Binary mixtures of homologous room-temperature ionic liquids: Temperature and composition evolution of the nanoscale structure

Using X-ray scattering from binary mixtures of [C(n)mim][NTf2] room temperature ionic liquids (RTILs) with n = 8, 12 we study their nanoscale layering and its evolution with temperature T and mole fraction x of [C(8)mim][NTf2]. The layers' lateral structure, dominated by the common headgroups' Coulomb interaction in the layer's polar slab, and by the chain-chain van der Waals interaction in the apolar slab, hardly changes. However, the longitudinal layer spacing, d(1), decreases with x, exhibiting domination by [C(12)mim][NTf2] at least up to x approximate to 0.5. The layering order's range decreases uniformly with x. d(1) is found to deviate positively from an ideal mixture spacing by up to less than or similar to 5%. The lateral spacings' deviations are 10-fold smaller, implying the nanoscale excess volume to be also less than or similar to 5%, 100-fold larger than those obtained from macroscopic molar density measurements. This gap is probably bridged at the larger length scales of these RTILs' hierarchical order. Increasing T decreases the d(1) deviations, but only marginally. The positive, and T-decreasing, d(1) deviations from ideality contrast strongly with the negative, 100-fold smaller, and T-increasing, deviations found for liquid normal-alkane mixtures of the same lengths, but fully agree with the positive similar-percent deviations obtained from the modified Vegard's law for soft-solid rotator phases of binary mixtures of alkanes and of alcohols. These results attest against a liquid-like chain packing in the apolar slab of the RTILs, but strongly support an interdigitated, roughly layer normal, chain packing. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

X-ray scattering was used to investigate the nanoscale layering of binary mixtures of [C(n)mim][NTf2] room temperature ionic liquids (RTILs) with n = 8, 12. It was found that the lateral structure of the layers is dominated by the Coulomb interaction of the common headgroups in the polar slab and the van der Waals interaction of the chain-chains in the apolar slab, with the longitudinal layer spacing decreasing with increasing mole fraction x. These deviations from ideal mixing and the hierarchical order of RTILs support an interdigitated chain packing in the apolar slab.