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_nmim][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, d_I, decreases with x, exhibiting domination by [C₁₂mim][NTf₂] at least up to x≈0.5. The layering order's range decreases uniformly with x. d_I 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 d_I deviations, but only marginally. The positive, and T-decreasing, d_I 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.