Anomalous temperature dependence in the structural organization of charge alternation in imidazolium-based ionic liquids of various alkyl chain lengths

We report on trends observed in the X-ray static structure factor obtained for four homologous imidazolium-based ionic liquids. In a series of low-energy (laboratory) and high-energy synchrotron X-ray diffraction experiments, the temperature dependent structure was investigated in 1-alkyl-3-methylimidazolium cations, with chain lengths of n = 4, 6, 8, and 10, paired with the trifluoromethylsulfonate (TF) anion. Interestingly, the charge alternation peak in the liquids displays an anomalous temperature evolution, increasing in amplitude as the temperature is raised. Analysis of the pair distribution function (PDF) evolution with temperature for all compositions attributes the observed anomalous behavior to a specific distance in the liquid. Extensive peak modelling of the structure factors confirms the anomaly for all compositions, which combined with the analysis of the temperature of the PDFs, suggests that the entropic tendency to decohere strict charge alternation overcomes enthalpic tendencies in these liquids.