Transition Metal Ions Solvation in Ionic Liquids: Coordination and Thermodynamics
Matteo BUSATO1, Paola D'ANGELO1, Marilena TOLAZZI2, Andrea MELCHIOR2
1Department of Chemistry, "La Sapienza" University of Rome, Rome, Italy
2Department Polytechnic of Engineering and Architecture, University of Udine, Udine, Italy
Among the various applications ionic liquids (ILs) have been studied for, several are those involving the presence of metal ions as solvated species in these media. For example, ILs have been proposed as advanced materials for metal ions separation, electrolytes in batteries and supercapacitors, and as new catalytic environments promoting organometallic reactions . In this framework, data about single-ion solvation are fundamental quantities that need to be known to design or improve new technologies. However, this fundamental knowledge is still missing for many metal species in several ILs.
The purpose of this research has been the achievement of a complete description of metal ions solvation in ILs from both a structural and thermodynamic point of view. A combined approach between theoretical methods like molecular dynamics (MD) simulations and density functional theory (DFT) calculations with X-ray absorption spectroscopy (XAS) measurements and advanced data analysis has been employed to study solutions of metal ions of industrial, environmental, and economic interest such as the Zn2+, Co2+, and Ag+ ions, in widely used ILs based on the [Tf2N]- (bis(trifluoromethylsulfonyl)imide) and [BF4]- (tetrafluoroborate) anions within the [Cnmim]+ (1-alkyl-3-methylimidazolium) cation. The coordination of the metal ions in these media (Figure 1a) was unraveled for the first time and described with unprecedented detail, while the obtained thermodynamic data have evidenced the impact of the entropic contribution and of the IL anion nature on the transfer process from the aqueous environment (Figure 1b), clarifying the dynamics at the base of an extraction procedure. The obtained results provided a fulfilling picture of the nature of the interaction between the studied metal ions and the ILs, representing a step forward in the understanding and development of ILs solutions as advanced processing media [2-4].
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