Even if almost 30 years have passed since the introduction of the Green Chemistry principles, the research on this field is still in its infancy. The present project is just ranked in this framework. In particular, its objectives are the development of low transition temperature mixtures (LTTMs) and their application as green solvents in some fields of the Analytical Chemistry, included that of the conservation and restoration of Cultural Heritage. LTTMs is a general term including the so-called type-III deep eutectic solvents (DESs) since both of them are the result of a self-association between an acceptor (HBA) and a donor (HBD) of H-bonds. Compared to ionic liquids, LTTMs are cheaper, easier to prepare and recyclable as the dictates of circular economy require. Most of them are also environmental friendly and non-toxic to the point of being defined ¿drinkable solvents¿. Moreover, thanks to the high number of combinations and stoichiometric flexibility, a variety of LTTMs can be designed with properties advantageously tailored. This also implicates their capability of solubilizing some compounds refractory to the conventional solvents. For these reasons, in this project, natural HBAs and HBDs will be selected to prepare new hydrophilic and hydrophobic LTTMs, liquid at room-temperature. Small angle X-ray scattering, differential scanning calorimetry, and other techniques will be used for their characterization and classification into LTTMs (glass transition and lack of any long-distance order) or into DESs (melting point). The obtained mixtures will then be tested as solvents for: i) the dispersive liquid-liquid microextraction of compounds of interest from environmental waters and biological fluids; ii) the cleaning of works of art surfaces, which is an innovative and almost unexplored topic in the literature. The whole project will be developed in close collaboration among the involved researchers to perfectly integrate their interdisciplinary expertise.
Even if several national and international research groups have actively been engaged in investigations aimed to unravel mechanisms of both eutectic formation and action as solvent systems, critical and compulsory endeavours have still to be pursued:
(i) Preparation of novel neoteric solvents, using natural or renewable components (HBDs and HBAs) with null or negligible toxicity. Thus far, most of the studies deal with well-known DESs, for example ChCl(urea)2 and ChCl(phenol)3. While ChCl is innocuous, many HBDs can exhibit a certain toxicity; for instance, phenol has a DL50 Phe=660 mg/Kg. Testing natural and safer HBDs and HBAs to compose new green mixtures, liquid at room temperature, will be one of our objectives. Moreover, according to the dictates of green economy, it will be also evaluated the possibility to use and recycle expired drugs whose active ingredient can act as an HBD; this could be the case of many nonsteroidal anti-inflammatory drugs.
(ii) Development of hydrophobic LTTMs, study of their stability over the time and understanding of their behaviour in water for a better use as extraction systems. As a matter of fact, up to now, research has been focused on hydrophilic and partially hydrophobic DESs, based on a relatively narrow range of compounds acting as HBAs, such as ChCl. The introduction of hydrophobic mixtures is expected to implicate a series of advantages. Surely, it will be possible to extend the extraction capability of these mixtures so to include emerging contaminants, drugs, hormones, and many other compounds with high values of logP. In fact, water is able to form hydrogen bond with both HBA and HBD and to disrupt LTTMs after a certain period of time, especially if one or both components are soluble in water. In most of the papers published till now, this aspect has almost been completely ignored in the applications involving the use of DESs for (micro)extraction purposes from aqueous samples [1].
(iii) Characterization of the novel mixtures to properly classify them as DESs or LTTMs. Over the very few last years, the literature is presenting an always growing number of papers dealing with the use of DESs to perform liquid-phase (micro)extractions. Nevertheless, when novel DESs were prepared and used, their characterization was often missing. In this project, the information from different investigation techniques (DSC, ESI-MS, SAXS, DLS) will be integrated to study and provide insights about the nature of DESs and LTTMs.
(iv) Testing the novel composed LTTMs to perform microextractions from real complex samples. In fact, most procedures developed till now involve applications to distilled water samples, while one of our goals will be the treatments of real systems such as environmental waters and biological fluids which have been almost ignored because of their matrix complexity. To the best of our knowledge, no work involving the explicit use of LTTMs as microextraction solvents has been published so far.
(v) Application of hydrophobic and hydrophilic LTTMs as innovative and safe solvents in the field of conservation and restoration of Cultural Heritage. For example, the cleaning of a varnish layer on painting surface could be accomplished with these kind of solvents, even supported by a polymeric matrix to facilitate its removal. Although some attempts have already been made with ILs [2], this topic is almost unexplored in the literature. On the other hand, applications to clean marble/stone surfaces with these solvents are completely missing in the literature.
All these points will be considered and faced in this research project thanks to the collaboration of an interdisciplinary team. The analytical methodologies and results obtained from this research will be fundamental for the application of LTTMs in several sectors of the Analytical Chemistry.
The future scientific impact of this project is expected to be high and able to expand the current knowledge on DESs and LTTMs as well as their factual use for microextraction and cleaning purposes. Regarding the immediate technological impact, the development of simple, fast, and safe extraction procedures will have considerable implications in each situation where high throughput analyses are required, for example routine clinical monitoring where the number of biological samples to be analysed is very high. Last but not least, unlike ILs, LTTMs can be recycled for subsequent reuses in agreement to both requirements of green chemistry and green/circular economy.
[1] C. Florindo, L. C. Branco, I. M. Marrucho, Fluid Phase Equilib. 448 (2017) 135-142.
[2] M.F. Pacheco, A. I.Pereira, L.C. Branco, A.J. Parola, J. Mater. Chem. A 1 (2013) 7016-7018.