Structure and thermal property of N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate-H2O mixtures

By in situ observations using simultaneous X-ray diffraction and differential scanning calorimetry method, complicated phase transitions were observed in N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF4] and H2O mixtures. In pure [DEME][BF4], two different crystal structures were determined below crystallization temperature, Tc. Two kinds of crystals correspond to two stages of melting upon heating. Tc decreases with increasing in the H2O content of [DEME][BF4]-H2O mixture. Around 6.7 mol% H2O, an amorphous solid, however, was formed without crystallization on cooling. Glass transition temperature, Tg, of the amorphous phase depends on cooling rate of the mixture. On heating, the amorphous solid transformed to a crystal accompanied by an exothermal peak. This unusual cold crystallization is induced by H2O molecules. Two different dynamic components were observed in a Raman spectrum of the amorphous phase, where the lower Raman band is crystal-like and the higher one is liquid-like. At higher H2O concentration, coexistence of the amorphous solid and crystal was realized below Tc, and the cold crystallization also occurred. In spite of a variety of phase transitions, the crystal structure of [DEME][BF4]-H2O mixtures is the same one as pure [DEME][BF4].