Endothermic freezing on heating and exothermic melting on cooling:
composition dependence and nature of the phenomenon in cyclodextrine, water, and 4-methylpyridine solutions.
E. Tombari
CNR-IPCF, UOS di Pisa
Generally, a liquid freezes exothermally on cooling and a crystal melts endothermally on heating.
Here we report an opposite occurrence - a liquid’s endothermic freezing on heating and the
resulting crystal’s exothermic melting on cooling at ambient pressure. The heat capacity, Cp
decreases on freezing and increases on melting, and the equilibrium temperature meets the
thermodynamic requirement. Melting on cooling takes longer than freezing on heating. A rapidly
cooled crystal state becomes kinetically frozen, evocative of a non-ergodic state. Both Cp and
enthalpy relax like those of glasses, though the viscosity is only a few centiPoise. The crystal state
belongs to energy minima higher than those of the melt, which has consequences for the use of
potential energy landscape, or inherent structures, for a thermodynamic description of a material.
Cp, and enthalpy and entropy change of  -cyclodextrin, H2O, and 4-methylpyridine solutions have
been studied during their freezing on heating, isothermal freezing, and the solid’s melting on
cooling. Freezing occurs in several endothermic steps on heating to 383 K and -cyclodextrin rich
solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its
steps merge. Decreasing the amount of  -cyclodextrin decreases the Cp change on freezing. The
endothermic freezing phenomenon differs from freezing of a pure liquid and is attributed to
formation of a solid inclusion compound and its incongruent way of exothermic melting.