Formation of metastable crystals of [C4mim][NTf2] and [C6mim][NTf2]

Heat capacities of crystalline 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][NTf2] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C6mim][NTf2] in the range 80 K–Tfus were measured in an adiabatic calorimeter. Anomalies in the heat-capacity curves for the both compounds occurred near 240 K. Positions of the anomalies depended on thermal history of the samples. More stable crystals had higher heat capacities in the range 220–260 K. Below 200 K heat capacities of all the crystals of the same compound were indistinguishable.