Phase behavior of room temperature ionic liquid - H2O mixtures: N, N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate

Phase behavior of room temperature ionic liquid (RTIL)-H2O mixtures (x = 0.0-98.5 mol% H2O) was investigated by in-situ observations of simultaneous X-ray diffraction and differential scanning calorimetry method, and Raman spectroscopy: The RTIL is N, N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF4]. We found that the crystal structure at low temperature is almost the same as that of pure [DEME][BF4] over the whole range of water concentration. Above 60 mol%, H2O crystal appears inside the RTIL crystals. The difference in the enthalpy between the liquid and the solid state was estimated as a function of H2O concentration. In Raman spectra, a nearly-free hydrogen bonded band of H2O appears clearly below 90 mol%. We correlate the existence of nearly-free bonds in the [DEME][BF4]-H2O mixtures with the abrupt change of the enthalpy at around 90 mol%.