Enthalpies of fusion, vaporisation and sublimation of crown ethers determined by thermogravimetry and differential scanning calorimetry

Abstract

The phase change enthalpies of 18-crown-6 ether (18C6), dibenzo-18-crown-6 ether (DB18C6), dibenzo-24-crown-8 ether (DB24C8) and dibenzo-30-crown-10 ether (DB30C10), which are chemical structures that are of interest in supramolecular chemistry, were determined by thermogravimetry (TGA) and differential scanning calorimetry (DSC). The vapour pressure data as a function of the temperature for the compounds under investigation were estimated by using thermogravimetry and applying the Langmuir equation; thus, the enthalpies of vaporisation were derived. The standard enthalpies of fusion were measured by differential scanning calorimetry then added to those of vaporisation to obtain reliable results for the enthalpy of sublimation at the fusion temperatures. For each crown ether, the heat capacities of the solid and liquid phase were also measured by DSC and that of the gas phase was theoretically estimated by DFT calculations, then all of them were applied to derive the enthalpies of sublimation at 298.15K, which were determined to be (139.7±3.6) kJmol−1 for 18-crown-6 ether, (190.6±3.6) kJmol−1 for dibenzo-18-crown-6 ether, (232.9±3.6) kJmol−1 for dibenzo-24-crown-8 ether and (255.7±5.3) kJmol−1 for dibenzo-30-crown-10 ether. The meaning of these results with regard to the energy of the cohesion in the solid of each crown ether is discussed herein.