THE METASTABLE T-P PHASE-DIAGRAM AND ANOMALOUS THERMODYNAMIC PROPERTIES OF SUPERCOOLED WATER

The metastable T-P phase diagram and the anomalies of the thermodynami c properties of supercooled water are calculated on the basis of a two -level thermodynamic model. Water is considered as a mixture of two co mponents which differ in atomic configurations and correspond to low-d ensity amorphous (Ida) and high-density amorphous (hda) ice. The expre ssion for the Gibbs potential of water is written in the form which is analogous to that of usual regular binary solutions. But this model c onsiders the concentration, c, of the components, as a pressure and te mperature-dependent internal parameter. There are only four constants in the expression for the Gibbs potential: the differences in the spec ific volumes, entropies, and energies of the two components and the mi xing energy of the components whose values are Delta V-0= -3.8 cm(3)/m ol, Delta S-0=4.225J/mol, Delta E-0=1037J/mol, and U=3824J/mol, respec tively. The lda-hda phase equilibrium line terminates at the critical point, T-cr= 230 K and P-cr= 0.173 kbar, the second critical point in the phase diagram of water. The anomalous thermal dependence of the sp ecific volume, the heat expansion coefficient, and the specific heat o f water calculated for the atmospheric pressure is in a good quantitat ive agreement with the available experimental data. Thus anomalous pro perties of supercooled water are well explained by the occurrence of t he second critical point close to the atmospheric pressure. The absolu te value of parameter c is not crucial for the thermal behavior of pro perties, instead, the anomalies in water are due to the dependence on pressure and temperature. The parameter c behavior is analyzed in vari ous pressure and temperature ranges around the second critical point. The thermal dependence of parameter c is very weak in the temperature range of 290-350 K at atmospheric pressure. As a consequence, the ther modynamic properties of water behave in this range like those of a nor mal liquid though water stays a mixture of two components, Ida-like an d hda-like, in an approximate proportion 2:3. (C) 1998 American Instit ute of Physics.