(Solid + liquid) phase equilibria and heat capacity of (diphenyl ether + biphenyl) mixtures used as thermal energy storage materials

Highlights: • A DSC calorimeter for measuring (solid + liquid) phase equilibrium and heat capacities is presented. • (Diphenyl ether + biphenyl) system presents a eutectic (solid + liquid) phase diagram with complete immiscibility in solid phase. • The T, x conditions of the eutectic point are x = (0.705 and 285.6) K, while 93.2 J · g−1 · K−1 is the latent heat of fusion. • The Wilson and NRTL equations, and the predictive UNIFAC model describes adequately the solid liquid phase equilibrium data. • The system exhibits high and positive excess heat capacities which can be correlated by using a Redlich–Kister equation. - Abstract: The (solid + liquid) phase equilibrium for eight {x diphenyl ether + (1 − x) biphenyl} binary mixtures, including the eutectic mixture were studied by using a differential scanning calorimetry (DSC) technique. A good agreement was found between previous literature and experimental values here presented for the melting point and enthalpy of fusion of pure compounds. The well-known equations for Wilson and the non-random two-liquid (NRTL) were used to correlate experimental solid liquid phase equilibrium data. Moreover, the predictive mixture model UNIFAC has been employed to describe the phase diagram. With the aim to check this equipment to measure heat capacities in the quasi-isothermal Temperature-Modulated Differential Scanning Calorimetry method (TMDSC), four fluids of well-known heat capacity such as toluene, n-decane, cyclohexane and water were also studied in the liquid phase at temperatures ranging from (273.15 to 373.15) K. A good agreement with literature values was found for those fluids of pure diphenyl ether and biphenyl. Additionally, the specific isobaric heat capacities of diphenyl ether and biphenyl binary mixtures in the liquid phase up to T = 373.15 K were measured