The Hydrogen Bonding Interactions between the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethyl Sulfate and Water

Abstract

1-Ethyl-3-methylimidazolium ethyl sulfate (EMIES) is a novel ionic liquid with potential industrial applications. Attenuated total reflectance infrared spectroscopy, ¹H NMR spectroscopy, and quantum chemical calculations were employed to investigate the molecular interactions between water and EMIES. The infrared spectra were analyzed by two methods: excess spectroscopy and two-dimensional correlation spectroscopy. This showed that the hydrogen bond involving the −SO₃ group in the ethyl sulfate anion (ES) was enhanced, while those involving the aromatic C−H groups of 1-ethyl-3-methylimidazolium cation (EMI) were weakened in the presence of water. During the process of increasing water concentration, the hydrogen bonding interaction between H₂O and SO is prior to that between H₂O and the C−H group on the imidazolium ring. At low concentrations, water interacts selectively with −SO₃ in the ethyl sulfate anion, while, at high concentrations (mole fraction of water equal or greater than 0.6), it can also form hydrogen bonds directly with the imidazolium ring. The following sequential order of interaction strength is established: EMI−water−ES > EMI−ES > ES−water > EMI−water.