Structural unfreezing and endothermic effects in liquids, beta-D-fructose

Both structural unfreezing and irreversible endothermic change between different molecular states of a material can occur on heating its liquid state and produce a double sigmoid-shaped endotherm in its apparent heat capacity, C-p. In such a case, the usual calorimetry is unable to distinguish between the molecular mechanisms of the two endothermic contributions. We report that this distinction can be made by measuring C-p and the complex heat capacity, C-p(*), simultaneously in a temperature-modulated scanning mode. When a sigmoid- shaped increase is observed in both C-p and C-p* of a liquid, the underlying process is structural unfreezing. P But, when this increase is observed in normal scanning and not in the temperature-modulated mode, the underlying process is either irreversible or too slow, compared to the modulation period. The method has been used for resolving the nature of tautomeric transformation in liquid beta-D-fructose. Simultaneous C-p and C-p(*) measurements show that, for normal scannin, C-p increases in a sigmoid-shaped manner with hysteresis, P 9 but for temperature-modulated scanning, the real component of C-p does not increase in this manner (i.e., the reversible component of the heat flow shows neither a sigmoid-shaped increase nor hysteresis). Therefore, the sigmoid-shaped C-p increase observed in normal scanning indicates an endothermic transformation in the beta-D-fructose liquid at T > T-g and not unfreezing of tautomeric transformation on the time scale of heating.