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

Abstract Phase behavior of room temperature ionic liquid (RTIL)-H 2 O mixtures ( x  = 0.0-98.5 mol% H 2 O) 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][BF 4 ]. We found that the crystal structure at low temperature is almost the same as that of pure [DEME][BF 4 ] over the whole range of water concentration. Above 60 mol%, H 2 O crystal appears inside the RTIL crystals. The difference in the enthalpy between the liquid and the solid state was estimated as a function of H 2 O concentration. In Raman spectra, a nearly-free hydrogen bonded band of H 2 O appears clearly below 90 mol%. We correlate the existence of nearly-free bonds in the [DEME][BF 4 ]-H 2 O mixtures with the abrupt change of the enthalpy at around 90 mol%.