Crystalline and amorphous phases in the binary system water–raffinose

Abstract

The melting points, glass transitions and enthalpies of fusion of crystalline hydrates in binary water–raffinose mixtures have been studied by differential scanning calorimetery (DSC) over the accessible composition range. X-Ray diffraction was used to identify the crystalline forms. From the liquidus and solidus curves and shapes of the DSC scans, a raffinose trihydrate has been identified, and lower hydrates are also believed to exist. The melting points and enthalpies of fusion of raffinose pentahydrate and trihydrate are 352.7±0.1 K, 88.0±0.4 kJ mol ^-1 and 358.4 K, 47.6 kJ mol ^-1 , respectively. In the phase coexistence curve at higher raffinose concentrations a minimum point is observed, which is thought to be the eutectic of trihydrate and a lower hydrate. The raffinose mole fraction of the eutectic is 0.347, and its melting point and enthalpy of fusion are 356.6 K and 51.4 J g ^-1 . The raffinose mole fraction of the ice–pentahydrate eutectic is 7.9×10 ^-3 , and its melting point and enthalpy of fusion are 270.4±0.2 K and 237.6±0.5 J g ^-1 , respectively. The glass-transition profile for the binary system has been measured over the experimentally accessible composition range and the data fitted to the Gordon–Taylor equation, from which the glass-transition temperature for anhydrous raffinose is obtained as 376.4 K. Rehydration and recrystallisation rates of anhydrous, amorphous raffinose taking place during storage over media of constant relative humidity have been studied by measurements of shifts in the glass-transition and melting temperatures. The kinetics of water uptake, and of the subsequent crystallisation of raffinose pentahydrate from the amorphous substrates, show a complex dependence on relative humidity and temperature, which requires further study.