Comparative Study of Imidazolium- and Pyrrolidinium-Based Ionic Liquids: Thermodynamic Properties

Yoshitaka SHIMIZU, Yoko OHTE, Yasuhisa YAMAMURA, Seiji TSUZUKI and Kazuya SAITO

Heat capacities of liquid, crystal(s) and liquid-quenched glass (LQG) of room temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([bmim][Tf2N]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([bmp][Tf2N]) were measured by adiabatic calorimetry. Melting points of [bmim][Tf2N] and [bmp][Tf2N] were 270.42 K 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][Tf2N] were observed at 59 K and 73 K. One thermal anomaly independent of thermal history was observed in a metastable crystalline [bmp][Tf2N]. Thermal properties related to LQG of [bmim][Tf2N] and [bmp][Tf2N] are similar to other glass formers and they are classified into 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 liquid phase are consistent to each other while those in crystal phase are deviated further as temperature increased. This result supports the authors' previous conclusion that the origin of low melting point is not curious property of the liquid but primarily related to properties of the crystal.
(J. Phys. Chem. B, 116, 5406-5413 (2012))

Is the Liquid or the Solid Phase Responsible for the Low Melting Points of Ionic Liquids? Alkyl-Chain-Length Dependence of Thermodynamic Properties of [Cnmin][Tf2N]

Yoshitaka SHIMIZU, Yoko OHTE, Yasuhisa YAMAMURA and Kazuya SAITO

To establish the alkyl-chain-length dependences of thermodynamic properties of typical ionic liquids [Cnmim][Tf2N], the heat capacities of compounds with n = 2 and 18 were measured by adiabatic calorimetry. The comparison with other ionic liquids and typical molecular substances reveals that the low melting point of [Cnmim][Tf2N] with a short alkyl chain mainly originates in the large fusion entropy arising from the low entropy of the crystalline phase.
(Chem. Phys. Lett., 470, 295 (2009))

Effects of Thermal History on Thermal Anomaly in Solid of Ionic Liquid Compound, [C4min][Tf2N]

Yoshitaka SHIMIZU, Yoko OHTE, Yasuhisa YAMAMURA and Kazuya SAITO

Heat capacity of [C4mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) was measured by adiabatic calorimetry.  Broad heat capacity anomaly and change in spontaneous endothermic effect were observed for crystal phase. The relation between these anomalies and thermal history after crystallization is described.
(Chem. Lett., 36, 1484 (2007))

Low Temperature Heat Capacity of Room Temperature Ionic Liquid, 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

Yoshitaka SHIMIZU, Yoko OHTE, Yasuhisa YAMAMURA, Kazuya SAITO and Tooru ATAKE

Heat capacities of liquid, stable crystal and liquid-quenched-glass of a room temperature ionic liquid, 1-hexyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide were measured between 5 and 310 K by adiabatic calorimetry. Heat capacity of the liquid at 298.15 K was determined for an IUPAC project as (631.6 +- 0.5) J K-1 mol-1. Fusion was observed at Tfus = 272.10 K for the stable crystalline phase with enthalpy and entropy of fusion of 28.34 kJ mol-1 and 104.2 J K-1 mol-1, respectively. The purity of the sample was estimated as 99.83 mol% by fractional melting method. The liquid could be supercooled easily and the glass transition was observed around Tg = 183 K, which was in agreement with the empirical relation, Tg = (2/3) Tfus. The heat capacity of the liquid-quenched-glass was larger than that of the crystal as a whole. In the lowest temperature region, however, the difference between the two showed a maximum around 6 K, and a minimum around 15 K, at which the heat capacity of the glass was a little smaller than that of crystal.
(J. Phys. Chem. B, 110, 13970 (2006))

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