Thermodynamic Study of Ionic Liquid Mixtures of 1-Hexyl-3-methylimidazolium Halide and Dimethyl Sulfoxide

This study measured densities and viscosities of ionic liquid (IL) mixtures comprising 1-hexyl-3-methylimidazolium halides ([C₆mim][Halide], where Halide = Cl^–, Br^–, and I^–) with dimethyl sulfoxide (DMSO) (288.15–323.15 K) and enthalpies of mixing (Δ_mixH) at 298.15 and 308.15 K. Excess volumes (V^E) and viscosity deviations (Δη) were calculated to investigate nonideal behavior. Negative V^E values indicate the formation of dense structures dominated by packing effects. Δη values suggest DMSO disrupts cation–anion interactions, causing deviations from ideality. Consistently negative Δ_mixH values confirm strong ion–solvent interactions, with ([C₆mim]I + DMSO) showing the most negative Δ_mixH values, attributed to stronger iodide–DMSO interactions and weaker cation–anion interactions. The nonrandom two-liquid model and Gibbs–Helmholtz equation successfully modeled Δ_mixH, yielding ARDs of 3.9%, 4.5%, and 3.2%, respectively. Qualitative analysis with cosolvents (H₂O, MeOH, IPA) reveals the influences of cosolvent and anion on mixture properties. In organic solvents, V^E and Δη trends correlate with solvent molar volume and polarity. Specific interactions lead to unique V^E variations in DMSO mixtures, whereas extensive hydrogen bonding causes abnormal V^E and Δη trends in aqueous IL mixtures. Δ_mixH variations reflect the competition between ion–solvent and cation–anion interactions, influenced by solvent dielectric constant, polarity, and anion size.