Heat Capacity Study and Thermal Property Determination of trans-Azobenzene

Azobenzene has been particularly concerned as promising candidates for solar thermal fuel and photo-switch for controllable latent heat release of phase change materials recently, due to its unique photo-isomerization reversibly between trans- and cis-isomers. The thermal property of azobenzene is closely involved in these solar and thermal energy related applications, and consequently needs to be extensively investigated and fully understood. Herein, we have reported the heat capacity study of trans-azobenzene using a combination of Physical Property Measurement System (PPMS) relaxation calorimeter and adiabatic calorimeter in the temperature range from 1.9 to 380 K. The heat capacity data have been fitted to a series of theoretical and empirical models, and the corresponding thermodynamic functions have been calculated based on the thermodynamic relations and fitting parameters. The standard molar heat capacity, entropy, and enthalpy of trans-azobenzene at 298.15 K have been determined to be 229.5 ± 1.1 J K^–1 mol^–1, 239.1 ± 1.2 J K^–1 mol^–1, and (35.56 ± 0.18) kJ mol^–1, respectively. Moreover, the other thermal properties of trans-azobenzene, such as thermal stability, thermal conductivity, formation enthalpy, phase transition temperature and enthalpy have also been measured using various thermal analysis and calorimetry technologies. These thermodynamic and thermal properties reported in this work can provide important fundamental basis for further studying and understanding azobenzene upon its structure transformation and functional performance, and consequently promote its application in solar thermal fuel and phase change materials for thermal energy utilization.