Irreversible freezing of water under isenthalpic conditions

When a liquid is supercooled, it will return, within a finite time elapsed, to stable thermodynamic equilibrium. The path towards equilibrium begins as a fast process which adiabatically drives the system to solid-liquid coexistence. Only at later stage will solidification proceed with the expected exchange of thermal energy with the external bath, eventually leading to the proper thermalization for assigned values of the volume or the pressure. On these bases, the usual comparison between the entropy of supercooled liquid and of the corresponding solid is in principle not meaningful, since the two states differ not only in their entropy but also in their enthalpy. Some aspects of the adiabating freezing of metastable water at constant pressure are presented. In particular, it is studied the thermal behaviour of the isobaric gap between the molar volume of water undercooled down to a given subfreezing temperature and the warmer ice-water mixture which eventually form s at equilibrium. The adopted scheme suggests that, for increasing undercooling at ambient pressure, the volume gap gradually reduces and vanishes at a temperature close to the current estimate for the homogeneous nucleation temperature. This behaviour is compared with the experimental observations for o-terphenyl (a system which does not display a volumetric anomaly) and with the indications from a model van der Waals fluid.