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.

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The current study focuses on 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF 4 ], and 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF 6 ]. The objective is to study the influence of pressure as well as that of the anion on several properties of this type of ionic liquid. The speed of sound and densities in pure ionic liquids (ILs) as a function of temperature and pressure have been determined. Several other thermodynamic properties such as compressibilities, expansivities, and heat capacities have been obtained. To the best of our knowledge, this research comprises both the first speed of sound data and the first evaluation of heat capacities at high pressures for ILs. Speed of sound measurements have been carried out in broad ranges of temperature (283 < T/K < 323) and pressure (0.1 < p/MPa < 150), sometimes inside the metastable liquid region using a nonintrusive microcell. The T-P melting line of [bmim][PF 6 ] has also been determined by an acoustic method. Density measurements have been performed for broad ranges of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 60) using a vibrating tube densimeter. The pressure dependence of the heat capacities, which is generally mild, is found to be highly dependent on the curvature of the temperature dependence of the density.

The fascinating, complex behavior of ionic liquids is analyzed using a molecular perspective that emphasizes the doubly dual nature of ionic liquids underlying the existence of cations and anions forming high- and low-charge-density regions. Our work bridges the liquid, gas, and solid phases spanning 5 years of research on themes as diverse as the vaporization, liquid-liquid demixing, solidification, and thermophysical behavior of ionic liquids and their mixtures and solutions.