ON POSSIBLE INTERPRETATIONS OF THE ANOMALOUS PROPERTIES OF SUPERCOOLED WATER

A search for ways to estimate thermodynamic properties of deeply super cooled water was undertaken to make it possible to analyze nucleation rates in droplets freezing in supersonic flow. It was found that the w ell-known anomalous behavior of supercooled water can be accounted for by the so-called ''two-state'' model that had been discredited 2 deca des ago. The model was found to be viable when applied in a form close ly related to one introduced by Speedy [J. Phys. Chern. 1984, 88, 3364 ]. Water is apportioned into equilibrium concentrations of high-and lo w-density components in somewhat the same way as described recently by Vedamuthu et al. [J. Phys. Chem. 1994, 98, 2222] except that criteria were imposed whereby the equilibrium constant inferred from the distr ibution was forced to obey the van't Hoff temperature and Gibbs-Poynti ng pressure relations. It was found that the expansivity, heat capacit y, and compressibility anomalies calculated by the model agreed well w ith those measured experimentally when the equilibrium was considered to be between relatively densely packed monomers and bulky aggregates containing five or six molecules. The model does not preclude a broade r distribution of oligomers, particularly when the larger species are somewhat less bulky than the model pentamers and hexamers. An appealin g feature of the model is that its results can be extrapolated to arbi trarily low temperatures, making it possible to estimate the heat and free energy of the freezing of water at the very low temperatures achi eved in a supersonic flow. Results of the model are compatible with Ta naka's recent extensive molecular dynamics simulations, which were ori ginally interpreted as corroborating the spinodal theory. Implications are also consistent with spectroscopic and X-ray scattering experimen ts. In addition, the model has something to say about Turnbull's relat ion for estimating interfacial free energies and their temperature dep endence. Strengths and weaknesses of the approach presented are discus sed, as is the question of whether the interpretation is distinct from the spinodal interpretation. What is different from the original spin odal interpretation is that the phase below the apparent instability t emperature is still liquid, not solid. The present paper does not purp ort to have established an accurate account of the molecular behavior responsible for water's anomalies. Its aim is to call attention to att ractive features of the two-state model that deserve further considera tion.