Composition dependence and the nature of endothermic freezing and exothermic melting.

The heat capacity, C-p, and enthalpy and entropy change of alpha-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 alpha-cyclodextrin rich solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its steps merge. Decreasing the amount of alpha-cyclodextrin decreases the C-p 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.