Friday, April 5, 2013

Why water prefers the single life!?

2:26 AM Author 2 comments

Does liquid water have a double life? New research says probably not.

Water is one of the most studied substances on Earth, yet it continues to surprise. One of the more controversial ideas in physical chemistry is that liquid water might exist in two structurally distinct forms — sometimes called the "two-liquid" model. Under this theory, as water is cooled toward its freezing point, it could transition between a high-density and a low-density liquid state. If true, this would explain several of water's well-known anomalous properties, like the fact that it expands when it freezes.

But a major new study has thrown cold water on the idea. Researchers conducted an extensive computational search for evidence of this two-liquid behavior, and found none. Their work suggests that liquid water is much more consistent and uniform in structure than the two-liquid model predicts — preferring, it seems, to stay as a single, well-behaved phase rather than splitting into two distinct forms.

The Origins of the Controversy

The two-liquid hypothesis gained traction because water really does behave oddly near its freezing point. Its heat capacity, compressibility, and viscosity all show unusual peaks and troughs that other simple liquids don't. Proponents of the two-liquid model argued these anomalies were signatures of a hidden phase transition deep in the supercooled regime — a region below 0°C where water can remain liquid if cooled carefully.

The problem is that experimentally accessing deeply supercooled water is extremely difficult. The liquid tends to freeze suddenly before you can measure it properly. So the debate has been fueled partly by the inability to directly probe the critical region where the hypothetical transition would occur.

What the New Research Shows

Using detailed computer simulations of water's molecular behavior, the team found no evidence of a second liquid phase appearing as the temperature dropped. The properties of simulated water changed smoothly and continuously — no sudden shift, no hidden critical point, no two-liquid scenario. The researchers argue this makes the single-liquid model by far the more likely description of what's actually happening.

This doesn't mean water is boring. Its hydrogen-bonding network is still remarkably complex and gives rise to all the properties that make it so essential to life. But the idea that it secretly lives a double life as two different liquids looks increasingly unlikely — at least according to this work.

Source: Physics World