Ice premelting during differential scanning calorimetry

Premelting at the surface of ice crystals is caused by factors such as temp erature, radius of curvature, and solute composition. When polycrystalline ice samples are warmed from well below the equilibrium melting point, surfa ce melting may begin at temperatures as low as -15 degrees C, However, it h as been reported (Bronshteyn and Steponkus, 1993. Biophys. J. 65:1853-1865) that when polycrystalline ice was warmed in a differential scanning calori metry (DSC) pan, melting began at about -50 degrees C, this extreme behavio r being attributed to short-range forces. We show that there is no driving force for such premelting, and that for pure water samples in DSC pans curv ature effects will cause premelting typically at just a few degrees below t he equilibrium melting point. We also show that the rate of warming affects the slope of the DSC baseline and that this might be incorrectly interpret ed as an endotherm. The work has consequences for DSC operators who use wat er as a standard in systems where subfreezing runs are important.