Although
physicochemical properties are essential to developing
new industrial ionic liquid (IL) applications, experimental data must
be obtained, especially under high-pressure operational conditions.
Therefore, density data were measured for three commercial ILs: methyltrioctylammonium
bis(trifluoromethylsulfonyl) imide [(C8)3C1N][NTf2]; 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)
imide [C3C1Pip][NTf2]; and 1-butylpyridinium
bis(trifluoromethylsulfonyl) imide [C5Py][NTf2] at high-pressure and wide temperature ranges [P = (0.2 to 100.0) MPa and T = (298.15 to 398.15)
K] by the intermediate of a tube vibrating method. The operational
range was chosen because it could cover many industrial applications.
The structural interaction between the IL cation and the anion is
significant in volumetric behavior. It was observed that the density
increases in the following order: [C5Py][NTf2] > [C3C1Pip][NTf2] > [(C8)3C1N][NTf2]. The Tammann–Tait
equation was used to evaluate the influence of pressure and temperature
on density, showing an average absolute relative deviation (%AARD)
of less than 0.032% compared to experimental data. From these data,
the isothermal compressibility (κT), isobaric expansivity
(αp), thermal pressure coefficient (γv), and internal pressure (P
i) were calculated.
Additionally, IL density was estimated using five different group
contribution methods reported in the literature, with better results
obtained for Gardas and Coutinho and Paduszyński and Domańska.