158
Keywords Deep eutectic solvents · DES · Hydrophobic · Extractions · Separations
5.1 Introduction
Deep eutectic solvents (DES) evolved from ionic liquids initially driven by the
search for lower cost and more readily available alternatives to “standard” ionic
liquids. From ionic liquids, deep eutectic solvents inherited ionic components, most
commonly organic halide salts such as choline chloride, that were paired with
hydrogen bond donors such as urea, alcohols, or acids (Abbott et al. 2003, 2004).
This design of mixtures, based on abundant and largely benign components, has
proven to be a successful strategy to produce liquids that have been explored across
a broad range of applications, mirroring the interest in ionic liquids in the preceding
decades. From their initial design, the components of deep eutectic solvents were
defined by their hydrogen bond donating and accepting abilities which, by definition, gave them high affinity to water (Abbott et al. 2003, 2004). Discussing the
structure of deep eutectic solvents in their liquid state, multiple competing
Coulombic and hydrogen bonding interactions are cited, which are synonymous
with hydrophilicity (Perkins et al. 2014; Wagle et al. 2015; Ashworth et al. 2016;
Hammond et al. 2016; Araujo et al. 2017; Stefanovic et al. 2017; Florindo et al.
2018a; Gilmore et al. 2018b).
Consequently, one area where the applications of deep eutectic solvents have
been less prominent than those of ionic liquids is in the generation of hydrophobic
deep eutectic solvents with controllable solvent characteristics, to be used as alternatives to volatile organic compounds for separations and biphasic reaction systems. In ionic liquids, hydrophobicity was often introduced through a
non-coordinating anion: early on with hexafluorophosphate, [PF 6 ]
−
, and later with
bis(trifluoromethanesulfonate)imide, [NTf 2 ]
−
, and other perfluorinated anions. This
had significant impact in driving the exploration of hydrophobic ionic liquids as
separation solvents (Huddleston et al. 2001). The design of deep eutectic solvents
called for the anion to be inexpensive, environmentally benign, and a good hydrogen bond acceptor, which effectively banned the use of perfluorinated anions, in
favor of hydrophilicity-inducing halides.
5.2 Design of Hydrophobic Deep Eutectic Solvents
5.2.1 From Hydrophobic Ionic Liquids to Hydrophobic Deep
Eutectic Solvents
The design of hydrophobic deep eutectic solvents, although hindered by the avoidance of perfluorinated anion, was nevertheless inspired by strategies that have been
used for producing hydrophobic ionic liquids. With the hindsight of two decades of
E. L. Byrne et al.
Keywords Deep eutectic solvents · DES · Hydrophobic · Extractions · Separations
5.1 Introduction
Deep eutectic solvents (DES) evolved from ionic liquids initially driven by the
search for lower cost and more readily available alternatives to “standard” ionic
liquids. From ionic liquids, deep eutectic solvents inherited ionic components, most
commonly organic halide salts such as choline chloride, that were paired with
hydrogen bond donors such as urea, alcohols, or acids (Abbott et al. 2003, 2004).
This design of mixtures, based on abundant and largely benign components, has
proven to be a successful strategy to produce liquids that have been explored across
a broad range of applications, mirroring the interest in ionic liquids in the preceding
decades. From their initial design, the components of deep eutectic solvents were
defined by their hydrogen bond donating and accepting abilities which, by definition, gave them high affinity to water (Abbott et al. 2003, 2004). Discussing the
structure of deep eutectic solvents in their liquid state, multiple competing
Coulombic and hydrogen bonding interactions are cited, which are synonymous
with hydrophilicity (Perkins et al. 2014; Wagle et al. 2015; Ashworth et al. 2016;
Hammond et al. 2016; Araujo et al. 2017; Stefanovic et al. 2017; Florindo et al.
2018a; Gilmore et al. 2018b).
Consequently, one area where the applications of deep eutectic solvents have
been less prominent than those of ionic liquids is in the generation of hydrophobic
deep eutectic solvents with controllable solvent characteristics, to be used as alternatives to volatile organic compounds for separations and biphasic reaction systems. In ionic liquids, hydrophobicity was often introduced through a
non-coordinating anion: early on with hexafluorophosphate, [PF 6 ]
−
, and later with
bis(trifluoromethanesulfonate)imide, [NTf 2 ]
−
, and other perfluorinated anions. This
had significant impact in driving the exploration of hydrophobic ionic liquids as
separation solvents (Huddleston et al. 2001). The design of deep eutectic solvents
called for the anion to be inexpensive, environmentally benign, and a good hydrogen bond acceptor, which effectively banned the use of perfluorinated anions, in
favor of hydrophilicity-inducing halides.
5.2 Design of Hydrophobic Deep Eutectic Solvents
5.2.1 From Hydrophobic Ionic Liquids to Hydrophobic Deep
Eutectic Solvents
The design of hydrophobic deep eutectic solvents, although hindered by the avoidance of perfluorinated anion, was nevertheless inspired by strategies that have been
used for producing hydrophobic ionic liquids. With the hindsight of two decades of
E. L. Byrne et al.