New aspects in the thermochemistry of solid-liquid phase transitions of organic non-electrolytes

Abstract

Thermochemistry of solid-liquid phase transitions plays an important role in solidification studies of polymers, metals, inorganic salts, and low-molecular organic compounds. Enthalpy of fusion is a source of information about intermolecular interactions in materials, crystallinity, mixtures phase diagrams shape, heat transfer in the solidification interval.The present paper is devoted to relationship between the fusion enthalpies of aromatic compounds at the melting temperature and the solution enthalpies in benzene at 298.15K. Sublimation and vaporization thermochemistry are used as an additional source of information about the latter relationship. Depending on the magnitude of deviation between the solution enthalpies in benzene at 298.15K and fusion enthalpies at the melting temperature, aromatic compounds are conventionally divided into three groups. For a large number of aromatic compounds not capable of self-association the equality between the fusion enthalpies at the melting temperature and the solution enthalpies in benzene at 298.15K is established. For self-associated aromatic compounds the solution enthalpy in benzene at 298.15K exceeds the fusion enthalpy at the melting temperature, and insertion of a substituent not forming inter- and intramolecular hydrogen bonds does not affect the difference between the solution and fusion enthalpies. The enthalpies of solution in benzene at 298.15K of several aromatic compounds not capable of self-association appear to be visibly less than the fusion enthalpies at the melting temperature. The reasons of different relationships between the fusion and solution enthalpies are analyzed using Kirchhoff's law for the fusion enthalpy.