Molecular Modeling of Ionic Liquids Combined with NMR and SAXS Studies

Molecular dynamics simulations were performed on piperidinium-based ([C4mpip⁺][Tf2N^-]) and pyrrolidinium-based ([C_nmpy⁺]][Tf₂N^-], n=3, 4, 6, 8, 10) ionic liquids at 1 atm over a wide range of temperatures. The temperature-dependent transport properties of free ionic liquids, including diffusion coefficients and reorientational correlation time were calculated and were found to agree very well with the results measured in NMR and Fluorescence anisotropy experiments. The computed small angle X-ray structure factors of ([C_nmpy⁺]][Tf₂N^-] at varying temperatures exhibited a good agreement with the corresponding SAXS experimental results.  Spatial heterogeneity of ([C_nmpy⁺]][Tf₂N^-] was also investigated. With the alkyl chain length increasing, the diffusion was slower. Additionally, the tail-tail radial distribution function showed that as the alkyl chain length increases, the nanoaggregation of cations becomes more significant. This result was also proved in the SAXS experiments.