Thermochimica Acta 464 (2007) 29–34 DSC approach for the investigation of mobile water fractions in aqueous solutions of NaCl and Tris buffer P. Kamasa a,∗ , M. Bokor a , M. Pyda b,c , K. Tompa a a Research Institute for Solid State Physics and Optics of Hungarian Academy of Sciences, POB 49, 1525 Budapest, Hungary b Department of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600, USA c Department of Chemistry, The University of Technology, 35 959 Rzesz´ ow, Poland Received 9 March 2007; received in revised form 3 August 2007; accepted 8 August 2007 Available online 15 August 2007 Abstract The fraction of mobile (unfrozen) water during phase transitions of NaCl–H 2 O 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Freezing and melting of solutions; Eutectic transitions; Tris; NaCl; Heat-flux DSC; NMR 1. Introduction Phase transitions at temperatures below freezing of water are important in the investigation of biological systems [1]. The investigated substances are usually aqueous solutions of proteins that also contain buffers and sodium chloride. The interactions of protein, buffer molecules and NaCl with water on a micro- scopic level have been investigated by proton NMR intensity measurements [2,3]. Liquid–solid transitions can be character- ized by the analysis of thermal reactions detected by DSC [4–6]. Enthalpies of freezing and melting are distinct from the thermal energy associated with heat capacity. Heat capacities are usually known and enthalpies of transitions can be determined. Struc- tural transformations of solutes occur over a longer temperature range and the enthalpy changes are slow. Freezing or melting is distributed over a temperature range due to the various water phases present in aqueous solution samples and the concentra- ∗ Corresponding author. E-mail address: kamasa@szfki.hu (P. Kamasa). tion changes of the dissolved substance during transition. Sharp enthalpy changes are observed at the melting point of the sol- vent. At cooling, the sample freezes from a supercooled state and this appears as a sharp transition. The present work is the introductory step leading up to the quantitative determination of hydration level of proteins. We studied the NaCl and buffer solutions in order to be able to dis- tinguish clearly the sources of measured effects. The behavior of the NaCl and buffer solutions without protein allows separating the effects connected with the hydration of NaCl, buffer and pro- tein. We used an approach that consisted of DSC supplemented by NMR intensity measurement [7]. 2. Experimental The TA Instruments TM heat-flux-type DSC 2920 cell was employed. One important parameter that should be taken into account when measuring phase changes using calorimetry is the heat-conducting path between the temperature sensor, the sam- ple and the heater. A comprehensive analysis of this parameter 0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2007.08.001