The employment of X-rays diffraction (XRD) techniques is here proposed for the first time to analyse the structural features of deep eutectic solvents (DESs), i.e. systems with an intrinsically glassy nature, upon variation of the nature of a metal (M) when M, in the generic cationic form M¿n+, is present either in the starting formulation of the DES, or as an extraneous species incorporated by DES after extraction from compounds containing metal cations. The classes of compounds which will be considered as metal cation sources for consideration of extraction with DESs are oxides, sulphides and the rest of the calchogenides, halogens, nitrides, phosphides and arsenides. In this context the presence of M¿n+ with its shell of coordination is expected to introduce structural order at a local level within DESs. As a consequence of that, the resulting profile of diffraction of metal-containing DESs (M-DESs) will display features which make it distinguishable from the diffraction patterns typical of amorphous systems. The structural characterization of the metal-containing DESs will be accompanied by the calculation of the stability of the resulting structures through advanced computational methods. Moreover, the present project proposal intends to determine also the electrical conductivity and the redox properties of M-DESs as function of the temperature with the purpose of evaluating their utilization as viscous, self-standing electrolytes in photoelectrochemical and electrochromic cells. The two types of electrochemical devices here considered require the high optical quality of the materials constituting the various components of the cell. For this reason we consider M-DESs as a particularly palatable choice of gel-like electrolyte because of the favourable combination of optical and conductivity properties. The spectral properties of M-DESs will be evaluated with Raman and UV-vis-NIR spectroscopies.
The community of the scientists dealing with DESs will recognize easily the great potential and usefulness of the fundamental information derived from the determination of the structures of M-DESs (either as viscous liquids, or amorphous/crystalline solids and single crystals) as planned in the first part of the project proposal (objective 1, vide supra). This is because the combined knowledge of the microscopic structure of M-DESs and the determination of the corresponding physical properties of interest (density, viscosity, electrical transport, dielectric constants and optical transmission and as a function the temperature) leads to the definition of structure-property relationships that are necessary to design DESs for a given purpose. For example, a preliminary study of the electrical conductivity for a series of oxalic acid-based DESs differing for the content of crystallization water and extent of ordered domains has been recently conducted by our research group. It has been found that such systems present two distinct regimes of charge transport due to the presence of a marked discontinuity in the profile of the energy of activation at a specific value of water content. The combination of such an information with the determination of the structure is allowing the modelling of the mechanism and the corresponding dynamics of charge transport at a microscopic level. It is envisaged that this type of analysis offers the possibility to predict the critical amount of water in the DES structure above which a polarized DES transports charge through the faster mechanism. Such a kind of information results particularly useful if researchers are interested in the use of DESs as electrolytic media. The results of this study have been reported in one submitted paper (title: Structures and thermodynamic properties of DESs from choline chloride and oxalic acid; authors: M. Bonomo, L.Gontrani, D. Dini, R. Caminiti) and in another manuscript in preparation (provisional title: Characterization of the structural and thermodynamic properties of phenol-based DESs; Authors: M. Bonomo, L.Gontrani, L. Bencivenni, D. Dini, R. Caminiti). Another potential of the knowledge of the geometry of metal coordination and the evaluation of the coordination number of M¿n+ in M-DESs will consist in the advancement of the level of control of the maximum content of metal that is storable by a specific formulation of DES under the given conditions of metal salt solubilisation and/or extraction from metal sources in different condensed states. The latter aspect can be of strategic importance when dealing with the definition of new valuable optimized procedures of metal processing following DES design for the finalities of disposal, recycling, immobilization or the controlled release of metals (present in different classes of compounds) at various scales of mass and/or molar ratio. The combined analyses of the electrolytic/redox properties of M-DESs and their metal solubilising properties have a technological impact in electroanalytical chemistry with the development of electrochemical metal sensors at graduated sensitivity, which are based either on an amperometric or potentiometric response with M-DESs at variable content of metals as analytes. As far as the applicative part of the project is concerned, the realization of a photoelectrochemical device like a DSC with DES as electrolyte represents an achievement of high innovation in itself. In fact, to our knowledge, the use of DESs in photoelectrochemical devices for solar energy conversion has not been reported in the available literature on DESs. In association with that, the research on the utilization of DESs in photoelectrochemistry will favour the progress of the knowledge on the general chemical-physical photostability of DES-based systems under solar irradiation and, more generally, on the changes of the solubilising properties of DESs with the intensity and wavelength of the incident light. Another implication of the realization of the present project is the progress of the knowledge about the effect of configuration and dimensionality of DESs on physical properties like the electrical conductivity and dielectric properties. In particular, the studies here proposed will advance the knowledge of the actual limits of the amount of DES constituents above which the system results an electrically and optically homogeneous medium.