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

The nanoscale structure of equimolar binary mixtures [C₁₂mim]_0.5[C_nmim]_0.5[NTf₂] was studied by small-angle 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 Å downshift, the increasing trend of the pure longer component's d_I, indicating its dominance of the layering. At low n,d_I 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 Å 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.