Raman and FTIR spectroscopic studies of 1-ethyl-3-methylimidazolium trifluoromethylsulfonate, its mixtures with water and the solvation of zinc ions

Abstract

In this paper we report on the interactions of the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) with water and the solvation of zinc ions in neat [EMIm]TfO and [EMIm]TfO-water mixtures investigated by FTIR and Raman spectroscopy. The structures and physicochemical properties of the [EMIm]TfO-water mixtures are strongly dependent on the interaction between cations, anions, and water. The structure was changed from ionic-liquid-like to water-like solutions upon addition of water. In addition, zinc salts can precipitate in 0.2 M Zn(TfO)2/[EMIm]TfO upon addition of 10 % (v/v) water, presumably as a result of polarity change of the solution. The average coordination number of TfO(-) per zinc ion calculated from Raman spectra is 3.8 in neat [EMIm]TfO, indicating that [Zn(TfO)4](2-), and [Zn(TfO)3](-) complexes are present in the solution. However, in the presence of water, water interacts preferentially with the zinc ions, leading to aqueous zinc species. The solvation of zinc ions in 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py(1,4)]TfO) was also investigated. In [Py(1,4)]TfO, there are, on average, 4.5 TfO(-) anions coordinating each zinc ion, corresponding to the weak interaction between [Py(1,4)](+) cations and TfO(-) anions. The species present in [Py(1,4)]TfO are likely a mixture of [Zn(TfO)4](2-) and [Zn(TfO)5](3-).

Keywords: IR spectroscopy; Raman spectroscopy; ionic liquids; solvation; water.