DSC approach for the investigation of mobile water fractions in aqueous solutions of NaCl and Tris buffer

The fraction of mobile (unfrozen) water during phase transitions of NaCl–H2O solution and tris(hydroxymethyl)aminomethane (Tris) buffer solution has been determined by differential scanning calorimetry (DSC). Measurements were carried out in the temperature range between −50 and +30 °C with a heating rate of 2 K min−1. The fraction of mobile water was estimated from the enthalpy of melting of the different frozen phases present in aqueous solution samples. Results are supplemented by proton NMR intensity measurements in the same temperature range. A small endothermic peak was detected in the DSC curves at the same temperature where the change in the NMR intensity occurs. We assume that activation/deactivation of rotational molecular motion of hydration water molecules occurs at the steps of the NMR intensity at higher and lower temperatures during heating and cooling, respectively. The rotational motion is probably the initial stage of the eutectic phase separation. Tris additive to NaCl solution causes thermal shifts of the small endothermic peak and in the related NMR intensity. These qualitative differences indicate interactions of Na+ and Cl− ions with the small organic molecule constituents.