Liquid Structure and the Ion-Ion Interactions of Ethylammonium Nitrate Ionic Liquid Studied by Large Angle X-Ray Scattering and Molecular Dynamics Simulations

Yasuhiro UMEBAYASHIa*, Wan-Lin CHUNGa, Takushi MITSUGIa, Shuhei FUKUDAa, Munetaka TAKEUCHIa, Kenta FUJIIb, Toshiyuki TAKAMUKUb, Ryo KANZAKIa and Shin-ichi ISHIGUROa

aDepartment of Chemistry, Faculty of Science, Kyushu University
Hakozaki, Higasi-ku, Fukuoka, 812-8581, Japan
bDepartment of Chemistry and Applied Chemistry, Faculty of Science and Engineering
Saga University, Honjo-machi, Saga 840-8502, Japan

(Received: July 1, 2008; Accepted for publication: September 16, 2008; Advance publication: November 22, 2008)

  Room temperature ionic liquids (RTILs) are useful as new solvents or materials owing to their favorable properties such as negligible vapor pressure, non-flammability, and wide electrochemical window. For further development of new RTILs, it is indispensable to understand macroscopic properties as solvent and/or liquids of RTILs at a molecular or atomistic level. Here, liquid structure and the ion-ion interactions of room temperature ionic liquid ethylammonium nitrate (EAN) were studied by means of large angle X-ray scattering (LAXS) experiment and molecular dynamics simulations. X-ray interference function for EAN shows a small peak of 0.62 A–1 indicating nano-scale segregation in the ionic liquid. X-ray radial distribution function as the form of D(r) - 4pr2r0 evidently shows a peak of 3.4 and broad ones of 4.7, 8 and 12 A, suggesting that EAN has long range ordering in the liquid state. The intra-molecular X-ray interference function was estimated on the basis of molecular geometries found in crystals to yield the inter-molecular X-ray pair correlation functionG_mathrm{inter}mathrm{LAXS}(r). In G_mathrm{inter}mathrm{LAXS}(r), peaks of 3.0, 3.4, and 4.7 A were found as the atom-atom correlation. The peak of 3.0 A can be assigned to the atom-atom correlation of N (C2H5NH3+)...O (NO3) in the NH...O hydrogen bonding. In addition, the peak of 3.4 A is also ascribable to C (C2H5NH3+)...O (NO3) correlations in the CH...O interactions. Molecular dynamics simulations based on newly developed force fields to describe a series of primary ammonium cation were performed to ascribe the peaks found in G_mathrm{inter}mathrm{LAXS}(r). The X-ray interference function imathrm{MD}(s) and the pair correlation function Gmathrm{MD}(r) derived from simulations were reasonably in agreement with the experimental ones. According to the partial atom-atom correlation functions derived from the simulations, the experimentally observed peaks of 3.0 and 3.4 A can be ascribable to the NH...O and the CH...O correlations in the closest ethylammonium and nitrate interaction, respectively. The CH...O interactions may play an important role in macroscopic properties of this kind of ionic liquids.

キーワード: Room temperature ionic liquids, Liquid structure, Large angle X-ray scattering, Molecular dynamics simulations

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