Chain-length dependent organisation in mixtures of hydrogenous and fluorous ionic liquids

As part of an ongoing study of the structure and properties of mixtures of ionic liquids in which one component has a hydrocarbon chain and the other a semiperfluorocarbon chain, we now report a study of the mixtures [C₈MIM]_1-[C₁₀MIM-F₁₇][Tf₂N], [C₁₀MIM]_1-[C₈MIM-F₁₃][Tf₂N] and [C₁₀MIM]_1-[C₁₀MIM-F₁₇][Tf₂N], where [C₈MIM][Tf₂N] is 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide, [C₁₀MIM][Tf₂N] is 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C₈MIM-F₁₃][Tf₂N] is 1-(1H,1H,2H,2H-perfluorooctyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide and [C₁₀MIM-F₁₇][Tf₂N] is 1-(1H,1H,2H,2H-perfluorodecyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide. The mixtures were investigated using small-angle X-ray (SAXS) and neutron (SANS) scattering complemented by molecular dynamics simulations (with viscosity and surface tension measurements also possible for the mixtures [C₁₀MIM]_1-[C₈MIM-F₁₃][Tf₂N]). Unlike previous studies of [C₈MIM]_1-[C₈MIM-F₁₃][Tf₂N], where no strong evidence of alkyl/fluoroalkyl chain segregation or triphilic behaviour was seen (Elstone et al., J. Phys. Chem. B, 2023, 127, 7394-7407), these new mixtures show the formation of small aggregates of varying sizes of each component, even though all were co-miscible across the full range of compositions. Thus, while a clear polar non-polar peak (PNPP) was observed at large or small values of x, at intermediate compositions the small-angle neutron scattering at low q was dominated by scattering from these small aggregates, while at other compositions, there was little or no evidence of the PNPP. The origins of this behaviour are discussed in terms of inter-chain interactions.