Thermophysical\nCharacterization of Protic Ionic Liquids:\nDensity Measurements, Group Contribution Method, and PC-SAFT Modeling\nApplication

The ionic liquid (IL) research area is growing exponentially,\nmainly\nto evaluate aprotic ionic liquids to replace conventional organic\nsolvents. However, the cost of producing ILs could be disadvantageous\nto their application. Alternatively, protic ionic liquids could reduce\nthe price of ionic liquids utilized in industry. However, more physicochemical\ncharacterization is needed for complete industrial application. In\nthis work, we studied four different protic ionic liquids: bis(2-hydroxyethyl)ammonium\npentanoate [BHEA][Pe], bis(2-hydroxyethyl)ammonium butanoate [BHEA][Bu],\nmethyl-2-hydroxyethylammonium acetate [m-2-HEA][Ac], and methyl-2-hydroxyethylammonium\npentanoate [m-2-HEA][Pe], in the temperature and pressure ranges of <i>T</i> = (303.15 to 403.15) K and <i>P</i> = (0.1 of\n100.0) MPa, respectively. Physicochemical characterization was performed\nby experimental density, determined by an Anton Paar SVM 3001 for\natmospheric pressure and an Anton Paar DMA HPM densimeter for higher\npressures, where the following increasing sequence for density was\nfound: [BHEA][Bu] > [m-2-HEA][Ac] > [BHEA][Pe] > [m-2-HEA][Pe].\nTo\nexplain our observations, we propose that two effects influence the\nstructures of PILs: (i) the packing effect and (ii) the hydrogen bond\ninteraction. Moreover, the experimental density data were used to\nobtain the Tammann–Tait equation parameters. The maximum average\nabsolute relative deviation (MAARD) between the experimental and calculated\ndata was 0.04%. Additionally, the isothermal compressibility, isobaric\nexpansivity, and internal pressure were calculated to understand\nintermolecular forces better. Also, the group contribution method\nproposed by Paduszyński and Domańska was applied, and\nwe obtained an MAARD of 1.45% for the density of [m-2-HEA][Pe] PIL.\nFinally, we utilized a new set of PC-SAFT parameters to estimate the\nPILs density, where we achieved a MAARD of 0.39%, compared to experimental\ndensity data.