Room temperature ionic liquids (RTILs) are salts made by an organic cation and an organic/inorganic anion, which are at the liquid state at 25 °C. They have attracted much attention as a more sustainable alternative to traditional solvents owing to their negligible vapor pressure, non-flammability, thermal stability, wide electrochemical windows, good solvation ability and lower toxicity. Due to these characteristics, RTILs have been proposed as new electrolytes for energy storage systems and as new media for catalysis, electrodepositions, chemical extractions and pharmaceutical research. Several of these applications also involve the presence of metal ions as solvated species in RTILs. In this framework, a deep knowledge of the structural solvation features of a metal ion in these media becomes essential to design new applications. However, this knowledge still lacks for many metal ions in several RTILs.
Available literature data about Ag(I) solvation in RTILs provide a very encouraging picture for the potential applications involving this metal ion in these solvents. This is particularly true for what concerns the employment of RTILs as receiving phase for chemical separations, since high extraction rates of Ag(I) have been obtained with RTILs carrying the [Tf2N]- (bis(trifluoromethanesulfonyl)imide) anion. Solvation thermodynamic data confirm this trend, showing favorable free energies of transfer from water to [C4mim][Tf2N] and [C4mim][BF4] (1-alkyl-3-butyl-imidazolium bis(trifluoromethanesulfonyl)imide) and tetrafluoroborate). However, no definite picture about Ag(I) coordination in these RTILs is available.
For these reasons, the purpose of this project is to study the solvation of Ag(I) in these RTILs from a structural point of view. In particular, both experimental (X-ray absorption spectroscopy - XAS) and theoretical methods (molecular dynamics simulations - MD) will be employed to obtain a detailed picture of the metal-solvent interactions in solution.
Let us consider the case of a "neat" IL solution, i.e. without the presence of auxiliary ligands or water. In this case, we can expect the anion (i.e. [Tf2N]-, [BF4]-) to coordinate the positively charged metal, provided that the IL cation has no potential coordinating substituents. Consequently, a negatively charged complex is formed in solution. The knowledge of this speciation is of a central importance for the application of ILs as electrolytes in new electrochemical devices (e.g. batteries, supercapacitors, solar cells), since the nature of the charged species formed by a metal ion in solution is directly related to its transference number and has a crucial influence on the working of electrochemical systems.[1,2] The employment of ILs in such devices is highly-desirable, since the purpose would be the substitution of the currently employed organic solvents, which are often highly volatile, flammable and toxic, with the non-volatile, non-flammable and less toxic ILs. In addition, ILs can be considered as ideal electrolytes also because of their high electrochemical windows and thermal stability.
Furthermore, if we take into account the case of a liquid-liquid extraction, the knowledge of the coordination of the extracted species both in the initial state (solution of origin, e.g. wastewaters) and in the final state (extracting phase, i.e. the IL) is an essential starting point to interpret the extraction mechanism and understand how to improve the process.
As regards IL applications in catalysis, here the catalytic species usually consist in the metal ion merely solvated by the IL (and therefore coordinated by the IL anion).[3] To this purpose, ILs with inert and weakly coordinating anions that do not compete with the substrate for the coordination to the metal (like the [Tf2N]-) are preferred. Also here, we can understand the importance of the nature of the catalytic species, i.e. of the metal ion coordination in solution. The possibility to regenerate the spent catalyst simply with the dissolution of a new amount of metallic salt in the IL is a key-advantage of such a process,[3] together with the recovery of the metal that can be performed by means of electrodeposition at the end of the catalysis.
For all these reasons, we believe that the combined theoretical/experimental evidence obtained in this project will help finding a rationale for the potential applications involving the Ag(I) ion in these more sustainable class of solvents, if not for metal ions and ILs in general.
As previously said, this study is deeply connected with the previous works of the Applicant, in particular with those concerning the Zn(II) and Co(II) ions in Tf2N-based RTILs.[4,5] In addition to help finding the coordination of the studied metal ions in ILs solutions, MD simulations were able to reproduce the positive unfavorable free energies of transfer from water to [Cnmim][Tf2N] for these metal ions in agreement with literature experimental data.[6,7] In addition, it was demonstrated that Zn(II) and Co(II) transfer from water to the ILs are favorable from an enthalpic point of view, but negative entropy of transfer opposites to the process. In other words, when the metal ion passes from the aqueous solution to the IL phase, bringing 6 [Tf2N]- anions from the solution bulk to the metal first solvation sphere means a great loss of disorder, therefore energy must be spent. Here we can note how the structural information about a metal coordination in IL solution is fundamental to interpret also the thermodynamic data. As regards the Ag(I) case, a favorable negative free energy of transfer from water is reported in literature data.[6,7] Therefore, it would be very interesting to understand where this favorable transfer, different from the Zn(II) and Co(II) cases, comes from. Our hypothesis is that Ag(I) should be present with a lower CN with respect to Zn and Co in ILs solution, reasonably between 2 and 4. In other words, bringing a lower number of ILs anions from the bulk to the metal first solvation shell could mean a smaller loss of disorder and this could make Ag(I) transfer from water to ILs favorable and its extraction feasible. However, the knowledge of the coordination occurring in IL solution would be essential to accomplish such a comparison.
References.
1. M. Armand et al., Nat. Mater., 2009, 8, 621-629.
2. D. R. MacFarlane et al., Energy Environ. Sci., 2014, 7, 232-250.
3. M. J. Earle et al., Chem. Commun., 2004, 1368-1369.
4. M. Busato et al., Phys. Chem. Chem. Phys., 2019, 21, 6958-6969.
5. M. Busato et al., J. Mol. Liq., 2020, 299, 112120.
6. A. Lewandowski et al., Phys. Chem. Chem. Phys., 2003, 5, 4215.
7. A. Lewandowski et al., J. Incl. Phenom., 2005, 52, 237-240.