THERMODYNAMIC CONTINUITY BETWEEN GLASSY AND NORMAL WATER

The enthalpy, the heat capacity, and Gibbs free energy of glassy water and of metastable water at 153 K have been examined by measuring the heat evolved on its crystallization to cubic ice. Measurements are mad e both isothermally and for slow heating, since the total heat evolved on crystallization decreases with the temperature. It is shown that a small fraction of metastable water persists up to 180 K when heated a t 30 K min(-1) and that the excess enthalpy of water at 153 K is 1.2 /- 0.1 kJ mol(-1). Free energy considerations suggest that a thermodyn amic continuity is possible only if the residual entropy of glassy wat er at 0 K is less than or equal to 5.3 J K-1 mol(-1), or the excess re sidual entropy over that of hexagonal ice is less than or equal to 1.9 J K-1 mol(-1). Precise estimates have been made by finding a variatio n of heat capacity with temperature between 153 and 236 K which satisf y the requirements of both the enthalpy and the third law of thermodyn amics. These give a value of 3.8-4.5 +/- 0.1 J K-1 mol(-1) for the res idual entropy of glassy water, or 0.4-1.1 +/- 0.1 J K-1 mol(-1) more t han that of hexagonal ice. On this basis, it is concluded that glassy water and normal water can be connected by a continuous thermodynamic path. Recent estimates of the excess residual entropy of water are con sistent with this value. Its absolute value can be determined only whe n its reversible transformation to a state of known entropy becomes ob servable.