Direct Raman evidence for a weak continuous phase transition in liquid water

This paper presents the Raman depolarization ratio of degassed ultrapure water as a function of temperature, in the range 303.4-314.4 K (30.2- 41.2 degrees C). The pressure of the sample was the vapor pressure of water at the measurement temperature. The data provide a direct indication of the existence of a phase transition in the liquid at 307.7 K, 5.8 kPa (34.6 degrees C, 0.057 atm). The minimum in the heat capacity, C-p, of water occurs at 34.5 degrees C, 1.0 atm (J. Res. Natl. Bur. Stand. 1939, 23, 1971()). The minimum in C-p is shallow, and the transition is a weak-continuous phase transition. The pressure coefficient of the viscosity of water changes sign as pressure increases for temperatures below 35 degrees C (Nature 1965, 207, 620(2)). The viscosity minimum tracks the liquid phase transition in the P, T plane where it connects with the minimum in the freezing point of pure water in the same plane (Proc. Am. Acad. Arts Sci. 1911-12, 47, 441(3)). Previously we argued (J. Chem. Phys. 1998, 109, 73794) that the minimum in the pressure coefficient of viscosity signaled the elimination of three-dimensional connectivity in liquid water. These observations coupled with recent measurements of the coordination shell of water near 300 K (Science 2004, 304, 9955) suggest that the structural component that changes during the phase transition is tetrahedrally coordinated water. At temperatures above the transition, there is no tetrahedrally coordinated water in the liquid and locally planar water structures dominate the liquid structure. Water is a structured liquid with distinct local structures that vary with temperature. Furthermore, liquid water has a liquid-liquid phase transition near the middle of the normal liquid range.