Ultraslow Dynamics at Crystallization of a Room-Temperature Ionic Liquid, 1-Butyl-3-methylimidazolium Bromide B

We studied the crystallization process of 1-butyl-3-methylimidazolium bromide ([C₄mim]Br) using measurements of supersensitive scanning calorimetry, free induction decay (FID) signals of ¹H NMR, and direct observation. These three methods provided consistent, complementary results, which showed extremely slow dynamics at crystallization. This sample does not crystallize during the cooling process, loses mobility, and changes to a coagulated state, which is not the thermodynamic glass state. The FID signals and direct observation in the heating process indicate that the coagulated sample liquefies just before crystallization. The crystallization of [C₄mim]Br does not occur from specialized locations such as the surface or wall of the sample tube but randomly in the liquid. The calorimetric measurements show that it takes 150 min for approximately 3 mg of this sample to crystallize perfectly. Conformational changes of the butyl group continue for approximately 330 min after crystallization. Such slow dynamics are thought to be due to the cooperative linking of crystallization and complex conformational changes in dense fields with high viscosity.