High-Pressure Densities and Derived Thermodynamic Properties of Imidazolium-Based Ionic Liquids

What is it about?

This work addresses the experimental measurements of the pressure (0.10 < p/MPa < 10.0) and temperature (293.15 < T/K < 393.15) dependence of the density and derived thermodynamic properties, such as the isothermal compressibility, the isobaric expansivity, the thermal pressure coefficient, and the pressure dependence of the heat capacity of several imidazolium-based ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4]; 3-methyl-1-octylimidazolium tetrafluoroborate, [omim][BF4]; 1-hexyl-3-methylimidazolium hexafluorophosphate, [hmim][PF6]; 3-methyl-1-octylimidazolium hexafluorophosphate, [omim][PF6]; 1-butyl-2,3-dimethylimidazolium hexafluorophosphate, [bmmim][PF6]; and 1-butyl-3-methylimidazolium trifluoromethansulfonate, [bmim][CF3SO3]. These ILs were chosen to provide an understanding of the influence of the cation alkyl chain length, the number of cation substitutions, and the anion influence on the properties under study. The influence of water content in the density was also studied for the most hydrophobic IL used, [omim][PF6]. A simple ideal-volume model was employed for the prediction of the imidazolium molar volumes at ambient conditions, which proved to agree well with the experimental results.

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Why is it important?

Experimental density data for six pure ILs in the temperature range (293.15 to 393.15) K and pressure range (0.10 to 10.0) MPa are presented, and the water content influence in the density of the most hydrophobic IL was also assessed. Density results show that it can be tailored by structural variations in the cation and anion. From the experimental data, a proportional molar volume increase with the −CH2 addition to the alkyl chain length of the 1-Cn-3-methylimidazolium-based ILs was observed, and a molar volume increase with the effective anion size was also observed. A simple ideal-volume model previously proposed was employed for the prediction of the imidazolium molar volumes at ambient conditions and proved to agree well with the experimental results. Water content, anion identity, and alkyl chain length can be a significant factor when considering the applications of a particular IL.

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