Melting and Crystallization Behaviors of an Ionic Liquid, 1-Isopropyl-3-methylimidazolium Bromide, Studied by Using Nanowatt-Stabilized Differential Scanning Calorimetry

We studied the melting and crystallization behaviors of 1-isopropyl-3-methylimidazolium bromide ([i-C(3)mim]Br) using a laboratory-made differential scanning calorimeter (DSC) with nanowatt stability and sensitivity. In addition to common behaviors for many ionic liquids such as premelting over a wide temperature range and excessive supercooling, we observed unique thermal phenomena: reversible phase transitions of domains in the premelting region, rhythmic crystallization and melting, and intermittent crystallization. The latter two phenomena were observed in the returning experiments from the premelting region at a cooling rate as slow as 0.05 mK s(-1). It is considered that these phenomena occurred because an [i-C(3)mim](+) ion is capable of taking plural stable conformations and the conformational changes occurred linking with melting and crystallization. We interpret these curious phenomena as the phase changes of the locally melted domains composed of different conformers, which significantly hindered the overall phase changes.