162
hydrogen bond donor-acceptor pairing strategy in their design, although interactions between the components are presumably hydrogen bonding with both alcohol
and carboxylic acid functionalities displaying donor and acceptor ability.
These DL-menthol/carboxylic acid hydrophobic deep eutectic solvents are radically different in composition to the ammonium salt-based deep eutectic solvents
from Kroon and co-workers (van Osch et al. 2015) and from typical choline
chloride- based deep eutectic solvents (Abbott et al. 2003). They also do not build on
traditional ionic liquid components, but crucially demonstrate the formulation of
functional hydrophobic liquids containing desirable properties – in this case, derived
from bio-sourced menthol and incorporating low cytotoxic biocompatible
components.
Subsequently, a very interesting variety of hydrophobic deep eutectic solvents
was proposed, comprised solely of long-chain carboxylic acids (Florindo et al.
2018a, b). These liquids take advantage of melting point depression in mixtures of
octanoic, nonanoic, decanoic, and dodecanoic acid (C 8 -C 12 ) that form room temperature liquids with eutectic points between 9 and 18 °C. Their design was inspired
by the behavior of mixtures of longer chain fatty acids (C 10 -C 18 ), which have slight
melt depression and eutectic points between 22 and 56 °C, and have been considered as phase change materials for thermal energy storage (Zhang et al. 2015).
Gilmore et al. (Gilmore et al. 2018b) have based their design on the widely used
hydrophobic extracting agent, trioctylphosphine, used for separating both metals
and organic molecules from aqueous media. Typically used in kerosene solution,
trioctylphosphine oxide was shown to form hydrophobic deep eutectic solvents,
acting as a hydrogen bond acceptor, when liquefied by the addition of 1–2 equivalents of a hydrogen-bonding component, such as phenol. Room temperature liquids
were formed over a wide compositional range (χ TOPO = 0.2–0.5), and trioctylphosphine oxide/phenol mixtures were significantly more fluid (with viscosities in the
range 12–43 mPa s at 25 °C) when compared to the ionic tetraalkylammonium
chloride/decanoic acid hydrophobic deep eutectic solvents (viscosities
173–783 mPa s at 25 °C) from Kroon and co-workers (van Osch et al. 2015).
Although the liquids were glass-forming, a eutectic point at χ TOPO = 0.33 was extrapolated from the variations in solid/liquid transition temperatures. The trioctylphosphine oxide/phenol deep eutectic solvent was demonstrated as an effective extractant
for uranyl ([UO 2 ]
2+
) species from aqueous acid due to the high concentration of
coordinating trioctylphosphine oxide in the liquids, much higher than that attainable
in kerosene. Furthermore, the eutectic formation strategy was also suggested as a
potential route to extract phenolic and other acidic hydrogen bond donors from
water, based on the positive partition coefficient of phenol from water.
Koon and co-workers (van den Bruinhorst et al. 2019) subsequently showed that
trioctylphosphine oxide could form room temperature liquids over a wide compositional range with N,N′-dihexylthiourea (χ TOPO = 0.3–0.5) and over a short compositional range with dodecanoic acid (χ TOPO = 0.4–0.5), both of these components
acting as hydrogen bond donors towards trioctylphosphine oxide. The eutectic point
was determined around χ TOPO = 0.4 in both cases.
E. L. Byrne et al.
hydrogen bond donor-acceptor pairing strategy in their design, although interactions between the components are presumably hydrogen bonding with both alcohol
and carboxylic acid functionalities displaying donor and acceptor ability.
These DL-menthol/carboxylic acid hydrophobic deep eutectic solvents are radically different in composition to the ammonium salt-based deep eutectic solvents
from Kroon and co-workers (van Osch et al. 2015) and from typical choline
chloride- based deep eutectic solvents (Abbott et al. 2003). They also do not build on
traditional ionic liquid components, but crucially demonstrate the formulation of
functional hydrophobic liquids containing desirable properties – in this case, derived
from bio-sourced menthol and incorporating low cytotoxic biocompatible
components.
Subsequently, a very interesting variety of hydrophobic deep eutectic solvents
was proposed, comprised solely of long-chain carboxylic acids (Florindo et al.
2018a, b). These liquids take advantage of melting point depression in mixtures of
octanoic, nonanoic, decanoic, and dodecanoic acid (C 8 -C 12 ) that form room temperature liquids with eutectic points between 9 and 18 °C. Their design was inspired
by the behavior of mixtures of longer chain fatty acids (C 10 -C 18 ), which have slight
melt depression and eutectic points between 22 and 56 °C, and have been considered as phase change materials for thermal energy storage (Zhang et al. 2015).
Gilmore et al. (Gilmore et al. 2018b) have based their design on the widely used
hydrophobic extracting agent, trioctylphosphine, used for separating both metals
and organic molecules from aqueous media. Typically used in kerosene solution,
trioctylphosphine oxide was shown to form hydrophobic deep eutectic solvents,
acting as a hydrogen bond acceptor, when liquefied by the addition of 1–2 equivalents of a hydrogen-bonding component, such as phenol. Room temperature liquids
were formed over a wide compositional range (χ TOPO = 0.2–0.5), and trioctylphosphine oxide/phenol mixtures were significantly more fluid (with viscosities in the
range 12–43 mPa s at 25 °C) when compared to the ionic tetraalkylammonium
chloride/decanoic acid hydrophobic deep eutectic solvents (viscosities
173–783 mPa s at 25 °C) from Kroon and co-workers (van Osch et al. 2015).
Although the liquids were glass-forming, a eutectic point at χ TOPO = 0.33 was extrapolated from the variations in solid/liquid transition temperatures. The trioctylphosphine oxide/phenol deep eutectic solvent was demonstrated as an effective extractant
for uranyl ([UO 2 ]
2+
) species from aqueous acid due to the high concentration of
coordinating trioctylphosphine oxide in the liquids, much higher than that attainable
in kerosene. Furthermore, the eutectic formation strategy was also suggested as a
potential route to extract phenolic and other acidic hydrogen bond donors from
water, based on the positive partition coefficient of phenol from water.
Koon and co-workers (van den Bruinhorst et al. 2019) subsequently showed that
trioctylphosphine oxide could form room temperature liquids over a wide compositional range with N,N′-dihexylthiourea (χ TOPO = 0.3–0.5) and over a short compositional range with dodecanoic acid (χ TOPO = 0.4–0.5), both of these components
acting as hydrogen bond donors towards trioctylphosphine oxide. The eutectic point
was determined around χ TOPO = 0.4 in both cases.
E. L. Byrne et al.