Study on the Thermodynamic Properties of Ionic Liquids 1-Hexyl-3-methylimidazolium Halide with Isopropanol Mixtures

In this work, densities and viscosities of the mixtures of ionic liquids (ILs) 1-hexyl-3-methylimidazolium halides ([C₆mim]X, where X = Cl^–, Br^–, and I^–) and isopropanol (IPA) were measured over the temperature ranging from 288.15 to 323.15 K to investigate the influence of anions, IL concentration, and temperature on the physical properties of the mixtures of ([C₆mim]X + IPA). Excess volumes (V^E) and viscosity deviations (Δη) were also calculated to study the nonideal behavior of ([C₆mim]X + IPA). The V^E and Δη values of ([C₆mim]X + IPA) are negative over the whole compositional range at all temperatures, indicating that IPA molecules preferentially coordinate with IL ions to form more densely packed structures. In addition, the molar enthalpies of mixing (Δ_mixH) for the mixtures were determined under 298.15 and 308.15 K, and the nonrandom two-liquid model along with the Gibbs–Helmholtz equation was introduced to describe Δ_mixH for the studied systems. The mixtures of ([C₆mim]Br + IPA) and ([C₆mim]I + IPA) showed endothermic behavior within the full range of compositions, while the mixtures of ([C₆mim]Cl + IPA) showed endothermic first and then changed to exothermic behavior with increasing IL mole fraction.