Temperature-dependent structure and transport of ionic liquids with short-and intermediate-chain length pyrrolidinium cations

We report on an observed connection between the structural evolution and dynamics in a series of pyrrolidium-based ionic liquids. Temperature-dependent X-ray diffraction experiments were compared to viscosity and conductivity measurements for 1-alkyl-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([CnPyrr][Tf2N], n = 2, 3, 4, 6, 8). We find that the temperature evolution of the charge alternation peak in the structure factor, S(q), is consistent with intuitive order decoherence for shorter chain lengths, but anomalous for longer chain lengths. Structure data show that [C2Pyrr][Tf2N] behaves fundamentally differently than [C4Pyrr][Tf2N], [C6Pyrr][Tf2N] and [C8Pyrr][Tf2N] over the temperature range investigated with [C3Pyrr][Tf2N] behaving as an intermediate case. Both the conductivity and viscosity mirror these trends providing insight into the fundamental connection between structural ordering and dynamics in these liquids. •The temperature dependence the structure and the dynamics of ionic liquids are investigated.•Low energy laboratory and high energy synchrotron X-ray diffraction was utilized.•Viscosity and conductivity data were taken.•Anomalous charge alternation ordering is identified.•A fundamental connection between the static liquid structure and dynamics is identified.