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 tetra fluoroborate, [DEME][BF4] and H2O mixtures. In pure [DEME][BF4], two different crystal structures were determined below crystallization temperature, T-c. Two kinds of crystals correspond to two stages Of melting upon heating. T-c 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, T-g, 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 T-c, 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]. (C) 2008 Elsevier B.V. All rights reserved.