Relationship between enthalpy and volume changes on melting: a case study of eight cyclic organic compounds

Abstract

The study of structural parameters that influence the thermodynamics of melting of organic compounds is many decades old. Our recent work revealed a relationship between fusion enthalpy, molar volume change on melting, and molecular structure. In this paper, we continued to analyze the relationship between the shape of the molecule and the thermodynamic properties of melting through a comprehensive study of eight cyclic organic compounds. The fusion enthalpies were determined by DSC, and the densities in the liquid state in the temperature range from T_m to 343 K were measured using a high-precision density meter. The predictive capability of the previously established linear correlation between the molecular sphericity parameter and the ratio between the enthalpy and molar volume changes on melting was confirmed using the experimental data in combination with literature values of crystal phase densities. The above ratios were predicted with RMS of 9 %, which is comparable with the combined experimental accuracy and beyond the known predictive approaches. In addition, the reliability of the molecular dynamics method in the evaluation of the liquid density at the melting temperature was tested and confirmed, which enables the replacement of the experimental procedure for the liquid density measurement when calculating the enthalpy-to-volume changes ratio according to the correlation found.