THE GIBBS-THOMSON EFFECT AND INTERGRANULAR MELTING IN ICE EMULSIONS -INTERPRETING THE ANOMALOUS HEAT-CAPACITY AND VOLUME OF SUPERCOOLED WATER

Calculations for the Gibbs-Thomson effect and the intergranular meltin g of the ice droplets in (water) emulsions at temperatures below 273.1 6 K show that water and ice coexist at thermodynamic equilibrium in an apparently frozen emulsion. The fraction of water at this equilibrium increases on heating, which alters further the thermodynamic properti es of the emulsion. As some of the ice in the emulsion has already mel ted, the increase in the enthalpy, H, and heat capacity, C-p, and the decrease in the volume measured on the normal melting at 273.16 K, are less than the values anticipated. The ratio of this increase in H, or C-p, on melting of the emulsion to the corresponding Value for pure i ce, underestimates the emulsion's water content which, when used for s caling the difference between the C-p of the unfrozen and frozen emuls ion at lower temperatures, as in earlier studies, leads to a larger C- p Of supercooled water than the actual value. Similar scaling of the c orresponding difference between the volume leads to higher volume, or lower density, than the actual value. A formalism for this premelting effect is given for both the adiabatic and differential scanning calor imetry (DSC), and its magnitude is calculated. New experiments show th at tile rise in the DSC signal, or equivalently in the apparent C-p ob served on heating the frozen emulsion, occurs over a temperature range much wider than the Gibbs-Thomson effect and intergranular melting pr edict, for which reasons are given. It is shown that C-p of the disper sant phase is also affected by the melting of ice droplets. There are four consequences of the premelting effects for all finely dispersed m aterials, for frozen water emulsions below 273.16 K: (i) water and ice coexist in the emulsion, (ii) its apparent C-p will increase with inc rease in the heat input used to measure it, (iii) the apparent C-p wil l increase with decrease in the average size of the droplets, and (iv) the apparent C-p will decrease on annealing the frozen emulsion for a period long enough to allow the ice-grain growth in the frozen drople ts. Calorimetry of emulsions has verified consequences (i) and (iv). T he corresponding effects on the molar volume are briefly discussed. A substantial fraction of tile anomalously high C-p and volume of superc ooled water is due to the observed premelting effects. (C) 1997 Americ an Institute of Physics. [S002 1-9606(97)50147-4].