4
proved by nuclear magnetic resonance (NMR) spectroscopy. What is interesting
about these solvents is that they are not only liquid at ambient temperature but also
tunable and highly solubilizing. After that, other deep eutectic solvents based on
ChCl and carboxylic acids were characterized and were also shown to have important solubilizing ability toward some metal oxides (Abbott et al. 2004a). Other liquids were also obtained when mixing ChCl with a hydrated metal salt like chromium
(III) chloride hexahydrate (Abbott et al. 2004b). Later on, an additional class of
ambient temperature solvents based on metal salts and hydrogen bond donors such
as amides (urea and acetamide) and diols (ethylene glycol and 1,6-hexanediol) were
reported, but it turned out that only a restricted number of metal salts and hydrogen
bond donors can lead to their formation (Abbott et al. 2007a).
Few years later, Choi et al. coined the term “natural deep eutectic solvents”
(NADES) (Choi et al. 2011). This category covers the deep eutectic solvents that are
made of primary metabolites such as organic acids, amino acids, sugars, polyols,
and choline derivatives (Choi et al. 2011; Dai et al. 2013). Besides, water can also
be part of natural deep eutectic solvents’ composition. They were introduced as a
way to explain the omnipresence of metabolites in high concentrations in cells.
Since different combinations of these candidates led to the formation of liquids
which also succeeded in the solubilization of some natural compounds, natural deep
eutectic solvents were proposed as a new cellular phase, together with water and
lipids. These mixtures might be engaged in the biosynthesis, storage, and transport
of some poorly water-soluble compounds as well as some other processes like dehydration, drought resistance, and cryoprotection. Further, their consideration is
highly encouraged owing to the advantages that they provide from an environmental
and economic point of view.
1.2 Classification
In order to differentiate between the possible eutectics, deep eutectic solvents were
classified into four types based on the general formula Cat
+
X
−
zY, where Cat
+
is
generally an ammonium, phosphonium, or sulfonium, while X is a Lewis base (usually a halide anion). Y represents a Lewis or Brønsted acid and z is the number of Y
molecules that interact with the corresponding anion (Fig. 1.1) (Abbott et al. 2007a;
Smith et al. 2014).
Type III eutectics are the most studied in the literature and are usually based on
ChCl and various hydrogen bond donors. ChCl has been extensively adopted since
it is relatively cheap, nontoxic, and biodegradable, considering it is approved as a
natural additive for several animal species (“Scientific Opinion on Safety and
Efficacy of Choline Chloride as a Feed Additive for All Animal Species,” 2011). In
fact, the first type III deep eutectic solvent was primarily based on ChCl. Since then,
a plethora of compounds have been successfully used in deep eutectic solvents’
formation. The hydrogen bond acceptors (HBA) mainly include quaternary ammonium or phosphonium salts, whereas the most common hydrogen bond donors are
T. El Achkar et al.
proved by nuclear magnetic resonance (NMR) spectroscopy. What is interesting
about these solvents is that they are not only liquid at ambient temperature but also
tunable and highly solubilizing. After that, other deep eutectic solvents based on
ChCl and carboxylic acids were characterized and were also shown to have important solubilizing ability toward some metal oxides (Abbott et al. 2004a). Other liquids were also obtained when mixing ChCl with a hydrated metal salt like chromium
(III) chloride hexahydrate (Abbott et al. 2004b). Later on, an additional class of
ambient temperature solvents based on metal salts and hydrogen bond donors such
as amides (urea and acetamide) and diols (ethylene glycol and 1,6-hexanediol) were
reported, but it turned out that only a restricted number of metal salts and hydrogen
bond donors can lead to their formation (Abbott et al. 2007a).
Few years later, Choi et al. coined the term “natural deep eutectic solvents”
(NADES) (Choi et al. 2011). This category covers the deep eutectic solvents that are
made of primary metabolites such as organic acids, amino acids, sugars, polyols,
and choline derivatives (Choi et al. 2011; Dai et al. 2013). Besides, water can also
be part of natural deep eutectic solvents’ composition. They were introduced as a
way to explain the omnipresence of metabolites in high concentrations in cells.
Since different combinations of these candidates led to the formation of liquids
which also succeeded in the solubilization of some natural compounds, natural deep
eutectic solvents were proposed as a new cellular phase, together with water and
lipids. These mixtures might be engaged in the biosynthesis, storage, and transport
of some poorly water-soluble compounds as well as some other processes like dehydration, drought resistance, and cryoprotection. Further, their consideration is
highly encouraged owing to the advantages that they provide from an environmental
and economic point of view.
1.2 Classification
In order to differentiate between the possible eutectics, deep eutectic solvents were
classified into four types based on the general formula Cat
+
X
−
zY, where Cat
+
is
generally an ammonium, phosphonium, or sulfonium, while X is a Lewis base (usually a halide anion). Y represents a Lewis or Brønsted acid and z is the number of Y
molecules that interact with the corresponding anion (Fig. 1.1) (Abbott et al. 2007a;
Smith et al. 2014).
Type III eutectics are the most studied in the literature and are usually based on
ChCl and various hydrogen bond donors. ChCl has been extensively adopted since
it is relatively cheap, nontoxic, and biodegradable, considering it is approved as a
natural additive for several animal species (“Scientific Opinion on Safety and
Efficacy of Choline Chloride as a Feed Additive for All Animal Species,” 2011). In
fact, the first type III deep eutectic solvent was primarily based on ChCl. Since then,
a plethora of compounds have been successfully used in deep eutectic solvents’
formation. The hydrogen bond acceptors (HBA) mainly include quaternary ammonium or phosphonium salts, whereas the most common hydrogen bond donors are
T. El Achkar et al.