Adiabatic freezing and nucleation rates in supercooled water

The irreversible return of a supercooled liquid to stable thermodynamic equilibrium often begins as a fast process which adiabatically drives the system to solid-liquid coexistence. Only at a later stage will solidification proceed with the expected exchange of thermal energy with the external bath. This considerations suggest that many of the indications from models moving from the Kauzmann’s observation of a cross-over temperature between entropies of the supercooled liquid and of the stable solid must be revised, because they move from misleading assumptions [1]. In this work we discuss some aspects of the adiabatic freezing of metastable water at constant pressure. In particular, we investigated the thermal behavior of the isobaric gap between the molar volume of supercooled water and that of the warmer ice-water mixture which eventually forms at equilibrium [2]. The available experimental data at ambient pressure, extrapolated into the metastable region within the scheme provided by the reference IAPWS-95 formulation, show that water ordinarily expands upon (partially) freezing under isenthalpic conditions. However, the same scheme also suggests that, for increasing undercoolings, the volume gap is gradually reduced and eventually vanishes at a temperature close to the currently estimated homogeneous ice nucleation temperature. This behavior is contrasted with that of substances which do not display a volumetric anomaly. The effect of increasing pressures on the alleged volume crossover from an expanded to a contracted ice-water mixture is also discussed. Finally, the results from an experiment, at the moment in progress, will be presented. In this experiment the velocity of propagation of the adiabatic nucleation through the sample volume (obtained by fast imaging technique) and the time required for locally raising the temperature to the coexistence value will be measured, as a function of the temperature of supercooling. Our preliminary results indicate that the front-propagation velocity increases, as the temperature decreases, from 4cm/s at 269 K to about 70 cm/s at 253 K. References 1) H.-J. Hoffmann, Mat.-wiss. u. Werkstofftech 43, 528 (2012). 2) F. Aliotta, P.V. Giaquinta, M. Pochylski, R.C. Ponterio, S. Prestipino, F. Saija, C. Vasi, to appear on J. Chem. Phys. 138 (2013).