Ion-ion interaction in room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate studied by large angle X-ray scattering experiment and molecular dynamics simulations

Large angle X-ray scattering (LAXS) experiment for 1-ethyl-3-methylimidazolium tetrafluoroborate [EMI+][BF4−] ionic liquid was carried out at 298 K, to reveal the closest ion-ion interaction in the ionic liquid. The intra-molecular atom-atom correlations based on the molecular geometries in crystals were subtracted from the total pair correlation function to yield the inter-molecular pair correlation function. In the inter-molecular pair correlation function, peaks of 3.4, 4.4 and 5.5 Å were successfully extracted as the closest ion-ion interactions. Molecular dynamics simulations based on the effective pair potentials were also performed to ascribe the peaks found in the experimentally evaluated inter-molecular pair correlation function at an atomistic level. The X-ray interference function derived from MD simulations and thus Fourier transferred X-ray weighted pair correlation function were reasonably agreement with those obtained by the experiment, the extracted peaks in the experimental inter-molecular pair correlation function can be appropriately attributed to the C (EMI+) - F (BF4−) atom-atom correlations. In addition, artificial MD simulations employed rigid models for the nonplanar and the planar EMI+ isomers were performed to obtain further insight into the effect of the conformational isomerism on the ion-ion interactions in the ionic liquid. It turned out that the ion-ion interaction, namely the anion orientation around the cation, depends on the conformational isomerism of the cation from the artificial MD simulations.