Thesis title: Deep Eutectic Solvents as Extraction Media for the Sustainable Recovery of Micronutrients from Food Waste: A Comprehensive Approach Integrating Computational Simulations and Practical Applications
The project focuses on tackling today’s challenges of reducing waste and minimizing pollution
from the agricultural and food industries. This involves addressing the proper handling
of waste byproducts and rethinking intensive production methods. One practical solution
is to make better use of agricultural byproducts by recovering through solvents extraction
the valuable bioactive compounds they contain. In recent years, the spotlight has shifted
toward the use of environmentally friendly "green" solvents as alternatives to traditional
organic ones. These innovative solvents are gaining attention for their potential to improve
extraction processes from natural, complex materials while being safer and more sustainable.
A class of alternative solvents is that of Deep eutectic solvents (DESs): they are mixtures
made from two or more components with a melting point much lower than expected from the
solid-liquid equilibrium phase diagram. This is due to a strong network of hydrogen bonds
formed within the mixture. The components of DESs are typically categorized as hydrogen
bond donors (HBDs) and acceptors (HBAs). In some cases, these mixtures do not have a
clear melting point but instead show a glass transition, classifying them as low transition
temperature mixtures (LTTMs).Thanks to their excellent solvating power, DESs and LTTMs
have become popular for many applications, especially in extraction processes, where they
improve efficiency, safety, and sustainability compared to traditional solvents. The approach
taken in studying these solvents includes a comprehensive analysis of the mixture structures,
their practical applications in extractions from real matrices, and, ultimately, an exploration
of how they interact with the extracted analytes. Thermal characterization through DSC
was performed on three systems formed by the three isomers of hydroxyphenol —catechol
(Cate), resorcinol (Reso), and hydroquinone (Hydro)— which were mixed with choline chloride
(ChCl) at a ChCl:HBD ratio of 1:x (x = 0.75, 1, 2, 3). The ChCl:Cate mixtures 1:0.75,
1:1, 1:2, and 1:3, ChCl:Reso 1:0.75, 1:1, and ChCl:Hydro 1:0.75, 1:1, and 1:2 were identified
as DESs due to their lower melting points observed in DSC, compared to predictions from
the ideal phase diagram, while the others either lacked a distinct melting point (ChCl:Reso
1:2, 1:3) or did not show a liquid phase of the mixture (ChCl:Hydro 1:3). Through combined
ATR-FTIR analyses and molecular dynamics (MD) simulations, it was possible to understand
how the bonds forming the mixture are stronger than those within the pure compounds.
Specifically, these bonds involve hydrogen interactions between the chlorine atom in ChCl
and the hydroxyl hydrogens of the two components. Additionally, it was observed that a
deep eutectic mixture is more easily formed at a ChCl:HBD molar ratio of 1:1. Similar
results were observed with the ChCl:4-methoxyphenol (4-MPh) 1:2 system, which, however,
does not qualify as a DES but rather as an LTTM due to the glass transition seen in its
DSC thermogram. This mixture was chosen based on preliminary tests as the most effective
for quercetin extraction, particularly from red onion skins. ChCl:4-MPh with 10% of water
was used in solid-liquid extractions enhanced by ultrasound (UAE), and subsequent quantitative
analysis via HPLC-MS/MS revealed a quercetin concentration of 7.41 mg/g, with
an extraction yield of approximately 98% from red onion peels. This not only outperformed the yield achieved with ethanol but also improved the analyte’s stability against oxidative
degradation, as confirmed by oxidation tests. The ChCl:4-MPh 1:2 and ChCl:2-MPh 1:2 systems,
along with the same mixtures supplemented with water at a ChCl:HBD 1:2:1.8 molar
ratio, were compared with ethanol regarding their ability to solvate quercetin. In the case of
ethanol, the primary interaction with quercetin involves hydrogen bonds formed between the
hydrogen atoms of the solvent and the hydroxyl groups of quercetin. On the other hand, the
eutectic solvents interact with hydroxyl groups of quercetin through hydrogen bonds with
the chlorine atom, which is unique to these mixtures. Adding water to the eutectic systems
did not result in significant changes in the structure surrounding quercetin. Particularly
noteworthy is the interaction between the oxygen atom double bonded to quercetin’s central
ring, which primarily binds with ethanol and 4-MPh. These observations were supported by
results from MD simulations , which also allowed the calculation of the ΔGsolv of quercetin
in the three pure solvents. Among the systems tested, ChCl:2-MPh (1:2) demonstrates the
most favorable thermodynamic contribution to quercetin solvation, marginally surpassing
the ChCl:4-MPh (1:2) system, while ethanol provides the least favorable contribution. Viscosity
measurements conducted on the eutectic mixtures with water revealed that, over a
temperature range from ambient to 50°C, the ChCl:4-MPh:H2O system is significantly less
viscous than ChCl:2-MPh:H2O. This difference helps explain why, despite computational
results, the system containing 4-MPh performed better in extraction tests. The study of
quercetin solvation, therefore, cannot rely solely on computational data. Experimental measurements,
combined with computational insights, provide a comprehensive understanding
of the systems, offering a synergy that has been maintained throughout the project.