Effect of Water on Structure of Hydrophilic Imidazolium-Based Ionic Liquid

The state of water in room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4-), has been investigated by measurements of absorption and desorption isotherms, attenuated total reflectance infrared (ATR-IR) spectroscopy, and H-2 NMR relaxation method. The absorption enthalpies of water for the ionic liquid were estimated from the absorption isotherms. The enthalpies in the water mole fraction range of: x(w) <= similar to 0.5 are lower than the enthalpy of vaporization for bulk water, but become larger than the value for bulk with increasing mole fraction of absorbed water. The ATR-IR spectra for EMI+BF4--water Solutions in the range of 0.09 <= x(w) <= 0.34 have revealed that individual water molecules hydrogen-bonded to the anions predominate in the solutions at x(w) <= similar to 0.2, while similar to 30% of water molecules are hydrogen-bonded among them in the solutions at x(w) > similar to 0.3. In addition, the activation energies for the rotational motion of a water molecule estimated from the H-2 NMR relaxation rates have indicated that the motion of water molecules in EMI+BF4--D2O solutions gradually becomes freer with increasing water content from x(w) = 0.10 to 0.30, but is retarded again at x(w) = 0.33. Therefore, all the present findings have suggested that the state of water molecules in EMI+BF4- significantly changes at x(w) approximate to 0.3. On the other hand, to directly observe the effect of water oil structure of EMI+BF4-, LAXS experiments have been made on EMI+BF4--water solutions. It has been suggested that the interactions between the C-2 atom within the imidazolium ring of EMI+ and BF4- are strengthened with increasing water content, while those at the C-4 and C-5 atoms weaken. Thus, the present LAXS experiments have clarified the beginning of formation of ion pair in EMI+BF4- by adding water at the molecular level.