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

Yusuke Imai a, Hiroshi Abe a,*, Takefumi Goto b, Yukihiro Yoshimura b, Yosuke Michishita a, Hitoshi Matsumoto a
a Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
b Department of Applied Chemistry, National Defense Academy, Yokosuka 239-8686, Japan

Chemical Physics 352 (2008) 224.

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][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].

Fig. 1. Molecular structures of the cation [DEME] and the anion [BF4].


Fig. 2. X-ray diffraction pattern at -80 oC (T < Tc) of pure ionic liquid [DEME][BF4]. Crystal consists of two kinds of crystal structures. Closed squares and open circles correspond to the calculated 2h values of orthorhombic and monoclinic, respectively.


Fig. 3. H2O concentration dependence of crystallization temperature, Tc, in DSC thermogram. Cooling rate was 10 oC/min. With increasing in the H2O content, Tc decreases monotonically and the exothems become smaller and broader. At 6.7 mol% H2O, the exotherm disappears. Considering the X-ray diffraction pattern at 6.7 mol% H2O (Fig. 4), glass transition occurs at Tg (inset). More H2O content (x = 12.0%) causes a weak and broad exothermal peak.


Fig. 4. X-ray diffraction patterns at several H2O concentrations at -80 oC (T < Tc): (a) crystal phase (C-phase) at 0.6 mol% H2O, (b) coexistence phase of amorphous and crystal ((A + C)-phase) at 12.0 mol% H2O and (c) amorphous phase (A-phase) at 6.7 mol% H2O.

Fig. 5. Cold crystallization on heating is observed at higher H2O concentrations. Heating rate was 3 oC/min. Exotherm from amorphous phase (6.7 mol% H2O) is larger than that from coexistence phase of amorphous and crystal (12.0 mol% H2O). Two kinds of melting points, Tm1 and Tm2, were observed by DSC measurements.

Fig. 6. X-ray diffraction patterns above cold crystallization temperature, Tcc and below Tcc. No additional Bragg reflections are observed on heating above Tcc.

Fig. 7. Phase diagrams on cooling and heating are determined by conventional DSC measurements and in situ X-ray diffraction and DSC measurements. L, A and C stand for liquid-, amorphous- and crystal-phases, respectively. Cold crystallization on heating occurs above xcc, which is the critical H2O concentration for the cold crystallization.
Fig. 8. H2O concentration dependence of (a) X-ray crystallinity and (b) enthalpy. eeExcessh enthalpies are observed at 0.6 and 2.9 mol% H2O. The dotted curve in (b) is drawn by expecting the enthalpy value from the X-ray crystallinity (see text).
Fig. 9. Cooling rate dependence of glass transition temperature, Tg, at 4.4, 6.7 and 8.8 mol% H2O. Pure amorphous phase appears at these concentrations. Tg depends on cooling rate and H2O concentrations. For a comparison, cooling rate dependence of crystallization temperature, Tc, at 0.0 mol% H2O is shown in the inset.
Fig. 10. Full width at half maximum (FWHM) of Bragg reflections at -80 oC (T < Tc). For comparison, FWHM of standard Si polycrystal is shown by open diamonds. FWHM of Bragg reflections from crystal phase (C-phase) is quite small. FWHM of amorphous and crystal coexistence ((A + C)-phase) is not so large. q dependence of FWHM of Bragg reflections is not dominant both in the C-phase and (A + C)-phase.
Fig. 11. H2O concentration dependence of Raman spectra for [DEME][BF4]-H2O mixtures in the regions of (a) 200-1500 cm-1 and (b) 2700-3200 cm-1. Raman spectrum of the liquid state (6.7%) was measured at 25 oC for a comparison with an amorphous phase. Other Raman spectra were observed at -80 oC. Amorphous phase at 6.7 mol% H2O provides two kinds of dynamic properties; liquid-like behavior around 3000 cm-1 and crystal-like behavior around 1000 cm-1.

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ab@nda.ac.jp
Department of Materials Science and Engineering
National Defense Academy

Last Modified: April 1, 2009