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

Abstract

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][BF 4] and H 2O mixtures. In pure [DEME][BF 4], 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 H 2O content of [DEME][BF 4]-H 2O mixture. Around 6.7 mol% H 2O, 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 H 2O 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 H 2O 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][BF 4]-H 2O mixtures is the same one as pure [DEME][BF 4].