Binary mixtures of homologous room-temperature ionic liquids: Nanoscale structure evolution with alkyl lengths' difference

The nanoscale structure of equimolar binary mixtures [C(12)mim](0.5)[C(n)mim](0.5)[NTf2] was studied by smal-langle X-ray scattering for n = 1 - 22 and T = 293 - 373 K. All mixtures exhibit local layering and layer-normal thermal contraction with increasing T, as found for the pure components. The layer-parallel spacings of the polar headgroups and of the alkyl chains vary minimally with n over the full n range. The layer-normal spacing d(I) at high n follows closely, with a 1-1.5 A downshift, the increasing trend of the pure longer component's d(I), indicating its dominance of the layering. At low n; dI at n = 1 greatly exceeds d(I) of the pure longer component, n = 12, and decreases sharply with increasing n, indicating a structure akin to lipid bilayer solutions. At intermediate n; d(I) is roughly constant, lying 1-1.5 A below d(I) of n = 12. Our layer spacings provide a near-unique opportunity to study the evolution over a wide n-range of the normalized d(I)'s molecular-scale deviation from ideality, yielding a linear dependence on the normalized n-difference squared. This may be related to the same-dependence interchange energy due to chain length mismatch, dominating binary alkane and alcohol mixtures. Finally, our d(I) also show the power P of the modified Vegard's Law to be linear in the mixtures' normalized n-mismatch, albeit with different low- and high-n slopes. (C) 2022 Elsevier B.V. All rights reserved.