253
that have been used for natural product extraction. Interestingly, they also present a
relatively wide electrochemical window, which has been used for metal deposition.
Type IV is a specific and relatively limited class of deep eutectic solvents that
do not contain an organic salt. Although non-hydrated inorganic salts generally do
not form deep eutectic solvents at low temperature, deep eutectic solvents have been
prepared at ambient temperature from transition metal chlorides such as ZnCl 2 associated with an organic HBD, such as urea or ethylene glycol. Metal halides are not
normally expected to dissociate in nonaqueous media; however, the structure
MCl (x−1)
+
.Y, MCl x
−
, where M is the metal atom and Y is the organic HBD, was proposed to account for the observed ionization and deep eutectic solvent formation.
The existence of a significant number of eutectics based solely on nonionic species (e.g., thymol:lidocaine or thymol:menthol) has very recently led Coutinho et al.
to define a fifth type of deep eutectic solvents (Abranches et al. 2019). The authors
show that phenols play a central role in type V deep eutectics, since their combination with any hydrogen bond acceptor is susceptible to produce a deep eutectic. This
is due to resonant character of phenols which make them good HBD and poor HBA,
then they prefer to interact with other HBA than with themselves.
In 2011, natural deep eutectic solvents were introduced as a particular class of
deep eutectic solvent, mainly of type III, prepared from biomolecules, such as choline chloride and betaine as the organic salt, and urea, organic acids, amino acids, or
sugars as the HBD (Hayyan et al. 2013; Dai et al. 2013b; Paiva et al. 2014; Aroso
et al. 2017; Silva et al. 2018). It can be also expected that many type V deep eutectic
solvents will be based on natural compounds. Remarkably, natural deep eutectic
solvents are ubiquitously present in living organisms both in the intra- and extracellular media where they may play a role in the synthesis and solubilization of poorly
soluble metabolites such as flavonoids, in enzymatic reactivity, and also in drought
tolerance (Dai et al. 2013b; Paiva et al. 2014). In this paradigm, natural deep eutectic solvents constitute a third type of natural liquid, separated from water and lipids
(Choi et al. 2011). Choi et al. investigated a series of natural deep eutectic solvents
formed from abundant biomolecules, including combinations of choline chloride
with citric, malic, maleic, and aconic acids; proline with citric acid; malic acid with
glucose; and mixtures of sugars (fructose:glucose, fructose:sucrose, glucose:sucrose,
etc.) (Choi et al. 2011). Natural deep eutectic solvents are regarded as an environmentally friendly alternative solvent for the extraction of biomolecules. Thus, the
formation of a glycerol-based natural deep eutectic solvent was evaluated for the
separation of glycerol from biodiesel (Abbott et al. 2007). Most type V deep eutectic solvents are also formulated from natural molecules and as such belong to the
family of natural deep eutectic solvents.
Despite the significant increase in the number and variety of deep eutectic solvents, research has mainly focused on the design of hydrophilic deep eutectic solvents. The first report of hydrophobic deep eutectic solvents, sometimes referred to
as HDES or HES, appeared only in 2015 (Florindo et al. 2019), but their major
interest as water-immiscible solvents for liquid-liquid extraction of natural compounds was rapidly identified, provided that their viscosity is sufficiently low and
their density far enough from that of water. Hydrophobic deep eutectic solvents can
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
that have been used for natural product extraction. Interestingly, they also present a
relatively wide electrochemical window, which has been used for metal deposition.
Type IV is a specific and relatively limited class of deep eutectic solvents that
do not contain an organic salt. Although non-hydrated inorganic salts generally do
not form deep eutectic solvents at low temperature, deep eutectic solvents have been
prepared at ambient temperature from transition metal chlorides such as ZnCl 2 associated with an organic HBD, such as urea or ethylene glycol. Metal halides are not
normally expected to dissociate in nonaqueous media; however, the structure
MCl (x−1)
+
.Y, MCl x
−
, where M is the metal atom and Y is the organic HBD, was proposed to account for the observed ionization and deep eutectic solvent formation.
The existence of a significant number of eutectics based solely on nonionic species (e.g., thymol:lidocaine or thymol:menthol) has very recently led Coutinho et al.
to define a fifth type of deep eutectic solvents (Abranches et al. 2019). The authors
show that phenols play a central role in type V deep eutectics, since their combination with any hydrogen bond acceptor is susceptible to produce a deep eutectic. This
is due to resonant character of phenols which make them good HBD and poor HBA,
then they prefer to interact with other HBA than with themselves.
In 2011, natural deep eutectic solvents were introduced as a particular class of
deep eutectic solvent, mainly of type III, prepared from biomolecules, such as choline chloride and betaine as the organic salt, and urea, organic acids, amino acids, or
sugars as the HBD (Hayyan et al. 2013; Dai et al. 2013b; Paiva et al. 2014; Aroso
et al. 2017; Silva et al. 2018). It can be also expected that many type V deep eutectic
solvents will be based on natural compounds. Remarkably, natural deep eutectic
solvents are ubiquitously present in living organisms both in the intra- and extracellular media where they may play a role in the synthesis and solubilization of poorly
soluble metabolites such as flavonoids, in enzymatic reactivity, and also in drought
tolerance (Dai et al. 2013b; Paiva et al. 2014). In this paradigm, natural deep eutectic solvents constitute a third type of natural liquid, separated from water and lipids
(Choi et al. 2011). Choi et al. investigated a series of natural deep eutectic solvents
formed from abundant biomolecules, including combinations of choline chloride
with citric, malic, maleic, and aconic acids; proline with citric acid; malic acid with
glucose; and mixtures of sugars (fructose:glucose, fructose:sucrose, glucose:sucrose,
etc.) (Choi et al. 2011). Natural deep eutectic solvents are regarded as an environmentally friendly alternative solvent for the extraction of biomolecules. Thus, the
formation of a glycerol-based natural deep eutectic solvent was evaluated for the
separation of glycerol from biodiesel (Abbott et al. 2007). Most type V deep eutectic solvents are also formulated from natural molecules and as such belong to the
family of natural deep eutectic solvents.
Despite the significant increase in the number and variety of deep eutectic solvents, research has mainly focused on the design of hydrophilic deep eutectic solvents. The first report of hydrophobic deep eutectic solvents, sometimes referred to
as HDES or HES, appeared only in 2015 (Florindo et al. 2019), but their major
interest as water-immiscible solvents for liquid-liquid extraction of natural compounds was rapidly identified, provided that their viscosity is sufficiently low and
their density far enough from that of water. Hydrophobic deep eutectic solvents can
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents