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

Diego Pontoni, Marco DiMichiel, Moshe Deutsch

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Abstract

Using X-ray scattering from binary mixtures of [Cnmim][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 [C8mim][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, dI, decreases with x, exhibiting domination by [C12mim][NTf2] at least up to x≈0.5. The layering order's range decreases uniformly with x. dI is found to deviate positively from an ideal mixture spacing by up to ≲5%. The lateral spacings’ deviations are 10-fold smaller, implying the nanoscale excess volume to be also ≲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 dI deviations, but only marginally. The positive, and T-decreasing, dI 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.

Original languageEnglish
Article number116587
JournalJournal of Molecular Liquids
Volume338
DOIs
StatePublished - 15 Sep 2021

Bibliographical note

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Ionic liquids
  • Layering
  • Mixtures
  • Vegard's Law
  • X-ray scattering

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