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 solventsor materials owing to their favorable properties such as negligiblevapor pressure, non-flammability, and wide electrochemical window.For further development of new RTILs, it is indispensable to understandmacroscopic properties as solvent and/or liquids of RTILs at a molecularor atomistic level. Here, liquid structure and the ion-ion interactionsof room temperature ionic liquid ethylammonium nitrate (EAN) werestudied by means of large angle X-ray scattering (LAXS) experimentand molecular dynamics simulations. X-ray interference function forEAN ˚ shows a small peak of 0.62 A–1 indicating nano-scale segregationin the ionic liquid. X-ray radial distribution ˚ function as theform of D(r) – 4πr 2 ρ0 evidently shows a peak of 3.4 and broadones of 4.7, 8 and 12 A, suggesting that EAN has long range orderingin the liquid state. The intra-molecular X-ray interference functionwas estimated on the basis of molecular geometries found in crystalsto yield the inter-molecular X-ray ˚ pair correlation function GLAXS(r). In GLAXS (r), peaks of 3.0, 3.4, and 4.7 A were found as theatom-atom inter inter ˚ can be assigned to the atom-atom correlationof N (C2 H5 NH3 + ) O (NO3 – ) correlation. The peak of 3.0 A ˚in the NH O hydrogen bonding. In addition, the peak of 3.4 A is alsoascribable to C (C2 H5 NH3 + ) O (NO3 – ) correlations in the CHO interactions. Molecular dynamics simulations based on newly developedforce fields to describe a series of primary ammonium cation wereperformed to ascribe the peaks found in GLAXS (r). The X-ray interferencefunction iMD (s) and the pair correlation function GMD (r) derivedfrom inter simulations were reasonably in agreement with the experimentalones. According to the partial atom-atom ˚ correlation functionsderived from the simulations, the experimentally observed peaks of3.0 and 3.4 A can be ascribable to the NH O and the CH O correlationsin the closest ethylammonium and nitrate interaction, respectively.The CH O interactions may play an important role in macroscopic propertiesof this kind of ionic liquids.