Determination of unfrozen matrix concentrations at low temperatures using stepwise DSC

The aim of the current study was to determine whether stepwise DSC (SW-DSC) is a suitable method for measuring the unfrozen matrix concentration (Cg) of binary aqueous solutions at temperatures as low as −50 °C. The optimal experimental conditions were determined using water. Reliable heat capacity values were determined at nominal scanning rates between 10 and 100 K min−1, sample weights between 8 and 15 mg, and with the sample completely covering the base of the DSC pan. These conditions were then applied to aqueous solutions of ethylene glycol, glycerol and sodium chloride. The apparent heat is the sum of all heat including latent heat, heat capacity and heat of dilution. The influence of each term on the apparent heat was discussed in detail. The apparent heat values of the frozen samples were then used to calculate the ice fraction in the solution and were expressed as the Cg. The calculated Cg values were similar to previously published values. This study showed that SW-DSC can be used to determine the Cg over a wide temperature range using only one single solution. This technique is advantageous for solutes that are not available in large quantities.