163
5.3 Applications
A common theme for all the hydrophobic deep eutectic solvent systems that have
been reported to date is that hydrogen bond donor and acceptor capability of the
deep eutectic solvent forming components is retained. For example, using either
carboxylic acids, alcohols, or phenols as one component and establishing immiscibility with water through the inclusion of long alkyl groups, e.g., tetraalkylammonium/phosphonium salts and trioctylphosphine oxide, fatty acids, and linear
alcohols.
The drivers for development and investigation of hydrophobic deep eutectic solvents have been to enable separations and partitioning from aqueous media, and
efforts along these lines can be generally divided between extraction of organic
compounds and inorganic metal species.
5.3.1 Extraction of Organic Compounds
The “gold standard” for understanding extraction and solute distribution between
aqueous and hydrophobic extracting phases is the octanol/water partition coefficient, LogP (Sangster 1989). The more hydrophobic a solute is, the greater its distribution coefficient is for partitioning to octanol from water. A recent work from
Dietz and co-workers (Kaul et al. 2019) reports on the measurement of distribution
coefficients in a range of representative organic molecules between hydrophobic
menthol/decanoic acid deep eutectic solvents and water using
14
C-labelled radiotracers and scintillation counting. They compared the results to the corresponding
octanol-water partition coefficients (Fig. 5.3) and showed that partitioning of solutes in this hydrophobic deep eutectic solvent/water and in octanol/water systems
were strongly correlated. This means that distributions of organic solutes between
hydrophobic deep eutectic solvents and water can be predicted and understood.
The authors have highlighted the potential to tune hydrophobic deep eutectic
solvents to modify the distribution ratios, suggesting that the perspectives are at
least comparable to those previously demonstrated with hydrophobic ionic liquids.
In this context, it must be noted that extraction of organic compounds with hydrophobic deep eutectic solvents is still in its very naissance, and even if the initial
results, discussed below, did not exceed industrial benchmarks, they must be taken
as a “learning set” for further improvements and fine-tuning, rather than a negative
outcome discouraging further trials.
In 2016, Kroon and co-workers (van Osch et al. 2016) first described the use of
their hydrophobic tetraalkylammonium salt-containing deep eutectic solvents to
extract volatile fatty acids (C 2 -C 4 ) from aqueous media. Extraction efficiencies
reported were greater than those using trioctylamine as a benchmark extractant as
shown in Table 5.1. In the trioctylamine system, extraction would be expected to
benefit from partial proton transfer from acids to the amine, resulting in the
5 Hydrophobic Deep Eutectic Solvents
5.3 Applications
A common theme for all the hydrophobic deep eutectic solvent systems that have
been reported to date is that hydrogen bond donor and acceptor capability of the
deep eutectic solvent forming components is retained. For example, using either
carboxylic acids, alcohols, or phenols as one component and establishing immiscibility with water through the inclusion of long alkyl groups, e.g., tetraalkylammonium/phosphonium salts and trioctylphosphine oxide, fatty acids, and linear
alcohols.
The drivers for development and investigation of hydrophobic deep eutectic solvents have been to enable separations and partitioning from aqueous media, and
efforts along these lines can be generally divided between extraction of organic
compounds and inorganic metal species.
5.3.1 Extraction of Organic Compounds
The “gold standard” for understanding extraction and solute distribution between
aqueous and hydrophobic extracting phases is the octanol/water partition coefficient, LogP (Sangster 1989). The more hydrophobic a solute is, the greater its distribution coefficient is for partitioning to octanol from water. A recent work from
Dietz and co-workers (Kaul et al. 2019) reports on the measurement of distribution
coefficients in a range of representative organic molecules between hydrophobic
menthol/decanoic acid deep eutectic solvents and water using
14
C-labelled radiotracers and scintillation counting. They compared the results to the corresponding
octanol-water partition coefficients (Fig. 5.3) and showed that partitioning of solutes in this hydrophobic deep eutectic solvent/water and in octanol/water systems
were strongly correlated. This means that distributions of organic solutes between
hydrophobic deep eutectic solvents and water can be predicted and understood.
The authors have highlighted the potential to tune hydrophobic deep eutectic
solvents to modify the distribution ratios, suggesting that the perspectives are at
least comparable to those previously demonstrated with hydrophobic ionic liquids.
In this context, it must be noted that extraction of organic compounds with hydrophobic deep eutectic solvents is still in its very naissance, and even if the initial
results, discussed below, did not exceed industrial benchmarks, they must be taken
as a “learning set” for further improvements and fine-tuning, rather than a negative
outcome discouraging further trials.
In 2016, Kroon and co-workers (van Osch et al. 2016) first described the use of
their hydrophobic tetraalkylammonium salt-containing deep eutectic solvents to
extract volatile fatty acids (C 2 -C 4 ) from aqueous media. Extraction efficiencies
reported were greater than those using trioctylamine as a benchmark extractant as
shown in Table 5.1. In the trioctylamine system, extraction would be expected to
benefit from partial proton transfer from acids to the amine, resulting in the
5 Hydrophobic Deep Eutectic Solvents