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

Abstract

Phase behavior of room temperature ionic liquid (RTIL)-H 2O mixtures ( x = 0.0-98.5 mol% H 2O) 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 2O 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 2O concentration. In Raman spectra, a nearly-free hydrogen bonded band of H 2O appears clearly below 90 mol%. We correlate the existence of nearly-free bonds in the [DEME][BF 4]-H 2O mixtures with the abrupt change of the enthalpy at around 90 mol%.