Effect of salts on the properties of aqueous sugar systems in relation to biomaterial stabilization - Part 2. Sugar crystallization rate and electrical conductivity behavior

Two of the sugars involved in the protection of living organisms, trehalose and sucrose, were employed to analyze the effect of different salts (MgCl2, CaCl2, KCl, NaCl) on sugar crystallization kinetics. The conductivity behavior in supercooled aqueous sugar salt systems was also studied to evaluate the extent of water salt interactions. Trehalose and sucrose crystallization, evaluated by differential scanning calorimetry (DSC), was delayed by the presence of salt without affecting the glass transition temperature, T-g, of the system. Sugar crystallization rates increased as the temperature increased above T-g. The crystallization kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation (JMAK), and it was found that the index n varied from 4 for pure sugars to less than 2 for sugar MgCl2 mixtures, suggesting that the presence of salts constrained the number of configurations for crystal growth. The electrical conductivity of NaCl and MgCl2 was measured in liquid and supercooled trehalose and sucrose aqueous solutions over a wide range of viscosity to find evidence of preferential solvation in the sugar water solutions. The results indicated that large positive deviations from the Walden rule occur in these systems, due to the higher tendency of the ions to move in water-rich regions. The observed delayed crystallization of sugar in aqueous solutions containing salt could be attributed to effects on the nucleation mechanism of ion-induced microheterogeneities in the supercooled solutions.