Thermal Properties of Freeze-Concentrated Sugar-Phosphate Solutions

In order to understand the effect of phosphate salts on the freeze-concentrated glass-like transition temperature (Tg′) of aqueous sugar solutions, two types of sugar (glucose and maltose) and five types of phosphate salts (Na3PO4, Na4P2O7, Na5P3O10, K3PO4, and K4P2O7) were employed, and the thermal properties of various sugar-phosphate aqueous systems were investigated using differential scanning calorimetry. The Tg′ of glucose increased with increasing sodium phosphates up to a certain phosphate ratio, decreasing thereafter. The maximum Tg′ value was slightly higher in the order of Na3PO4 > Na4P2O7 ≥ Na5P3O10. Maltose-sodium phosphate also showed a similar trend as glucose-sodium phosphate samples. However, the degree of Tg′-rise of maltose systems was much less than that of glucose. It is thought that the Tg′ elevated by the molecular interaction between sugar and phosphate ions will be reduced by hydrated sodium ions. In comparisons between potassium phosphate and sodium phosphate, it was found that sugar-potassium phosphates showed the lower maximum Tg′ at a lower phosphate ratio than sugar-sodium phosphates. In addition, the Tg′ of potassium phosphates dropped sharply in comparison with sodium phosphates at the high phosphate ratio. These results suggest that potassium phosphates are lower Tg′ than sodium phosphates, and that potassium ion plays a better plasticizer than sodium ion. A certain amount of sodium phosphates (Na3PO4 and Na4P2O7) caused devitrification. Potassium phosphates, however, did not show devitrification which can be explained by the fact that potassium ion can be dynamically restricted by sugar.