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

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 angstrom(-1) indicating nano-scale segregation in the ionic liquid. X-ray radial distribution function as the form of D(r) -4 pi r(2)rho(0) evidently shows a peak of 3.4 and broad ones of 4.7, 8 and 12 angstrom, 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 function G(inter)(LAXS)(r). In G(inter)(LAXS)(r), peaks of 3.0, 3.4, and 4.7 angstrom were found as the atom-atom correlation. The peak of 3.0 angstrom can be assigned to the atom-atom correlation of N (C2H5NH3+)center dot center dot center dot O (NO3-) in the NH center dot center dot center dot O hydrogen bonding. In addition, the peak of 3.4 angstrom is also ascribable to C (C2H5NH3+)center dot center dot center dot O (NO3-) correlations in the CH center dot center dot center dot 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(inter)(LAXS)(r). The X-ray interference function i(MD)(s) and the pair correlation function G(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 angstrom can be ascribable to the NH center dot center dot center dot O and the CH center dot center dot center dot O correlations in the closest ethylammonium and nitrate interaction, respectively. The CH center dot center dot center dot O interactions may play an important role in macroscopic properties of this kind of ionic liquids.