X-ray Diffraction Study of Ionic Liquid Based Mixtures

Yusuke Imai,† Hiroshi Abe,*,† and Yukihiro Yoshimura‡
Department of Materials Science and Engineering, and Department of Applied Chemistry,
National Defense Academy, Yokosuka 239-8686, Japan

J. Phys. Chem. B 2009, 113, 2013.

Abstract
Crystal structures in N, N-diethyl-N-methyl-N-2-methoxyethylammonium tetrafluoroborate, [DEME][BF4], are determined by X-ray diffraction method. In [DEME][BF4]-based mixtures, various kinds of crystal structures and superstructures are induced by additives, i.e., H2O, CH3OH, C2H5OH, and C6H6. Most of the crystal structures in the mixtures are related to that of pure [DEME][BF4], though unit cells in 1.1 and 6.1 mol % C6H6 are different from the pure one. Further, [DEME][BF4]-C6H6 mixtures require the two kinds of superstructures, where volume per [DEME][BF4] is contracted by a small amount of additives. Also, volume contraction and the coexistence of two kinds of superstructures are seen in 0.9 mol % H2O. The superstructures are derived from orientational or displacive modulations of [DEME][BF4] molecules. In spite of a small amount of additives, molecular interactions of [DEME][BF4] are influenced extensively.

Figure 1. X-ray diffraction pattern at -80 oC (T < Tc) of pure ionic liquid [DEME][BF4]. Crystal structure is assigned by monoclinic or orthorhombic. Open circles and closed triangles correspond to the calculated 2θ values of monoclinic and orthorhombic, respectively.


Figure 2. (a) Two kinds of unit cells. (b) Molecular structure of cation, [DEME]. Each unit cell has a relation each other. aM, bM, and cM reveal lattice constants of monoclinic, and aO, bO, and cO exhibit those of orthorhombic.


Figure 3. X-ray diffraction patterns of [DEME][BF4]-H2O mixtures depend on additions. Crystal structures of (a) 0.6 mol %, (b) 0.9 mol%, and (c) 2.9 mol % H2O are calculated. Only the crystal structure of 0.9 mol % H2O is not calculated by unit cells of pure [DEME][BF4]. Open squares reveal a'O × b'O × 2c'O modulated lattice and open triangles exhibit 2a'O × b'O × 2c'O modulated lattice.


Figure 4. H2O concentration dependence of volume per [DEME][BF4], V4. Modulated lattices such as 2V4 and 4V4 are normalized by the number of molecules for a comparison. V4 at 0.9 mol % H2O is extraordinary small.

Figure 5. X-ray diffraction patterns of (a) 0.8 mol % and (b) 7.3 mol % CH3OH. Open circles reveal orthorhombic lattice pf pure [DEME][BF4]. [DEME][BF4]-7.3 mol % CH3OH has the modulated lattice based on unit cell of pure [DEME][BF4]. Closed triangles are calculated by aO × bO × 2cO.
Figure 6. X-ray diffraction patterns of (a) 1.2 mol % and (b) 6.7 mol % C2H5OH. Open circles of 1.2 mol % and 6.7 mol % are calculated 2θ values using aO × bO × cO and aM × bM × cM, respectively.

Figure 7. X-ray diffraction patterns of (a) 1.1 mol % and (b) 6.1 mol % C6H6. Highly modulated lattice and different monoclinic unit cell are required to explain the complicated X-ray diffraction pattern of 1.1 mol %. The different monoclinic lattice (a'M × b'M × c'M) is shown in Figure 8. Open circles and closed triangles in Figure 7a shows modulated lattice (a'M × 2b'M × 2c'M) and orthorhombic (aO × bO × cO). At 6.1 mol %, only simple monoclinic lattice (a'M × b'M × c'M) is enough to fit the X-ray diffraction pattern as shown in Figure 7b.
Figure 8. Different monoclinic lattice (a'M × b'M × c'M) has orientational relation with the orthorhombic one (aO × bO × cO).
.Figure 9. Mass effect such as lattice expansion is not seen in volume per [DEME][BF4], V4. x is concentration and M is atomic weight of each additions. Except for 0.9 mol % H2O and 1.1 mol % C6H6, no distinct volume change is found in [DEME][BF4]-based mixtures.
Figure 10. Geometrical relationship between the orthorhombic lattice and its sublattice is drawn on aO-cO plane. The sublattice is rhombic and the edge length of the rhombic is equal to 2bO.

References

(1) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391.
(2) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Chem. Lett. 2003, 32, 498.
(3) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett.2003, 32, 740.
(4) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 948.
(5) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnston,S.; Seddon, K. R.; Rogers, R. D. Chem. Commun. 2003, 1636.
(6) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard,O.; Hardacre, C.; Rogers, R. D. Chem. Commun. 2003, 476.
(7) Hanke, C. G.; Lynden-Bell, R. M. J. Phys. Chem. B 2003, 107,10873.
(8) Malham, I. B.; Letellier, P.; Turmine, M. J. Phys. Chem. B 2006, 110, 14212.
(9) Sato, T.; Masuda, G.; Takagi, K. Electrochim. Acta 2004, 49, 3603.
(10) Imai, Y.; Abe, H.; Goto, T.; Yoshimura, Y.; Kushiyama, S.; Matsumoto, H. J. Phys. Chem. B 2008, 112, 9841.
(11) Imai, Y.; Abe, H.; Goto, T.; Yoshimura, Y.; Michishita, Y.; Matsumoto, H. Chem. Phys. 2008, 352, 224.
(12) Gardon, M.; Pinheiro, C. B.; Chapuis, G. Acta Crystallogr. B 2003,59, 527.
(13) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.; Sheppard, O.; Soper, A. K. J. Phys. Chem. B 2005, 109, 1593.
(14) Harper, J. B.; Lynden-Bell, R. M. Mol. Phys. 2004, 102, 85.

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

Last Modified: April 1, 2009