A good molecular relation – the basis for the best solvent match

Plant extracts have been widely used as bioactive compounds in several pharmacological applications. For instance, polyphenols have been researched as potential anti-inflammatory and anti-oxidative materials. Presently, several traditional methods for the extraction of polyphenols are available. Unfortunately, these techniques have disadvantages such as high cost, high energy consumption and polluting solvents that limit their use. Therefore, the development of green extraction techniques for phenolic compounds is highly desirable.

Ionic liquids with structures consisting of organic cations and organic or inorganic anions have recently emerged as green solvents. Their properties can be customized through the choice of moieties to obtain the desired solvent behavior. Also, ionic liquids have the ability to dissolve a wide spectrum of organic and inorganic compounds thus a promising alternative to conventional solvents used in numerous extraction techniques. However, the search for ideal solvents is still based on a trial and error approach.

To this note, Raquel Barrulas and Tiago Paiva, led by Dr. Marta Corvo from the NOVA University Lisbon employed the use of nuclear magnetic resonance (NMR) to study the profile of ionic liquids molecular interactions with model compounds capable of mimicking polyphenols. To simultaneously study all the possible interactions, several model compounds were selected to understand the behavior of phenol moiety towards hydrophobic interactions and achieve a rationalization. The main objective was to eliminate the slow and unsustainable trial and error process. The work is published in the journal, Separation and Purification Technology.

The research team further went ahead to validate their hypothesis to justify the possibility of predicting solvent behavior of ionic liquids to see the accuracy of the nuclear magnetic resonance technique. Specifically, matcha green tea was extracted with ionic liquids aqueous solution using the ultrasound-assisted extraction considering the positive balance between time and efficiency. The total polyphenolic content was determined. The effects of the anions and cations on the extraction process were investigated first through nuclear magnetic resonance followed by a proof of the matcha green tea extraction concept.

When extracting polyphenols from the matcha Japanese tea, the ionic liquids that exhibited the strongest molecular interactions were proven to have the highest efficiency. Matcha tea is extremely rich in these compounds. Both the cations and anions exhibited influence on the behavior of the solvent. The solubility of polyphenols in ionic liquids agreed well with the molecular interactions in the presence of suitable models. The best ionic liquid solvents for the matcha polyphenols were tricyanomethanide, dicyanamide, and triflate imidazolium derivatives.

The authors successfully located the π-π stacking and hydrophobic interactions for cation cases and H-bond acceptor interaction for cases of anions. Through the nuclear magnetic resonance approach, it was possible to profile the molecular interactions prevailing between ionic liquids and model compounds. Additionally, they identified the structural features in the ionic liquids responsible for the different interactions. This also enabled the efficient selection of the most preferred solvent to avoid the time-consuming trial and error process.

In summary, the demonstrated nuclear magnetic resonance methodology, with the study of molecular interactions provides a suitable platform for the selection of ionic liquids. Thus, Dr. Marta Corvo the lead author in a statement to Advances in Engineering expressed her confidence that the method could be extended to other target molecules by identifying adequate model compounds. It will also pave the way for future works to enable a complete assessment of the extraction process.