Raman and FTIR Spectroscopic Studies of 1-Ethyl-3-methylimidazolium Trifluoromethylsulfonate, its Mixtures with Water and the Solvation of Zinc Ions

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 0.2M 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,Py-4]TfO) was also investigated. In [Py-1,Py-4]TfO, there are, on average, 4.5TfO(-) anions coordinating each zinc ion, corresponding to the weak interaction between [Py-1,Py-4](+) cations and TfO- anions. The species present in [Py-1,Py-4]TfO are likely a mixture of [Zn(TfO)(4)](2-) and [Zn(TfO)(5)](3-).