Toward Understanding the Structural Heterogeneity and Ion Pair Stability in Dicationic Ionic Liquids

The structural and dynamical properties of dicationic ionic liquids (DILs) 1,3- bis[3- methylimidazolium-1-yl]alkane bis(trifluoromethylsulfonyl)amide [C_n(mim)₂](Tf₂N)₂  were investigated by molecular dynamics (MD) simulation in combination with small-angle X-ray scattering (SAXS) measurement. Enhanced spatial heterogeneity is observed with the prolongation of chain length in DILs, accompanied with an increased heterogeneity order parameter (HOP). The structure function calculated from MD simulation agrees qualitatively with that obtained from SAXS. Temperature variation results in slight influences on the local structural assembly of DILs and all the three peaks at 0.4 Å^-1, 0.9 Å^-1 and 1.4 Å^-1 of the structure function shift towards low Q as temperature increases. The change in peak position by the temperature is also supported by the temperature-independent HOP values. The prepeak shifts are in opposite directions in DILs and MILs as temperature increases. This is revealed in MD simulation for the first time, possibly indicating an RTIL-specific prepeak shifting. Furthermore, the high ion pair/ion cage stability in DILs is indicative of the high thermal stability and insensitivity of structural heterogeneity to temperature variation, which can likely be attributed to the stronger Columbic network interaction in DILs due to the additional charges in dications.