Melting Points and Phase Diagrams

What constitutes an ionic liquid, as distinct from a molten salt It is generally accepted that ionic liquids have relatively low melting points, ideally below ambient temperature [1, 2]. The distinction is arbitrarily based on the salt exhibiting liquidity at or below a given temperature, often conveniently taken to be 100 °C. However, it is clear from observation that the principle distinction between the materials of interest today as ionic liquids (and more as specifically room-temperature ionic liquids) and conventional molten salts is that ionic liquids contain organic cations rather than inorganic ones. This allows a convenient differentiation without concern that some molten salts may have lower melting points than some ionic liquids . 

It should also be noted that terms such as high temperature and low temperature are also subjective, and depend to a great extent on experimental context. If we exclusively consider ionic liquids to incorporate an organic cation, and further limit the selection of salts to those that are liquid below 100 °C, a large range of materials are still available for consideration. 

It should be emphasized that ionic liquids are simply organic salts that happen to have the characteristic of a low melting point. Many ionic liquids have been widely investigated with regard to applications other than as liquid materials as electrolytes, phase-transfer reagents [12], surfactants [13], and fungicides and biocides [14, 15], for example. 

The most simple ionic liquids consist of a single cation and single anion. More complex examples can also be considered, by combining of greater numbers of 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

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Melting point J.D. Holbrey, R.D. Rogers, Melting Points and Phase Diagram, in P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, Wiley VCH, Weinheim 2003, p. 41. 

Physicochemical Properties of Ionic Liquids Melting Points and Phase Diagrams... 

Equation 13.9 can be used to calculate various thermodynamic properties of polymers, in particular, the equilibrium melting point and phase diagrams. It can be used to calculate the mixing free energy and the binodal L-L curve, as well as the coexistence L-S curve in polymer solutions, as introduced... 

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