A Tale of Two Waters
Despite being the most familiar of chemicals, with a formula so short and famous that most kids can reel it off, water is also one of the most bizarre liquids known. Now, a group of researchers has observed another strange property of water: a strikingly dense form that may confirm a theory that water has two liquid phases at low temperatures, in addition to the liquid phase that comes out of your kitchen tap.
Water does things few other liquids can. It can absorb large amounts of heat without warming up much. And when it chills down, it becomes more compressible--the reverse of most liquids. Perhaps strangest of all, its density rises as it cools, peaks at 4°C and then decreases, instead of decreasing steadily with falling temperatures like most other liquids. Physicists have suspected that such anomalous properties stem from a dual personality; water, they propose, really has two liquid phases at low temperatures: a high-density liquid (HDL) and a low-density liquid (LDL). The two have never been isolated because they should be able to exist only below –45°C, where water is normally frozen.
In the 21 May Physical Review Letters, Harald Reichert of the Max Planck Institute for Metal Research in Stuttgart, Germany, and colleagues report that they may have observed HDL water. The team focused x-rays on the border between a block of ice and a block of silica (silicon dioxide) at –17°C and measured what bounced back. This revealed a very thin layer of water--just five molecules thick--at the interface between the ice and silica. The liquid layer was 17% denser than water at room temperature. The researchers note that in such a thin layer, the water molecules can't move around very much, and so their effective temperature is much lower than –17°C, perhaps close to –45°C, the proposed phase transition between HDL and LDL water.
“This work is a big step” toward proving the two-phase theory of liquid water, says Eugene Stanley of Boston University, who was one of the first to propose the theory. Reichert says that such ultrathin layers of water are a common phenomenon, and understanding the behavior of HDL and LDL water may lead to insights into water's behavior in diverse situations, from ice skating to ion movement across biological cell walls.