An Old Theory That Liquid Water Has Two Phases Gains Support

In everyday life, we encounter water in solid, liquid, and gasseous states. However, at very cold temperatures, liquid water probably has two phases, which would explain its behavior under conditions closer to those we are familiar with. Andrey Bocharov/Shutterstock.com

Water has some very rare properties. Its solid phase is less dense than its liquid one, for example, which is why icebergs float. It also flows more easily when compressed. These features are beneficial, perhaps even essential for life. We know these oddities are the product of localized arrangements driven by hydrogen bonds, but we have not managed to form a complete explanation. Now, an old theory about why water has such unusual properties has gained some new evidence.

Decades ago, scientists proposed that if there are two different forms of very cold liquid water, it would explain some of the features we don't understand. If so, there should be a critical point on a temperature-pressure graph where these two meet, just as there is for the conversion of water to ice. However, attempts to find this point have failed, leaving the theory of different water forms controversial. Now Princeton's Professor Pablo Debenedetti claims to have found the critical point's location.

Another water oddity is that it can be cooled well below 0ºC (32ºF) without solidifying, provided it lacks any impurities around which ice crystals can start to form. This supercooling is a popular science demonstration and upercooled water droplets in high-altitude clouds help shape the weather.

Under the conditions available in a school science lab, supercooling means taking water a few degrees below zero. However, under high pressures and very clean conditions, it's possible to go a lot lower than that, and this liquid-liquid critical point is thought to be at temperatures that would daunt even the bravest penguin. Unfortunately, it's hard to make measurements under these conditions, so nothing has been experimentally confirmed.

In Science, Debenedetti and colleagues report modeling the cooling of water under 2,000 times the atmospheric pressure at sea level. The two models agree that somewhere between 170 and 190 Kelvin (-103ºC and -83ºC  / -153ºF and -117ºF ) a critical point exists. One of the paper's authors, Professor Francesco Sciortino of Sapienza University of Rome, proposed something along these lines in 1992, but at the time computers were not remotely powerful enough to offer anything beyond crude confirmation.

Two computer simulations (top and bottom panels) showed large swings in density as supercooled water oscillates between two liquid phases. Fig B (right): The simulations revealed a critical point between the two liquid phases on graphs of temperature and pressure. PG Debenedetti et al, Science 

The complexity of the task of modeling the interactions of so many molecules is so great that even today 1.5 years' worth of research computers' time went into this paper. "Now I can sleep well, because after 25 years, my original idea has been confirmed," Sciortino said in a statement.

The paper concludes that when water gets cold enough, it swings between tetrahedral structures of five loosely bound molecules and a sixth molecule squeezing inside, producing wild swings in density. Although this occurs in conditions far from our everyday experience, Debenedetti said a critical point's existence influences the properties of a substance under quite different temperatures and pressures.

Even the best computer models are not the same as the real world, and work will continue to confirm these claims experimentally. The models differ by about 10 percent on the temperature at which the critical point lies, so they provide only a rough guide on where experimentalists should be looking.

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