Water Network in Room-Temperature Ionic Liquid: N,N-Diethyl-N-Methyl-N-2-Methoxyethyl Ammonium Tetrafluoroborate

H. ABE, Y. IMAI, T. GOTO, Y. YOSHIMURA, M. AONO, T. TAKEKIYO, H. MATSUMOTO, and T. ARAI

Department of Materials and Science and Engineering, Department of Applied Chemistry, Department of Applied Physics,
National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 41A, (2010) 1137.

Abstract

Crystal structures of room-temperature ionic liquid (RTIL)-H2O mixtures are determined by the X-ray diffraction method. The RTIL is N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF4]. At 0.9 mol pct H2O, two kinds of superstructures occur simultaneously without a strain. Also, the volume of the unit cell is very small only at 0.9 mol pct additives. This relates to the composite domain structure, including a twin-related one, as an elastic anomaly. At other water concentrations, such an extraordinary behavior is not observable. By assuming a sublattice having an equivalent lattice constant, a water network at 1 mol pct H2O is simulated using a Monte Carlo (MC) method. The network develops over the medium range in the simulation box.

Fig. 1.
X-ray diffraction patterns (h-2h scan) of pure [DEME][BF4], -0.6, -0.9, and -2.9 mol pct H2O at -80 oC. Open circles and closed triangles represent monoclinic and orthorhombic, respectively. At 0.9 mol pct, the calculated 2h values of two kinds of superstructures coincide with the observed ones.


Fig. 2.
FWHM of Bragg reflections along the radial direction (h-2h scan) at pure 0.9 and 2.9 mol pct H2O derived from the results of Fig. 1. Closed triangles show FWHM of Bragg reflections of standard Si polycrystalline for comparison.


Fig. 3.
Rocking curves along the transverse direction (h scan) on Debye rings measured at -80 oC.
Fig. 4.
Schematic crystal domain formations are drawn at each water concentration. Ellipsoidal shapes and a pair of parallelograms reveal conventional domains and twin-related ones, respectively. Characteristic twin-related domains appear inside the conventional domains at 0.9 mol pct.
Fig. 5.
Three possible water molecules on a sublattice, aO, bO, and cO, reveal orthorhombic lattice constants. Orthorhombic lattice on a aO-cO plane is expressed by dotted lines. The sublattice on the orthorhombic grids is represented by two parallelograms, i.e., P0-P1-Q1-P3 and P0-P2-Q2-P3. The sublattice possesses the equivalent lattice constant, 2.7 nm. Considering double layers along the bO direction, one sublattice contains 12 orthorhombic unit cells. Under the specific constraint of one water molecule in two sublattices, three types of water bonding on the sublattice such as P1-Q1 and P2-Q2, P0-P3, and Q1-P3-Q2 are indicated by bold red lines.
Fig. 6.
Water network in two-dimensional MC simulations. Light blue grids represent the orthorhombic lattice on a aO-cO plane. Closed and open circles correspond to bound and unbound water molecules on the orthorhombic lattice, respectively. Red lines correspond to 2.7-nm bonding (=aW), where pairs of water molecules are trapped on the lattice sites: (a) including no assumption (randomly located), (b) assumption 1 (r>aW), and (c) assumptions 1+2 (r>aW and hopping) (the text provides details). By repulsive water interactions, water network develops over long range.
Fig. 7.
Water network in 3-D MC simulations. Closed red circles are 2.7-nm bonding water. Pairs of the bonding water are expressed by red lines. Open circles are nonbonding water molecules. (a) Only with assumption 1, constraint of r>aW (=2.7 nm) is employed in the simulations. Geometrical restrictions on aO-cO planes suppress the development of water network on the planes. Along the bO direction, chain networks develop relatively. (b) Assumptions 1 and 2 are considered: constraints of r>aW and hopping. By random walk of nonbonding water, anisotropic water network disappears.
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ab@nda.ac.jp
Department of Materials Science and Engineering
National Defense Academy

Last Modified: April 1, 2009