Local order and long range correlations in imidazolium halide ionic liquids: a combined molecular dynamics and XAS study

Abstract

A thorough characterization of the structural properties of alkylimidazolium halide ionic liquids (ILs), namely 1-alkyl-3-methylimidazolium bromide ([C_nmim]Br with n = 5, 6, 8, 10) and iodide ([C₆mim]I), has been carried out by combining molecular dynamics simulations and EXAFS spectroscopy. The existence of a local order in [C_nmim]Br ILs has been evidenced, with anions and imidazolium head groups forming a local three-dimensional bonding pattern that is common to all the [C_nmim]Br IL family, regardless of the length of the alkyl chain attached to the cation. On the other hand, upon alkyl chain elongation significant differences have been highlighted in the long-range structure of these ILs. Theoretical X-ray structure factors have been calculated from MD simulations and a low q peak has been found for all [C_nmim]Br ILs, indicating the existence of long-range structural correlations. The low q peak moves to smaller q values corresponding to longer distances, increases in intensity and sharpens with increasing alkyl chain length on the cation. Similarities and differences between the ion three-dimensional arrangements in [C₆mim]Br and [C₆mim]I were highlighted and the structural arrangement of Br⁻ and I⁻ was found to be different in the proximity of the most acidic hydrogen atom of the imidazolium ring: the I⁻ ion is preferentially located above and below the ring plane, while the Br⁻ ion has a high probability also to be coplanar with the imidazolium ring. A quantitative analysis of the Br and I K-edge EXAFS spectra of alkylimidazolium halide ILs has been carried out based on the microscopic description of the systems derived from MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the alkylimidazolium halide ILs investigated in this work.