Application of fast scanning calorimetry to the fusion thermochemistry of low-molecular-weight organic compounds: Fast-crystallizing m-terphenyl heat capacities in a deeply supercooled liquid state

Abstract

Fusion enthalpy temperature dependence is related to the difference in heat capacity of the liquid and solid. Below the melting temperature, it is hard to measure the liquid heat capacity using conventional methods due to fast crystallization.Based on an indirect solution calorimetry approach, we previously concluded that the temperature dependence of heat capacities below the melting temperature is the extrapolation of the linear function above it. In this study, we employed a fast scanning calorimetry technique to test the validity of this conclusion.Three compounds were studied: two organic glass formers, benzophenone and o-terphenyl, for which the liquid and supercooled liquid heat capacities were known to be accurately described by a linear function of temperature, and m-terphenyl, which had never been studied in the supercooled liquid state. The results were in good agreement with the literature for liquid benzophenone and o-terphenyl heat capacities above and below the melting temperature. The heat capacity of the supercooled liquid and glassy m-terphenyl was obtained for the first time. The measured molar heat capacity of supercooled liquid m-terphenyl was the linear extrapolation of the temperature dependence of the heat capacity of the melt found in the literature.The molar heat capacity of the solid, the enthalpy and fusion temperature of m-terphenyl were determined by conventional DSC. Kirchhoff’s integral, calculated from the measured molar heat capacities of solid and liquid m-terphenyl, was in accordance with the fusion and solution thermochemistry data.