Research on the use of neoteric solvents has been motivated by an increase in pollution-controlled legislation and more regulations of common solvents, especially related to the treatment of biomass. The most widely used neoteric solvent in biomass treatment, and especially for the extraction of essential oils and other bioactive compounds from plants, is supercritical carbon dioxide. A great reason to use CO2 in its supercritical condition is to expand the spectrum of solvent solubility, polarity, and volatility [1].
Ionic liquids (ILs) and the deep eutectic solvents (DESs) are very attractive for extraction and separation sciences as replacements for volatile organic solvents. Unfortunately, traditional ILs, despite their high chemical flexibility and non-volatility, are not green solvents.
DESs represent another type of tailor-made solvent that use both hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) compounds, available from renewable resources, resulting in natural deep eutectic solvents (NADESs) which have preferable intrinsic characteristics, low cost, easy preparation, and low toxicity. The HBDs and HBAs of a DES dictate the solvent properties that have a direct impact on the extraction efficiency. The role of water in DESs’ composition is to alter the pH, viscosity, and polarity, which results in a significant increase in extraction efficiency.
There are studies that have shown the excellent performances of DESs in the extraction of biomolecules (such as polyphenols, iridoids, and alkaloids), providing comparable or even higher efficiency than conventional solvents [2,3].
A variety of analytical methods could be applied for the characterization of obtained extracts, including HPLC-DAD-MS, which is well suited for analysing the most complicated samples.
Author Contributions
Conceptualization, M.K. (Mihkel Koel) and M.V.; methodology, M.V. and M.K. (Mihkel Koel); validation, M.K. (Mihkel Koel), M.K. (Maria Kuhtinskaja) and M.V.; investigation, M.V., P.S.-R. and M.K. (Maria Kuhtinskaja); data curation, M.V.; writing—original draft preparation, M.K. (Mihkel Koel); writing—review and editing, M.V., P.S.-R. and M.K. (Maria Kuhtinskaja); project administration, M.V.; funding acquisition, M.V. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the Estonian Research Council (Grant IUT33-20) and the R&D project SS22004 “Evaluation of antioxidant and antibacterial activity of plant extracts”, funded by the Tallinn University of Technology.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data will be available on request from authors.
Acknowledgments
The Estonian Center of Analytical Chemistry (ECAC) is acknowledged for advice on methodology.
Conflicts of Interest
The authors declare no conflict of interest.
References
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