Physicochemical and thermodynamic properties of pyrrolidinium-based ionic liquids and their binary mixtures with carboxylic acids

Abstract Two oxopyrrolidinium-based ionic liquids (ILs) with different functional groups namely, epoxypropane and dihydroxy propane in cation, were synthesized in this work. Propanoic acid and butanoic acid were mixed with the synthesized ILs to create a series of binary liquid mixtures with various mole fractions. The physicochemical properties of density (ρ) and speed of sound (u) of the binary systems were measured in the range of 0.1–1.0 mol fraction at temperatures from 293.15 to 313.15 K at a 5 K interval under atmospheric pressure (p = 101 kPa). Based on these experimental data, the excess molar volume, (VmE), isentropic compressibility, (ks), deviation in isentropic compressibility (∆ks) and intermolecular free length (Lf) were then calculated. Furthermore, the interactions such as H-bond, ionic interaction, dipole-dipole interactions and weak van der Waals interactions among the IL and carboxylic acid were inferred through the experimental and derived data. These interactions appear to play a major role in determining the potential usefulness of these IL-acid binary systems. The negative values of VmE and ∆ks may be indicative of strong interactions between IL and acid. A long carbon chain of acid appears to enhance the interaction strength that could be attributed to the increase in viscosity of systems due to the longer alkyl chain and big size of acid. The derived thermodynamic properties, including VmE, ∆ks and Lf were fitted in Redlich-Kister polynomial equation and standard deviation.