Relationship between low-Q peak and long-range ordering of ionic liquids revealed by high-energy X-ray total scattering.

The structures of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([CnmIm(+)][TFSA(-)]) ionic liquids (alkyl-chain length n = 4, 8, 10, and 12) have been studied by high-energy X-ray total scattering at T = 298-453 K. The low-Q peaks observed in the X-ray structure factors S(Q)s at 0.2 < Q < 0.4 A(-1) for n≤ 8 were almost unchanged up to T = 453 K, whereas they shifted to lower Q values and their peak intensity decreased for n = 10 and 12. The radial distribution functions G(r)s for n≤ 8 showed no temperature dependence, but the G(r)s in the long r region showed a significant temperature dependence, particularly for r = 15-25 Å for n = 10 and 12. To discuss the relationship between the low-Q peak intensity in S(Q) and the profile of G(r) at long range, we propose a new function S(Qpeak)(r). It is elucidated that the low-Q peak observed for n = 12 is a signature of long-range ordering in the r range 15-25 Å, which corresponds to both of the anion-anion correlations at the first neighbor and the alkyl group aggregates among the C12mIm(+) cations, whereas the anion-anion correlations at the first neighbor are predominant for reproducing the low-Q peaks in the n = 8 and 10 systems.