(705b) Liquid-Liquid and Liquid-Solid Transitions in Supercooled Water

Liquid water’s distinctive thermophysical properties, such as its unusually high heat capacity and melting and boiling temperature, and its low compressibility, set it apart from many other liquids and shape its role in a variety of important processes ranging from climate regulation to energy production. Consequently, there has been sustained scientific interest over the past several decades in uncovering the microscopic origins and broader implications of water’s unusual behavior.  One hypothesis posits that water’s thermodynamic anomalies arise due to the presence of a second critical point associated with a first-order phase transition between a low density and high density liquid phasein the deeply supercooled regime [1]. Because the region of the phase diagram where this hypothetical critical point would occur is below the homogenous nucleation temperature of bulk water, obtaining direct experimental evidence to falsify the second critical point hypothesis has so far proved to be a significant challenge.

            Using state of the art free energy calculations, we demonstrated that a liquid-liquid transition occurs in the ST2 water model [2].  Our free energy calculations also predict that ice nucleation, should it occur, takes places within the low-density liquid polymorph.  Although these results shed light on the reversible phase behavior of the model, they provide limited information regarding kinetics and mechanisms underlying the liquid-liquid and liquid-solid phase transitions. To gain this insight, we perform extended molecular dynamics simulations to track the temporal evolution of ST2 water as it undergoes these distinct phase transitions. We find that upon thermally quenching ST2 water into the predicted region of metastable liquid coexistence, the system rapidly phase separates into two immiscible liquids.  While crystallization is also observed, it occurs on time scales that are orders of magnitude longer than those required to form a liquid-liquid interface.  Finally, we analyze these processes using several structural order parameters to characterize the microscopic events that occur as the system transitions from one phase to another.

 [1]. P. H. Poole, F. Sciortino, U. Essmann, and H. E. Stanley, Nature, (1992) 360, 324.

 [2]. F.H. Stillinger and A. Rahman, J Chem Phys, (1974) 60, 1545.