Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) have been combined to study the coordination of the Co²⁺ and Ni²⁺ ions in ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide ([Tf₂N]⁻) anion and having different organic cations, namely, 1-butyl-3-methylimidazolium ([C₄mim]⁺), 1,8-bis(3-methylimidazolium-1-yl)octane ([C₈(mim)₂]²⁺), N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium ([choline]⁺), and butyltrimethylammonium ([BTMA]⁺). Co and Ni K-edge XAS data have been collected on 0.1 mol L^–1 Co(Tf₂N)₂ and Ni(Tf₂N)₂ solutions and on the metallic salts. MD simulations have been carried out to obtain structural information on the metal ion coordination. The analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the solutions has been carried out based on the atomistic description provided by MD, and the studied ILs have been found to be able to dissolve both the Co(Tf₂N)₂ and Ni(Tf₂N)₂ salts giving rise to a different structural arrangement around the metal ions as compared to the solid state. The combined EXAFS and MD results showed that the Co²⁺ and Ni²⁺ ions are surrounded by a first solvation shell formed by six [Tf₂N]⁻ anions, each coordinating in a monodentate fashion by means of the oxygen atoms. The nature of the IL organic cation has little or no influence on the overall spatial arrangement of the [Tf₂N]⁻ anions, so that stable octahedral complexes of the type [M(Tf₂N)₆]^4– (M = Co, Ni) have been observed in all the investigated ILs.

The changes upon methanol (MeOH) addition in the structural arrangement of the highly eco-friendly deep eutectic solvent (DES) formed by choline chloride (ChCl) and sesamol in 1:3 molar ratio have been studied by means of attenuated total reflection Fourier transform infrared spectroscopy, small- and wide-angle X-ray scattering (SWAXS), and molecular dynamics simulations. The introduction of MeOH into the DES promotes the increase of the number of Cl–MeOH hydrogen bonds (HBs) through the replacement of sesamol and choline molecules from the chloride anion coordination sphere. This effect does not promote the sesamol–sesamol, choline–choline, and sesamol–choline interactions, which remain as negligible as in the pure DES. Differently, the displaced sesamol and choline molecules are solvated by MeOH, which also forms HBs with other MeOH molecules, so that the system arranges itself to keep the overall amount of HBs maximized. SWAXS measurements show that this mechanism is predominant up to MeOH/DES molar ratios of 20–24, while after this ratio value, the scattering profile is progressively diluted in the cosolvent background and decreases toward the signal of pure MeOH. The ability of MeOH to interplay with all of the DES components produces mixtures with neither segregation of the components at nanoscale lengths nor macroscopic phase separation even for high MeOH contents. These findings have important implications for application purposes since the understanding of the pseudophase aggregates formed by a DES with a dispersing cosolvent can help in addressing an efficient extraction procedure.

Molecular dynamics simulations and X-ray absorption spectroscopy were employed to study Zn²⁺ ions in [C_nmim][Tf₂N] (n = 2, 4).

The metal-based deep eutectic solvent (MDES) formed by NiCl₂·6H₂O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV-vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni²⁺ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni²⁺ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni-Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl₂·6H₂O 1:3.5 MDES, the Ni²⁺ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.