Glass-softening of trehalose and calorimetric transformations in its liquid state

Abstract

To gain insight into the molecular relaxation processes and interaction of water with trehalose (α-d-Glucopyranosyl-α-d-glucopyranoside), changes in the state of trehalose dihydrate with temperature, water content and thermal treatment at ambient and vacuum conditions have been investigated by differential scanning calorimetry, systematically over a broad temperature range from 313 to 523K. Also, structural relaxation and temperature range of glass-softening of its partially dehydrated and completely dehydrated liquid state have been investigated and analysed in terms of the non-exponential, non-linear manner of structural relaxation. The heat released on the removal of water from trehalose dihydrate is 13.8kJmol−1, which is ∼66% of that expected from evaporation of water alone. The difference is attributed partly to the exothermic processes of dissolution of trehalose in the water released on heating before evaporation and partly to conformer transformation. The onset temperature of water release increased by ∼10K when the sample had been partially dried in vacuum. The glass-softening temperature, Tg, of the apparently dry trehalose was found to be in the range 386–402K. It increased when the molten trehalose was thermally cycled between 413 and 467K, as it did when it was heated to temperatures above 513K, where the trehalose partially decomposed. In addition to the glass-softening endotherm, the trehalose showed several endothermic features in the liquid state, whose magnitude varied with the thermal treatment. Amongst these, the most prominent feature appeared in the 440K range. The endotherms observed on heating liquid trehalose were attributed to the kinetic unfreezing of the tautomeric forms and various other conformers of the trehalose molecule and the effects of H-bonding between them. Their concentration changed on annealing the liquid, which in turn changed the viscosity. Thus Tg of trehalose depended upon the thermal history of the melt. The endothermic features in liquid trehalose appeared similar to those already observed in liquid fructose, galactose and fructose–glucose mixture. Structural relaxation of trehalose was found to be as non-exponential as for polymers, but less non-linear than for most glasses.