Thermophysical properties and thermodynamic phase behavior of ionic liquids

Abstract

The thermal stability of many tested ionic liquids (ILs) was investigated by the TGA and DTA curves over the wide temperature range from 200 to 780 K. The TGA curves have mainly a sigmoid shape, which can be split into three segments. The thermal decomposition of the samples was higher than 500 K. For the ammonium salts, C 2BF 4, or C 2PF 6, or C 2N(CN) 2, or C 4Br, the temperatures of the decompositions were 583.5, 556.1, 545.1 and 525.3 K, respectively. Generally, it was found that the temperature of decomposition of investigated ionic liquid is strongly depended on the type of cation and the anion. Phase equilibria and thermophysical constants were measured also for the dialkoxy-imidazolium ILs, [(C 4H 9OCH 2) 2IM][BF 4], [(C 8H 17OCH 2) 2IM][Tf 2N], [(C 10H 21OCH 2) 2IM][Tf 2N] and for pyridinium IL, [Pyr][BF 4]. The characterization and purity of the compounds were obtained by the elemental analysis, water content (Fisher method) and differential scanning microcalorimetry (DSC) analysis. From (DSC) method, the melting points, the enthalpies of fusion, the temperatures and enthalpies of solid–solid phase transitions and the half C p temperatures of glass transition of all investigated ionic liquids were measured. The phase equilibria of these salts with common popular solvents: water, or alcohols or n-alkanes, or aromatic hydrocarbons have been measured by a dynamic method from 290 K to the melting point of IL, or to the boiling point of the solvent in the whole mole fraction range, x from 0 to 1. These salts mainly exhibit simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperatures (UCST), not only with aromatic hydrocarbons, cycloalkanes and n-alkanes but also with longer chain alcohols. For example the C 2BF 4 salt show simple eutectic system with water and simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperature with alcohols. The solid–liquid phase equilibria, SLE curves were correlated by means of the different G Ex models utilizing parameters derived from the SLE. The root-mean-square deviations of the solubility temperatures for all calculated data depend on the particular system and the equation used.