Glass transition temperature ionic liquid melting point

A number of molten salt systems [e.g., the simple ionic system Ca(N03)2-KN0j], have the property of being able to be supercooled, i.e., to remain liquid at temperatures below the melting point down to a final temperature. This is called the glass transition temperature, and at this temperature the salts form what is called a glass. This glass is only apparently solid. It is a highly disordered substance in which a liquid structure... 

Similarly, ionic liquids bearing a hydroxyl functionality - which leads to unfavorable intermolecular hydrogen bondings - can be used as a liquid at low temperamre. Salts 30-33 are easily synthesized from 1-methylimidazole and 2-chloroethanol they are solid at room temperature. However, because of a weak tendency to crystallize, they enter into a metastable supercooled liquid state when cooled and can therefore be used as solvents at low temperature. The glass transition temperatures are very low, between -72°C (30) and -111°C (33, Table 2.5). The influence of the anion on the glass transition temperatures, as opposed to the melting points, is not clear, as the chlorine salt 33 is a metastable liquid at lower temperature than the hexafluorophosphate and tetrafluoroborate salts 30 and 31. ... 

The phase behavior of ionic liquids can be complicated. Some are crystalline at low temperatures and show a sharp transition from crystal to liquid state (a true melting point) as the temperature is raised, but others exist as a glass at low temperatures and convert to a liquid at the glass-liquid transition temperature, denoted by a small change in heat capacity. Still others are glasses at very low temperatures, transform to crystals as the temperature is raised, and finally become liquid at a still higher temperature. See Reference 3 for a discussion of the types of phase behavior. 

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