Theoretical model clarifies the low-temperature phase behavior of liquid water

The study, published in the journal Proceedings of the National Academy of Sciences (PNAS), was co-directed by Giancarlo Franzese, Associate Professor with the Department of Fundamental Physics of the UB's Faculty of Physics, and H. Eugene Stanley, Director of the Center for Polymer Studies at Boston University, with contributions from Kevin Stokely and Marco G. Mazza of Boston University.

Water exhibits highly anomalous macroscopic and microscopic behaviour in comparison to that of other fluids, and can remain in a liquid state at temperatures as low as -92ºC (at a pressure of 2 kbars). Until now, scientists had used one of four distinct scenarios to account for the molecular behaviour of water at low temperatures. As Franzese explains, “the four interpretations are based on different hypotheses. In particular, they can be distinguished by the assumed presence or absence of two liquid phases with different densities, and by the possible existence of a critical point.”

The anomalous behaviour of water at low temperatures is largely due to the properties of its hydrogen bonds, and the proposed model uses two key physical components to determine which of the four scenarios best describes water - the value of the hydrogen bond's directional component, and the value of the cooperative component. According to Franzese, “We have found that the balance between these two components determines which of the scenarios is valid. In addition, from our experimental estimates we can conclude that the correct scenario is the one that assumes the presence of a critical point between the two liquids.”

Water in its liquid state is the foundation of all life on Earth and is therefore central to many branches of science from molecular biology to geochemistry. The results of the study are particularly relevant to specific fields such as cryogenics, cryobiology, and the cryogenic storage of food products and organic material including stem cells, ovules and sperm at temperatures between -80ºC and -196ºC. Most proteins lose their functionality at below -53ºC, an effect that recent findings suggest is due to a change in the water dynamics, thus underlining the importance of establishing the precise molecular behaviour of water at low temperatures.