Venturing deep into water’s no man’s land

Water is known to have many unique properties, including an increasing compressibility when chilled when as a supercooled liquid. One hypothesis that seeks to explain these observations states that water can occur as a liquid in two distinct high- and low-density phases, between which a coexistence line can be drawn, ending at a liquid-liquid critical point (LLCP). The LLCP, however, lies in a region of the phase diagram largely inaccessible using conventional experimental techniques due to rapid freezing. A new model looks to make inroads into this so-called no man’s land.

Publishing their work in The Journal of Chemical Physics, a team of researchers demonstrated that recent experimental measurements of water’s diffusion constant, at temperatures and pressures within no man’s land, are consistent with the LLCP hypothesis based on simulations using the explicit three-body (E3B3) model of liquid water.

Unlike many models for liquids, which involve only the interactions of pairs of molecules, the E3B3 model also considers the interactions of three molecules at once, in order to model the cooperative hydrogen bonding found uniquely in water.

After calculations of the diffusion constant over a wide range of temperatures and pressures, the team concluded that the recently published measurements are consistent with an LLCP having a critical pressure of over 600 atmospheres.

The researchers hope their work will encourage others to find novel ways to experimentally study the no man’s land region of water’s phase diagram, especially at elevated pressures.

Source: “Communication: Diffusion constant in supercooled water as the Widom line is crossed in no man’s land,” by Yicun Ni, Nicholas J. Hestand, and J. L. Skinner, Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5029822 .