254
meet the requirement of green solvents, and therefore they constitute a promising
alternative to organic solvents. They also show major advantages over ionic liquids,
such as lower prices and easy preparation. Interestingly, hydrophobic deep eutectic
solvents enable the extraction of metal ions from water (van Osch et al. 2016), and
membranes impregnated with hydrophobic deep eutectic solvents can be used in
separative devices. An increased CO 2 capture in hydrophobic deep eutectic solvents
compared to hydrophilic ones is also reported (Zubeir et al. 2018). Hydrophobic
deep eutectic solvents often make use of fatty acids as HBD; the hydrophobicity can
also be brought about by constituents such as terpenes. A comprehensive list of
hydrophobic deep eutectic solvents, can be found in a recent review (Dwamena 2019).
It is important to note that the term deep eutectic solvent and associated concepts
are commonly used well beyond their strict definition, as stated in the beginning of
this section. Two issues in particular should be discussed, which concern type III
deep eutectic solvents principally. First, water can be part of the components of a
deep eutectic solvent as hydrogen bond donor (HBD) added in stoichiometric
amount. In this case, water is an integral part of the deep eutectic solvent and is
extremely difficult or even impossible to remove (Dai et al. 2013b; Aroso et al.
2017). However, in many extraction studies, relatively large amounts of water are
added to reduce the viscosity of the solvent or to facilitate the preparation of the
mixture. Although spectroscopic analyses (NMR, FTIR) show that a moderate
excess of water does not destroy the deep eutectic solvent structure, this point is
rarely controlled (Dai et al. 2015). The other issue concerns the use of liquid hydrogen bond donor, such as glycerol, in type III deep eutectic solvent. Although liquid
HBD may interact with organic salts in the same way as their solid analogues, in the
absence of the visual criterion of liquefaction, it is difficult to distinguish a simple
dissolution from the formation of a eutectic. As an alternative, Abbott et al. have
used the lowest freezing point to define the “eutectic composition” of mixtures of
glycerol with various quaternary ammonium salts (Abbott et al. 2007). Nevertheless,
even if the thermodynamic notion of eutectic can be questioned in these two situations, it remains that excellent solvation properties can be observed and used.
Deep eutectic solvents present favorable features as solvents, such as low vapor
pressure, nonflammability, and low or negligible toxicity. Compared to ionic liquids, deep eutectic solvents are generally more environmentally friendly, easier to
prepare, and radically less expensive. Until recently, the two main applications of
deep eutectic solvents have been metal deposition and synthesis media. However,
with the emergence of natural deep eutectic solvents, applications as extraction
media for natural products are becoming increasingly important. Indeed, natural
deep eutectic solvents cover a large range of polarities from more polar than water
to equivalent to methanol (Ruesgas-Ramón et al. 2017). They proved to be excellent
solvents for metabolites of medium of low polarity that are poorly soluble in water,
particularly phenolic compounds (Dai et al. 2013c). This is due to the composition
of the media, which are rich both in ions and in HBD moieties. Their physicochemical and solubilizing properties can be tailored by adjusting the molecular structures
and molar proportions of the components, including water (Dai et al. 2013b).
Interestingly deep eutectic solvents can be combined with most of the
L. Percevault et al.
meet the requirement of green solvents, and therefore they constitute a promising
alternative to organic solvents. They also show major advantages over ionic liquids,
such as lower prices and easy preparation. Interestingly, hydrophobic deep eutectic
solvents enable the extraction of metal ions from water (van Osch et al. 2016), and
membranes impregnated with hydrophobic deep eutectic solvents can be used in
separative devices. An increased CO 2 capture in hydrophobic deep eutectic solvents
compared to hydrophilic ones is also reported (Zubeir et al. 2018). Hydrophobic
deep eutectic solvents often make use of fatty acids as HBD; the hydrophobicity can
also be brought about by constituents such as terpenes. A comprehensive list of
hydrophobic deep eutectic solvents, can be found in a recent review (Dwamena 2019).
It is important to note that the term deep eutectic solvent and associated concepts
are commonly used well beyond their strict definition, as stated in the beginning of
this section. Two issues in particular should be discussed, which concern type III
deep eutectic solvents principally. First, water can be part of the components of a
deep eutectic solvent as hydrogen bond donor (HBD) added in stoichiometric
amount. In this case, water is an integral part of the deep eutectic solvent and is
extremely difficult or even impossible to remove (Dai et al. 2013b; Aroso et al.
2017). However, in many extraction studies, relatively large amounts of water are
added to reduce the viscosity of the solvent or to facilitate the preparation of the
mixture. Although spectroscopic analyses (NMR, FTIR) show that a moderate
excess of water does not destroy the deep eutectic solvent structure, this point is
rarely controlled (Dai et al. 2015). The other issue concerns the use of liquid hydrogen bond donor, such as glycerol, in type III deep eutectic solvent. Although liquid
HBD may interact with organic salts in the same way as their solid analogues, in the
absence of the visual criterion of liquefaction, it is difficult to distinguish a simple
dissolution from the formation of a eutectic. As an alternative, Abbott et al. have
used the lowest freezing point to define the “eutectic composition” of mixtures of
glycerol with various quaternary ammonium salts (Abbott et al. 2007). Nevertheless,
even if the thermodynamic notion of eutectic can be questioned in these two situations, it remains that excellent solvation properties can be observed and used.
Deep eutectic solvents present favorable features as solvents, such as low vapor
pressure, nonflammability, and low or negligible toxicity. Compared to ionic liquids, deep eutectic solvents are generally more environmentally friendly, easier to
prepare, and radically less expensive. Until recently, the two main applications of
deep eutectic solvents have been metal deposition and synthesis media. However,
with the emergence of natural deep eutectic solvents, applications as extraction
media for natural products are becoming increasingly important. Indeed, natural
deep eutectic solvents cover a large range of polarities from more polar than water
to equivalent to methanol (Ruesgas-Ramón et al. 2017). They proved to be excellent
solvents for metabolites of medium of low polarity that are poorly soluble in water,
particularly phenolic compounds (Dai et al. 2013c). This is due to the composition
of the media, which are rich both in ions and in HBD moieties. Their physicochemical and solubilizing properties can be tailored by adjusting the molecular structures
and molar proportions of the components, including water (Dai et al. 2013b).
Interestingly deep eutectic solvents can be combined with most of the
L. Percevault et al.