Solvation of Zn2+ ion in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids: a molecular dynamics and X-ray absorption study

Abstract

Molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) were employed to study the solvation of Zn²⁺ ion in dry [C_nmim][Tf₂N] (n = 2, 4; 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) room temperature ionic liquids (RTILs). For [C₄mim][Tf₂N] also a water-saturated system was considered. The thermodynamic parameters of single ion solvation and transfer from water to the RTIL, as well as the structural information, were calculated by means of MD simulations and our study evidences that both structural and thermodynamic features can be reproduced by MD in good agreement with the experimental data. In the case of [C₄mim][Tf₂N], enthalpies and entropies of solvation and transfer from water to the RTIL were also obtained. The Zn²⁺ ion results more favorably solvated in water than in the RTIL and this seems to be caused by the more negative entropy of solvation in the latter liquid. This can be explained by the higher ordering imposed by the metal ion when solvated in [C₄mim][Tf₂N] with respect to water. When the transfer of Zn²⁺ in a water/[C₄mim][Tf₂N] biphasic system is considered, the results show a positive value for the free energy of transfer and the metal ion always prefers to remain solvated in water than in the water-saturated RTIL phase. This theoretical result was confirmed by the analysis of XANES (X-ray absorption near edge structure) and the EXAFS (extended X-ray absorption fine structure) spectra of Zn(Tf₂N)₂ solutions in [C₄mim][Tf₂N] in the presence of water.