Theoretical and experimental investigation of the interactions between [emim]Ac and water molecules

Density functional theory (DFT) calculations, atom in molecules (AIM) theory, natural bond orbital (NBO) analysis and infrared (IR) spectroscopy were performed to investigate the interactions between water molecules and ionic liquid 1-ethyl-3-methylimidazolium acetate ([emim]Ac). It was found that [emim]Ac interacts with water molecules mainly via H-bonds, and the anionic part of [emim]Ac plays a major role in the interaction with H2O. The energies of H-bonds were estimated from spectral shifts of hydroxy antisymmetric stretching vibration. Moreover, the experimental results also indicated that hydroxy of water mainly interacts with the COO− of [emim]Ac. Further studies indicated that the intensity of hydroxy stretching vibrations tend to be stronger with the increase of the concentration of water. In addition, the frequency of hydroxy stretching vibrations showed clearly red-shift, and the COO− vibrational frequency gradually shifted to the lower wavenumber region, which were indicative of extended hydrogen bonded network.

► The anion of [emim]Ac plays a major role in the interaction with H2O. ► Hydroxy of water mainly interacts with the COO− of [emim]Ac. ► The energies of H-bonds are estimated from shifts of Hydroxy stretching vibration. ► The intensities of hydroxy stretching vibrations tend to be stronger. ► The COO− vibrational frequency showed clearly red-shift.