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

Abstract

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 0.62Å -1 indicating nano-scale segregation in the ionic liquid. X-ray radial distribution function as the form of D(r) -4πr 2 ρ 0 evidently shows a peak of 3.4 and broad ones of 4.7, 8 and 12Å, 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 LAXS inter (r). In G LAXS inter (r), peaks of 3.0, 3.4, and 4.7Å were found as the atom-atom correlation. The peak of 3.0Å can be assigned to the atom-atom correlation of N (C 2 H 5 NH 3 + ) O (NO 3 -) in the NH O hydrogen bonding. In addition, the peak of 3.4Å is also ascribable to C (C 2 H 5 NH 3 + ) O (NO 3 -) 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 LAXS inter (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 3.4Å 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.

Acidity and Basicity of Aqueous Mixtures of a Protic Ionic Liquid, Ethylammonium Nitrate

“…It is supposed that KS varies in a complicated manner by varying the solvent composition, because, the C2H5NH3 + ion is hydrogen-bonded with three NO3 -ions in neat EAN, according to our recent study by means of X-ray scattering and MD simulation, 20 and the hydrogen-bonded structure is ruptured upon the addition of water to EAN, since the ability of water to form hydrogen bonds is stronger than that of NO3 -. Typical potentiometric titration curves obtained in the mixtures of EAN mole fraction x = 0.5 and 0.9 are shown in Fig.…”

Physicochemical and Acid-base Properties of a Series of 2-Hydroxyethylammonium-based Protic Ionic Liquids

“…71,72 Hence, the hydrogen bonding between cation-cation and cation-anion plays essential roles in macroscopic properties of the PILs composed of cations with the 2-hydroxyethyl group(s). Large angle neutron 63 and X-ray 71,73 scattering experiments with the aid of molecular simulations may provide direct structural evidences at an atomistic level, and are now going on. (n = 0 -3) decreased with increasing the number of the 2-hydroxyethyl groups.…”

Structural Aspect on Li Ion Solvation in Room-Temperature Ionic Liquids

“…Soper et al firstly applied such combination of experimental and simulation data to the structural investigation of ILs, i.e., the large-angle neutron diffraction with empirical potential structure refinement (EPSR) based on Monte-Carlo simulations [41,42]. We also reported liquid structure of various IL systems such as imidazolium- [27,32,35], pyrrolidinium- [26,43], and ammonium [30,44]-based ILs studied by large-angle X-ray scattering or high-energy X-ray diffraction (LAXS and HEXRD, respectively) with the aid of molecular dynamics (MD) simulations. In general, the liquid structure and ion-ion interaction obtained from X-ray/neutron experiment or computer simulation study are discussed in terms of radial distribution function given by Furrier transformation of the structure factor, S(q) where q denotes the scattering vector.…”