Mixing and decomposition behavior of {[4bmpy][Tf2N] + [emim][EtSO4]} and {[4bmpy][Tf2N] + [emim][TFES]} ionic liquid mixtures

• Densities, viscosities, refractive indices, thermal decomposition, and specific heats of two IL mixtures were studied.
• Viscosity, refractive index, and thermal decomposition of {[4bmpy][Tf2N] + [emim][EtSO4]} showed high deviations from ideality.
• Viscosity and thermal decomposition of {[4bmpy][Tf2N] + [emim][EtSO4]} were not intermediate between those of pure ILs.
• The {[4bmpy][Tf2N] + [emim][TFES]} IL mixture showed a quasi-ideal decomposition and mixing behaviors.
• The reliability of predictive methods of the thermophysical properties of the mixtures was evaluated.

Mixing ionic liquids (ILs) has been revealed as a useful way to finely tune the properties of IL-based solvents. The scarce available studies on IL mixtures have shown a quasi-ideal behavior of their physical properties. In this work, we have performed a thermophysical characterization of two binary IL mixtures, namely {4-methyl-N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([4bmpy][Tf2N]) + 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4])} and {[4bmpy][Tf2N] + 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate [emim][TFES]}. Both binary IL mixtures have been recently proposed as promising solvents in the (liquid + liquid) extraction of aromatic hydrocarbons from mixtures with alkanes. Densities, viscosities, refractive indices, thermal stability, and specific heats of the {[4bmpy][Tf2N] + [emim][EtSO4]} and {[4bmpy][Tf2N] + [emim][TFES]} IL mixtures have been measured as a function of both temperature and composition. Dynamic viscosities, refractive indices, and thermal stability of the {[4bmpy][Tf2N] + [emim][EtSO4]} mixture have exhibited strong deviations from the ideality, in contrast with the quasi-ideal properties of the {[4bmpy][Tf2N] + [emim][TFES]} mixture and the behavior of the imidazolium and pyridinium-based IL mixtures studied hitherto. The reliability of predictive methods of the thermophysical properties of the mixtures has also been evaluated.

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