Influence of temperature on thermodynamics for binary mixtures of short aliphatic protic ionic liquids

Two binary mixtures of protic ionic liquids comprising formate, acetate and propionate anions and 2-hydroxyethyl ammonium, bis(2-hydroxyethyl) ammonium and tris(2-hydroxyethyl) ammonium cations have been studied in terms of volumetric and acoustic properties as a function of temperature. The corresponding derived properties have been computed from the experimental data and fitted to temperature dependent Redlich–Kister and Redlich–Mayer equations; accurate results being obtained. Other properties such as intermolecular free length, acoustic impedance, geometrical volume, collision factor and isothermal coefficient of pressure excess molar enthalpy were computed due to their importance in the study of specific molecular interactions. The new experimental data were used to test the prediction capability of different models for density (Mchaweh–Nasrifar–Moshfeghian (MNM) model and the modified Heller temperature dependent equation (MHE)) and ultrasonic velocity (different empirical equations, collision factor theory (CFT) and free length theory (FLT)). The high non-ideality of these mixtures points to strong contractive behaviour at any temperature and composition. The obtained results indicate that ionic liquid interactions into mixture are stronger than in the ionic pure components for both mixtures at any studied condition. Despite the strong ion interaction, the tested models showed, at least, the qualitative prediction capability.