Low-Temperature Heat Capacity of 1-Alkyl-3-Methylimidazolium Bis(oxalato)borate Ionic Liquids and the Influence of Anion Structural Characteristics on Thermodynamic Properties

Ionic liquids are very promising replacements for the traditional volatile organic solvents because of their unique physicochemical properties and designability.1 The cation and anion of ILs can be varied and combined in nearly infinite ways which lead to a quite large number of different compounds with unique desirable properties. Correspondingly, the properties of ILs including viscosity, solubility and melting point etc. could be adjusted. Therefore, understanding the relationship between basic physical properties and molecular structure characteristics of ILs is one of key issues for ILs’ design, evaluation and successful use. In this work, two chelated orthoborate ionic liquids (ILs) 1-butyl-3-methylimidazolium bis(oxalato)borate ([Bmim][BOB]) and 1-hexyl-3-methylimidazolium bis(oxalato)borate ([Hmim][BOB]) were prepared2 and studied by using adiabatic calorimetry and differential scanning calorimetry. Their thermodynamic properties and phase change behavior were compared with other series of [Bmim] based ionic liquids. Results indicate that their heat capacities increase with the increasing in atom number of anion. Glass-transition temperature of ILs is normally affected by ion size and the charge dispersion degree. High content of strong electro negative atoms and Cn or Cnv (n = 1, 2, 3,…,∞) point group symmetry in the anion is recommended for the design room temperature ILs with wide liquid temperature range