Thermophysical Characterization of Protic Ionic Liquids: Density Measurements, Group Contribution Method, and PC-SAFT Modeling Application

The ionic liquid (IL) research area is growing exponentially, mainly to evaluate aprotic ionic liquids to replace conventional organic solvents. However, the cost of producing ILs could be disadvantageous to their application. Alternatively, protic ionic liquids could reduce the price of ionic liquids utilized in industry. However, more physicochemical characterization is needed for complete industrial application. In this work, we studied four different protic ionic liquids: bis(2-hydroxyethyl)ammonium pentanoate [BHEA][Pe], bis(2-hydroxyethyl)ammonium butanoate [BHEA][Bu], methyl-2-hydroxyethylammonium acetate [m-2-HEA][Ac], and methyl-2-hydroxyethylammonium pentanoate [m-2-HEA][Pe], in the temperature and pressure ranges of T = (303.15 to 403.15) K and P = (0.1 of 100.0) MPa, respectively. Physicochemical characterization was performed by experimental density, determined by an Anton Paar SVM 3001 for atmospheric pressure and an Anton Paar DMA HPM densimeter for higher pressures, where the following increasing sequence for density was found: [BHEA][Bu] > [m-2-HEA][Ac] > [BHEA][Pe] > [m-2-HEA][Pe]. To explain our observations, we propose that two effects influence the structures of PILs: (i) the packing effect and (ii) the hydrogen bond interaction. Moreover, the experimental density data were used to obtain the Tammann–Tait equation parameters. The maximum average absolute relative deviation (MAARD) between the experimental and calculated data was 0.04%. Additionally, the isothermal compressibility, isobaric expansivity, and internal pressure were calculated to understand intermolecular forces better. Also, the group contribution method proposed by Paduszyński and Domańska was applied, and we obtained an MAARD of 1.45% for the density of [m-2-HEA][Pe] PIL. Finally, we utilized a new set of PC-SAFT parameters to estimate the PILs density, where we achieved a MAARD of 0.39%, compared to experimental density data.