Thermodynamic Study of Ionic Liquids 1‑Hexyl-3-methylimidazolium Halide with Methanol Mixtures

To investigate the effects of anion size, solvent content, and temperature on their physical properties, in this work, densities and viscosities of three binary mixtures, including ionic liquids (ILs) composed of the cation 1-hexyl-3-methylimidazolium (i.e., [C₆mim]⁺) and the halide anions (i.e., Cl^–, Br^–, and I^–), along with methanol (MeOH), were measured across the entire concentration range at temperatures from 288.15 to 323.15 K, and enthalpies of mixing were determined at temperatures of 298.15 and 308.15 K. Excess volumes and viscosity deviations were calculated to study the nonideal behavior of the mixtures. The results of excess volumes suggest that a combination of intermolecular interactions and packing effects contributes to their nonideal behaviors, with packing effects likely dominating the negative excess volume. Viscosity deviations and enthalpies of mixing indicate that MeOH molecules intend to coordinate with IL ions (IL cations and IL anions) in the IL-rich region, with stronger interactions observed between [C₆mim]Cl and MeOH compared to ([C₆mim]Br + MeOH) and ([C₆mim]I + MeOH) mixtures. Additionally, the nonrandom two-liquid model and the Gibbs–Helmholtz equation were combined to describe the enthalpies of mixing for the studied mixtures reliably.