What determines the homogeneous freezing temperature of water?

Abstract

One of water's puzzles is what determines the lowest temperature at which liquid water and aqueous solutions can be cooled before freezing to ice. The increase of ice nucleation rates as water is supercooled correlates with a dramatic increase in the heat capacity and compressibility of supercooled water, raising the question of whether a structural transformation occurring within the liquid phase controls the rate of nucleation of ice. We used molecular simulations to elucidate the mechanism and compute the rates of water freezing and their relation to the thermodynamics and structure of supercooled water. A main finding of our study is that the kinetics of water freezing is controlled by a structural transformation in supercooled liquid water that steeply increases the fraction of four-coordinated water molecules in the liquid, blurring the boundary between the liquid and crystal states. The results of the molecular simulations and of calculations using classical nucleation theory with available experimental data, indicate that the crystallization rate of bulk water reaches a maximum at ∼224 K, a few degrees below the experimental temperature of homogeneous nucleation of ice. We predict that below this temperature supercooled bulk liquid water crystallizes at a rate faster than it can equilibrate, therefore is beyond its limit of metastability. Our results provide a microscopic foundation to the known correlation between nonequilibrium freezing temperatures and the thermodynamics of supercooled liquid water. We will discuss the effect of confinement of water in nanopores and as nanodroplets on the mechanisms and temperatures of crystallization of ice as well as on the structure of the ice formed at temperature close to the limit of supercooling.