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 <i>n</i>-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 <i>n</i>, have been observed and described by many researchers. Different correlations for odd and even <i>n</i> 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 <i>n</i> = 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 <i>n</i>-alkanes, not just in whether <i>n</i> is odd or even. These detailed findings promise a better understanding of the thermodynamics of phase transitions.