Composition dependence and the nature of endothermic freezing and exothermic melting

摘要

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.