Structure and dynamics of aromatic and alkyl substituted Imidazolium-based ionic liquids

The structure and dynamics of two ionic liquids based on imidazolium cations with the common anion dicyanamide, [N(CN)(2)](-), were studied and compared: one with an aromatic side chain, 1-benzyl-3-methylimidazolium dicyanamide [BzC(1)Im] [N(CN)(2)], and another with a normal alkyl chain, 1-n-heptyl-3-methylimidazolium dicyanamide [C(7)C(1)Im][N(CN)(2)]. We use classical molecular dynamics (MD) simulations in order to unveil subtle structural differences between these two systems. The collective dynamics were analyzed by considering the spectra of the mass current correlation functions, which were analyzed within the framework of the Zwanzig-Mori formalism. This allowed to compare the effect of aromatic or non-aromatic substituents on the high-frequency collective dynamics and its relationship to a low-frequency transport property such as viscosity. It is found that even though the aromaticsubstituted system is a denser and more viscous liquid, the high-frequency dynamics at finite momentum transfers and frequencies, i.e. within the length and timescales of the intermediate-range order (IRO), is only slightly altered. In the low-wavevector limit, however, there are significant differences in the vibrational density of states and viscosity. The scenario put forward by the MD simulations is that even though the role of ionic packing dominates over specific interactions, the different structural motifs within the IRO range imply signatures on the macroscopic dynamics of aromatic and aliphatic substituted imidazolium ionic liquids. (C) 2021 Elsevier B.V. All rights reserved.