Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence.

The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties. To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis(fluorosulfonyl)amide ([Pip1,4][FSA]) was selected as the first sample. In addition to the rotational flexibility of the alkyl group, the [Pip1,4](+) cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations. Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip1,4][FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion. Each phase change is also governed by very slow dynamics. The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations. Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 38.8 kJ/mol. It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings.