UV-vis spectroscopic study of room temperature ionic liquid-water mixtures: N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate

Masami Aonoa, Yusuke Imaia, Hiroshi Abea, Hitoshi Matsumotoa, Yukihiro Yoshimurab

aDepartment of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan
bDepartment of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

Thermochimica Acta 532 (2012) 179-182.

Abstract
N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4], and H2O mixtures were investigated in optical properties by UV-visible spectroscopy. The concentration region of H2O in the mixtures was 0-100 mol%. At room temperature, the mixtures of H2O exhibit absorption peaks at around 280 nm and 750 nm, which depend on H2O concentrations. At 90-95 mol% H2O, the optical transmittance decreased over the whole region of UV-visible light. In X-ray diffraction, correlation over the medium-range such as network-forming materials develops at 90-95 mol%. Also, in quenched glass, low glass transition temperature appeared at these H2O concentrations. The specific glass is induced by different molecular aggregation in liquid. Therefore, an optical anomaly is connected with the specific water network over the medium-range order.

Fig. 1
DSC thermogram of the [DEME][BF4]-H2O mixture at 95 mol% on slow cooling and heating.


Fig. 2
X-ray diffraction pattern of (a) the [DEME][BF4]-95 mol% H2O mixture at -45 oC, (b) the [DEME][BF4]-95 mol% H2O mixture at -80 oC, and (c) the pure [DEME][BF4] at -80 oC. The arrows denote Bragg reflections of ice crystal. A peak at 38o is assigned aluminum as the sample holder.
Fig. 3.
H2O concentration dependence of glass transition temperature by rapid quenching.
Fig. 4.
Optical transmittance spectra of pure [DEME][BF4] and pure H2O collection at (a) 200-800 nm and (b) 700-800 nm. The transmittance of H2O and [DEME][BF4]is revealed by dashed and solid line, respectively.
Fig. 5.
Optical transmittance spectra as a function of water concentration at (a) 0-100 mol% and (b) 90-100 mol%.
Fig. 6.
H2O concentration dependence of the optical absorption peak position in (a) short wavelength region and (b) long wavelength region. The open and closed triangles indicate pure H2O and pure [DEME][BF4], respectively.
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