Experimental evidences for molecular origin of low-Q peak in neutron/x-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide ionic liquids

Short- and long-range liquid structures of [C(n)mIm(+)][TFSA(-)] with n = 2, 4, 6, 8, 10, and 12 have been studied by high-energy x-ray diffraction (HEXRD) and small-angle neutron scattering (SANS) experiments with the aid of MD simulations. Observed x-ray structure factor, S(Q), for the ionic liquids with the alkyl-chain length n > 6 exhibited a characteristic peak in the low-Q range of 0.2-0.4 angstrom (-1), indicating the heterogeneity of their ionic liquids. SANS profiles I-H(Q) and I-D(Q) for the normal and the alkyl group deuterated ionic liquids, respectively, showed significant peaks for n = 10 and 12 without no form factor component for large spherical or spheroidal aggregates like micelles in solution. The peaks for n = 10 and 12 evidently disappeared in the difference SANS profiles Delta I(Q) [= I-D(Q) - I-H(Q)], although that for n = 12 slightly remained. This suggests that the long-range correlations originated from the alkyl groups hardly contribute to the low-Q peak intensity in SANS. To reveal molecular origin of the low-Q peak, we introduce here a new function; x-ray structure factor intensity at a given Q as a function of r, S-Qpeak(r). The S-Qpeak(r) function suggests that the observed low-Q peak intensity depending on n is originated from liquid structures at two r-region of 5-8 and 8-15 angstrom for all ionic liquids examined except for n = 12. Atomistic MD simulations are consistent with the HEXRD and SANS experiments, and then we discussed the relationship between both variations of low-Q peak and real-space structure with lengthening the alkyl group of the C(n)mIm. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672097]