Since determining experimentally a wide variety of thermophysical properties-even for a very small portion of the already known room temperature ionic liquids (and their mixtures and solutions)-is an impossible goal, it is imperative that reliable predictive methods be developed. In turn, these methods might offer us clues to understanding the underlying ion-ion and ion-molecule interactions. 1-Butyl-3-methylimidazolium tetrafluoroborate, one of the most thoroughly investigated ionic liquids, together with water, the greenest of the solvents, have been chosen in this work in order to use their mixtures as a case study to model other, greener, ionic liquid aqueous solutions. We focus our attention both on very simple methodologies that permit one to calculate accurately the mixture's molar volumes and heat capacities as well as more sophisticated theories to predict excess properties, pressure and isotope effects in the phase diagrams, and anomalies in some response functions to criticality, with a minimum of information. In regard to experimental work, we have determined: (a) densities as a function of temperature (278.15 < T/K < 333.15), pressure (1 < p/bar < 600), and composition (0 < x IL < 1), thus also excess molar volumes; (b) heat capacities and excess molar enthalpies as a function of temperature (278.15 < T/K < 333.15) and composition (0 < x IL < 1); and (c) liquid-liquid phase diagrams and their pressure (1 < p/bar < 700) and isotopic (H 2 O/D 2 O) dependences. The evolution of some of the aforementioned properties in their approach to the critical region has deserved particular attention.
The current study focuses on several phosphonium-based ionic liquids, namely, trihexyltetradecylphosphonium chloride, trihexyltetradecylphosphonium acetate, and trihexyltetradecylphosphonium bis{(trifluoromethyl)sulfonyl}amide. The objective was to study the influence of pressure as well as that of the anion on several properties of this type of ionic liquids. Densities in pure ionic liquids as a function of temperature and pressure have been determined. Other thermodynamic properties, such as the isothermal compressibility, the isobaric expansivity, and the thermal pressure coefficient, have been calculated. Density measurements have been performed at a broad range of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 65) using a vibrating tube densimeter. A simple ideal-volume model was employed for the prediction of the molar volumes of the phosphonia at ambient conditions, which proved to compare favorably with the experimental results.
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