Effects of anion and alkyl chain length of cation on the thermophysical properties of imidazolium-based ionic liquid

This study focused on the effects of anions and the alkyl chain length of cations on the thermophysical properties of imidazolium-based ionic liquid (ILs). The data were obtained from references and the NIST database. The studied parameters include the melting temperature (T-m), glass transition temperature (T-g), decomposition temperature (T-d), enthalpy of fusion (Delta H-m), and thermal conductivity (lambda). The variations of T-m with the anions in [C16MIM][A] where A = Br, Cl, BF4, PF6, and TFO, generally decreased with increasing anion radius, except for A = PF6, due to strong hydrogen bonds for the sake of an F atom. The values of the critical temperatures (T-m, T-g, and T-d) generally show a strong variation with the number of carbon atom or alkyl chain length (the number of n in [CnMIM][A] for A = BF4, NTf2, and PF6). The variation of T-m corresponding to n shows non-monotonous variations. This characteristic is the result of the combination of interaction potentials in the crystalline and liquid phases. The variation of T-g corresponding to n shows even-odd alternation, presumably due to the competition between the electrostatic and van der Waals forces. Many types of ILs have a relatively high T-d value, enabling them to remain in the liquid state above 400 degrees C, they have excellent catalytic activity and dynamic properties. The variation of T-d with n appears different to that of T-m. Furthermore, the variation of Delta H-m with n seems to follow that of T-m. This behaviour is in accordance with the thermodynamic relation between Delta H-m, T-m, and the entropy of the system. The values of k varied weakly with the alkyl chain length, and strongly depend on the type of anion. (C) 2020 Elsevier Ltd. All rights reserved.