255
extraction-assisted techniques such as microwave, ultrasound, high pressure, etc.
Their main limitation is their viscosity, which is generally higher than those of most
conventional solvents. Viscosity is an important parameter when considering a solvent for extraction, since it controls diffusion and mixing (Ruesgas-Ramón et al.
2017). However, the viscosity can be considerably lowered by adding small amounts
of water or increasing the temperature.
7.3.2 Hydrogen Bonds
Hydrogen bonds (H bonds) play a major role both in the formation and in the solubilization properties of organic molecules in type III deep eutectic solvent. For this
reason, we will recall here their main features. A H bond is an electrostatic force of
attraction between a hydrogen atom covalently bound to a more electronegative
atom (particularly N, O, or F) and another electronegative atom bearing a lone pair
of electrons, the former being the hydrogen bond donor and the latter the hydrogen
bond acceptor. H bonds are generally considered as electrostatic dipole-dipole
interactions. However, they also share some features with covalent bonds. They are
directional and the interatomic distances are shorter than the sum of the van der
Waals radii. Their strength varies from a few kJ.mol
−1
to more than 100 kJ.mol
−1
,
which places them in between the van der Waals and covalent interactions. Thanks
to their directionality, H bonds enable molecular recognition, which is ubiquitous in
biomolecules. Thus, it strongly contributes to the structure of DNA and proteins and
to substrate-protein association. The existence of hydrogen bonds in type III deep
eutectic solvents was revealed by NMR spectroscopy (Abbott et al. 2003; Dai et al.
2013b). More recently, the variety of hydrogen bonds that can be formed between
the different components was evidenced to play an important role in the freeze point
depression (Migliorati et al. 2019). The ability of the solvent to form hydrogen
bonds is particularly of interest for the selective extraction of polyphenols since
these compounds are generally rich in hydrogen bond donor groups (Choi et al.
2011; Ruesgas-Ramón et al. 2017). In addition, ternary deep eutectic solvents containing polyphenols in their formulation such as the poorly soluble caffeic acid
(e.g., choline chloride:caffeic acid:ethylene glycol) were also prepared as a basis of
molecularly imprinted materials for the recovery of the target polyphenol (Fu et al.
2017a, b).
7.3.3 Deep Eutectic Solvent and Extraction of Polyphenols
The polarity of polyphenols ranges from LogP ~ −2.5 for the most polar ones (catechin) to LogP = 5 for amentoflavone (Bohn 2014), while LogP is equal to 10.5 for
α-tocopherol (vitamin E), which bears a long alkyl chain. Moreover, polyphenols
may also exhibit a polymeric structure (e.g., tannins or marine algae polyphenols)
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents
extraction-assisted techniques such as microwave, ultrasound, high pressure, etc.
Their main limitation is their viscosity, which is generally higher than those of most
conventional solvents. Viscosity is an important parameter when considering a solvent for extraction, since it controls diffusion and mixing (Ruesgas-Ramón et al.
2017). However, the viscosity can be considerably lowered by adding small amounts
of water or increasing the temperature.
7.3.2 Hydrogen Bonds
Hydrogen bonds (H bonds) play a major role both in the formation and in the solubilization properties of organic molecules in type III deep eutectic solvent. For this
reason, we will recall here their main features. A H bond is an electrostatic force of
attraction between a hydrogen atom covalently bound to a more electronegative
atom (particularly N, O, or F) and another electronegative atom bearing a lone pair
of electrons, the former being the hydrogen bond donor and the latter the hydrogen
bond acceptor. H bonds are generally considered as electrostatic dipole-dipole
interactions. However, they also share some features with covalent bonds. They are
directional and the interatomic distances are shorter than the sum of the van der
Waals radii. Their strength varies from a few kJ.mol
−1
to more than 100 kJ.mol
−1
,
which places them in between the van der Waals and covalent interactions. Thanks
to their directionality, H bonds enable molecular recognition, which is ubiquitous in
biomolecules. Thus, it strongly contributes to the structure of DNA and proteins and
to substrate-protein association. The existence of hydrogen bonds in type III deep
eutectic solvents was revealed by NMR spectroscopy (Abbott et al. 2003; Dai et al.
2013b). More recently, the variety of hydrogen bonds that can be formed between
the different components was evidenced to play an important role in the freeze point
depression (Migliorati et al. 2019). The ability of the solvent to form hydrogen
bonds is particularly of interest for the selective extraction of polyphenols since
these compounds are generally rich in hydrogen bond donor groups (Choi et al.
2011; Ruesgas-Ramón et al. 2017). In addition, ternary deep eutectic solvents containing polyphenols in their formulation such as the poorly soluble caffeic acid
(e.g., choline chloride:caffeic acid:ethylene glycol) were also prepared as a basis of
molecularly imprinted materials for the recovery of the target polyphenol (Fu et al.
2017a, b).
7.3.3 Deep Eutectic Solvent and Extraction of Polyphenols
The polarity of polyphenols ranges from LogP ~ −2.5 for the most polar ones (catechin) to LogP = 5 for amentoflavone (Bohn 2014), while LogP is equal to 10.5 for
α-tocopherol (vitamin E), which bears a long alkyl chain. Moreover, polyphenols
may also exhibit a polymeric structure (e.g., tannins or marine algae polyphenols)
7 Extraction of Plant and Algal Polyphenols Using Eutectic Solvents