High-Accuracy and High-Resolution Calorimetry Revealing New Correlations of Phase Change Enthalpy, Entropy, and Number of Carbon Atoms n in n-Alkanes

Correlations between material properties are useful in engineering, and in addition, the underlying common mechanisms allow for a better understanding of the origins of the properties. Properties related to phase changes are an example, being important, e.g., in thermodynamic applications. For n-alkanes used, e.g., as phase change materials for thermal energy storage, linear correlations between enthalpy changes and entropy changes in phase transitions, as well as the number of carbon atoms n, have been observed and described by many researchers. Different correlations for odd and even n were found, though still with significant outliers. In this work, data from high-accuracy and high-resolution calorimetry were used for the analysis of enthalpy and entropy changes for alkanes with n = 14 to 30. The analysis shows more, and different, correlations than described in the previously published literature. Specifically, the 'outliers' have a physical and chemical origin, rooted in the phase transitions present in the specific n-alkanes, not just in whether n is odd or even. These detailed findings promise a better understanding of the thermodynamics of phase transitions.