properties of the reagents (thermal-sensitive substances might require freeze-drying instead of heating due to thermal instability).  
Compounds that are preferred for their natural origin and green character, and that have been heavily employed in NaDES synthesis are shown in Table 1. All the molecules in Table 1 have been a part of DES used in flavonoid extraction.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Hydrogen Bond Donors (HBD)**} & \multicolumn{2}{c}{**Hydrogen Bond Acceptors (HBA)**} \\ \hline \multicolumn{1}{c}{Urea  } & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{Urea  } & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{Choline Chloride} & \multicolumn{1}{c}{} & \\ \hline Glycerol  & \multicolumn{1}{c}{} & \\ \multicolumn{1}{c}{Holyene Glycol} & \