The PVT surface of water: critical phenomena near 0.195 GPa, 182 K

Abstract This paper uses data on the low temperature, high pressure properties of liquid water to identify the probable location of critical behavior on the PVT surface of the liquid. It points out the fact that five collections of information in the literature point to, or are consistent with, the region near 0.195 GPa, 182 K being a location for critical phenomena on the surface of liquid water. The point 0.195 GPa, 182 K is: the lowest temperature on the homogeneous nucleation curve; the intersection point of the fitted curves for temperature of maximum density and homogeneous nucleation; the extrapolated intersection of the hysteresis curves for the low density amorphous (LDA) and high density amorphous (HDA) water phase transition; and the lowest temperature on the calculated spinodle for water. The decompression induced melting behavior of ice IV tracks to the same point within experimental error. These observations are used along with reports of the density of water and ice as a function of pressure and temperature to construct a PVT surface for amorphous water in the range 0–0.35 GPa, 160–320 K. Extrapolation and structure-based criteria are used in areas where experimental data are lacking. Structure-based criteria refer to the chemical structure of the liquid, understood in terms of the packing density of water molecules. The volume expansion at low temperature, one atmosphere, for supercooled water is extrapolated by means of a fourth-order polynomial to the specific volume of ice Ih expanded by 2.3%. The specific volume difference between ice Ic and LDA at the same temperature and pressure is approximately 2.3%. The extrapolated temperature associated with this volume, 223 K, is shown as the limit of the homogeneous nucleation temperature, THL. The specific volume of amorphous water will decrease with decreasing temperature below THL. At this point the liquid volume will have expanded to its limit without the rupture of large numbers of hydrogen bonds. Because volume expansion, without major loss of stability, is no longer possible for the liquid it will behave like ice Ih and decrease in volume with decreasing temperature. The available data for the homogeneous nucleation temperature of water as a function of pressure have a low temperature limit in the PT plane near 0.195 GPa, 182 K. This value corresponds, within the error of the estimate, to the minimum in the conjectured stability limit of water. The corresponding point on the D2O PVT surface occurs at 0.211 GPa, 187 K. A tricritical point would be compatible with the topography of the H2O PVT surface near 0.195 GPa, 182 K. Features of the PVT surface of water are discussed in light of recent high pressure melting experiments, and other experimental data that point to critical phenomena in the low temperature PVT surface of water. The line of continuous transitions emanating from the high temperature side of a tricritical point would provide a reasonable basis for understanding the decompression induced melting behavior of H2O, and D2O, and the heat capacity of the supercooled liquid as a function of temperature.