Water Network in Room-Temperature Ionic Liquid: N,N -Diethyl- N -Methyl- N -2-Methoxyethyl Ammonium Tetrafluoroborate
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INTRODUCTION
ROOM-TEMPERATURE ionic liquids (RTILs) are well-known new solvents in ‘‘green chemistry.’’ The curious nature of RTILs is represented by almost zero vapor pressure. The RTILs are distilled at low pressure without decomposition.[1] The RTILs are widely applied as clean solvents and catalysts[2] in photochemistry and separation chemistry. Many studies of the crystal structures at low temperature have been carried out to obtain the molecular interactions in RTILs. For instance, the orientational order of molecules is reflected in space group in crystallography. In X-ray powder patterns, two kinds of crystal structure of 1-butyl-3-methylimidazolium chloride, [C4mim]Cl, were found.[3] Crystal structural studies have also been performed using single crystals. The crystal structure of [C4mim]Cl was determined to be monoclinic (P21/n, Z = 4).[4] The detailed analysis HIROSHI ABE, Associate Professor, MASAMI AONO, Research Associate, YUSUKE IMAI, Graduate Student, and HITOSHI MATSUMOTO, Professor, Department of Materials Science and Engineering, and TAKASHI ARAI, Professor, Department of Applied Physics, are with the National Defense Academy, Yokosuka 239-8686, Japan. Contact e-mail: [email protected] TAKEFUMI GOTO, Graduate Student, YUKIHIRO YOSHIMURA, Associate Professor, and TAKAHIRO TAKEKIYO, Assistant Professor, are with the Department of Applied Chemistry, National Defense Academy, Yokosuka 239-8686, Japan. This article is based on a presentation given in the symposium ‘‘Neutron and X-Ray Studies of Advanced Materials,’’ which occurred February 15–19, 2009, during the TMS Annual Meeting in San Francisco, CA, under the auspices of TMS, TMS Structural Materials Division, TMS/ASM Mechanical Behavior of Materials Committee, TMS: Advanced Characterization, Testing, and Simulation Committee, and TMS: Titanium Committee. Article published online July 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
suggests that each chloride anion is hydrogen bonded to three different imidazolium cations. On the other hand, the crystal structure of [C4mim]Br is found to be orthorhombic (Pna21, Z = 4).[5] Recently, RTIL-based mixtures are gathering interest and are in progress for applications. Benzene and toluene as additives into the RTIL cause a monoclinic structure, though pure [C4mim]Cl and [C4mim]Br are assigned to orthorhombic (Pna21, Z = 4) and [C4mim]Cl/benzene and [C4mim]Cl/ toluene are monoclinic (P21/c, Z = 4).[6] Also, cationbenzene ‘‘sandwich’’ packing is realized in 1,3-dimethylimidazolium hexafluorophosphate, [C1mim][PF6]50 mol pct C6H6 mixture.[7] In contrast, the pure [C1mim][PF6] forms zigzag chains. It has been discovered that the benzene disturbs the RTIL network. Two kinds of RTILs, N,N-diethyl-N-methyl-N-2methoxyethyl ammonium tetrafluoroborate and bis(trifluoromethylsulfonyl)imide, [DEME][BF4], and [DEME][TFSI] are typically synthesized for electrochemical capacitors, which have quite wide potential windows.[8] [DEME][BF4] is hydrophilic and [DEME][TFSI] is hydrophobic. Fundamental physical properties of both [
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