title: Comparative study of imidazolium- and pyrrolidinium-based ionic liquids: thermodynamic properties.
abstract: Heat capacities of liquid, crystal(s) and liquid-quenched glass (LQG) of room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([bmim][Tf(2)N]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([bmp][Tf(2)N]) were measured by adiabatic calorimetry. The melting points of [bmim][Tf(2)N] and [bmp][Tf(2)N] were 270.42 and 265.82 K, respectively. Heat capacity anomalies depending on thermal history after crystallization were observed above 200 K in both compounds. Two thermal anomalies due to glass transitions in crystalline [bmim][Tf(2)N] were observed at 59 and 73 K. One thermal anomaly independent of thermal history was observed in a metastable crystalline [bmp][Tf(2)N]. Thermal properties related to LQG of [bmim][Tf(2)N] and [bmp][Tf(2)N] are similar to those of other glass formers, and they are classified as fragile liquids. Heat capacities of [bmim](+) and [bmp](+) due to normal modes of the intramolecular vibration were evaluated through DFT calculations. The comparison between experimental and calculated heat capacity differences shows that the trends in the liquid phase are consistent with each other, while those in the crystal phase deviate further as the temperature is increased. This result supports the authors' previous conclusion that the origin of the low melting point is not a curious property of the liquid but primarily related to properties of the crystal.
authors.affiliation: National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 3, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8563, Japan.
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Shimizu
authors.forename: Yoko
authors.initials: Y
authors.lastname: Ohte
authors.forename: Yasuhisa
authors.initials: Y
authors.lastname: Yamamura
authors.forename: Seiji
authors.initials: S
authors.lastname: Tsuzuki
authors.forename: Kazuya
authors.initials: K
authors.lastname: Saito