A new method for heat capacity determination in supercooled liquid state using fast scanning calorimetry: Thermochemical study of 9,9'-bifluorenyl

Abstract

© 2020 Elsevier B.V. Fast scanning calorimetry (FSC) enables to prevent the crystallization of many low-molecular-mass organic compounds. In spite of extremely high cooling rates accessible by fast scanning calorimetry, limitations remain on the measurements of rapidly crystallizing and volatile compounds in the supercooled liquid state. We developed a new method for heat capacity determination, which requires measuring the heat flow rates only on cooling at one fixed scanning rate of a number of samples of different mass. The molar heat capacity is derived from the slope of the heat flow rates plotted against the sample amount, divided by the cooling rate. The total uncertainty of the method was determined to 5 %. The validity was confirmed using benzophenone as a reference compound. Then this approach was applied to study the heat capacity of 9,9′-bifluorenyl in the supercooled liquid state between 350 and 550 K; respective values were obtained for the first time. The relationship between the fusion enthalpy of 9,9′-bifluorenyl at Tm and solution enthalpy in benzene at 298.15 K was analyzed using combination of Hess's and Kirchhoff's laws of thermochemistry. Consistency between the thermochemical data derived independently by solution, fast scanning, and differential scanning calorimetry was established and confirmed the validity of the new method for heat capacity determination by FSC.