Structural and Thermodynamic Properties of Transition Metal Ions in Room Temperature Ionic Liquids

Room temperature ionic liquids (RTILs) are salts made by an organic cation and an organic or inorganic anion, which are at the liquid state at 25 °C. RTILs have attracted much attention as new sustainable solvents owing to some unique properties they usually possess, such as a practically negligible vapor pressure, non-flammability, high thermal stability, wide electrochemical windows, good solvation ability and supposed low toxicity. These features make RTILs good candidates for the substitution of classical organic solvents in many technological applications. For these reasons, they are currently studied as new media for chemical separations, electrodepositions, electrolytes for batteries and supercapacitors, catalysis and pharmaceutical research. Several of these applications also involve the presence of metal ions as solvated species in RTILs. In this field, structural and thermodynamic data about single-ion solvation are fundamental quantities that need to be known to improve new technologies. However, this fundamental knowledge still lacks for many metal species in several ionic liquids. The aim of this thesis is to obtain a complete description of metal ions solvation in RTILs both from a structural and thermodynamic point of view. Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) experiments have been performed to study solutions of metal ions of industrial, environmental and economic interest such as Zn2+, Co2+, Ag+ in widely used RTILs like those based on the [Tf2N]- (bis(trifluoromethylsulfonyl)imide) and [BF4]- (tetrafluoroborate) anions within the [Cnmim]+ (1-alkyl-3-methylimidazolium) cation. MD simulations have been carried out on Zn2+ in [Cnmim][Tf2N] (n = 2, 4) and [C4mim][BF4]. The obtained thermodynamic data are in good agreement with literature experimental values and indicate the goodness of the employed protocol. The calculated Gibbs free energies of transfer (ΔGtrans) from water to the [Cnmim][Tf2N] RTILs suggest that Zn2+ is more favorably solvated in aqueous solution than in this class of ionic liquids, while the opposite is found for [C4mim][BF4]. The obtained single-ion solvation enthalpies and entropies provided an interpretation of the different contributions to the calculated free energies. In addition, XAS experimental results allowed to understand the coordination of Zn2+ in water-saturated [C4mim][Tf2N], representing the real-operating condition in a liquid-liquid extraction. A similar picture has been obtained for Co2+ in [C4mim][Tf2N]. MD calculated ΔGtrans showed that the metal ion is still more favorably solvated in water than in the RTIL because of an unfavorable entropic contribution. XAS experiments and data-fitting allowed to obtain Co2+ coordination in dry [C4mim][Tf2N]. The metal resulted to be bound by six monodentate anions forming the [Co(Tf2N)6]4- octahedral species. In addition, water is found to preferentially coordinate the metal when present at high concentrations in the RTIL, as provided by UV-Vis data. As regards the study about Ag+ in RTILs, a totally different picture with respect to Zn2+ and Co2+ has been obtained. MD results showed that this ion is more favorably solvated both in [C4mim][Tf2N] and [C4mim][BF4] with respect to water, and this encourages the employment of these RTILs as extracting phase for this metal. Ag+ resulted coordinated by four or five RTILs anions, depending on the employed interaction potential. However, when considering the transfer of Ag+ from water to the RTILs, great care must be taken because of a possible change in the coordination number. Indeed, preliminary XAS data suggest a linear coordination for this metal ion in aqueous solution, differently from the tetrahedral model that is usually accepted and reproduced by the current classical potentials. Ab initio MD simulations with the Car-Parrinello method seemed to confirm this observation.