Water can separate Into two different liquids

The dazzling beauty of a snowflake is a testament to the amazing shapes water can form below freezing point, separating into two different liquids.

The beauty of a snowflake is a testament to the shapes water can form below freezing point. Researchers have found that it separates into two different liquids.

Placed under pressure, the elegant dance of the H2O molecule contorts into something bizarre at super chilly temperatures. They virtually tie themselves in knots to avoid transforming into ice.

Researchers from the University of Birmingham in the UK and Sapienza Università di Roma in Italy examined the behaviour of molecules in pressurised liquid water placed under conditions that usually cause it to crystallise.

They identified key features of two different liquid states. This was based on modelling the behaviour of water as a suspension of particles. One is ‘topologically complex’, linked in an overhand knot similar to a pretzel. The other is in a more low-density formation of simpler rings.

“This colloidal model of water provides a magnifying glass into molecular water. It enables us to unravel the secrets of water concerning the tale of two liquids,” says University of Birmingham chemist Dwaipayan Chakrabarti.

Theories laid down in the 1990s have hinted at the kinds of molecular interactions that could happen when water is supercooled – chilled to temperatures below its typical freezing point without solidifying.

Scientists have been pushing the limits on cooling water without it flipping into a solid state for years, eventually holding it in a chaotic liquid form at an insanely cold –263 degrees Celsius (–441 degrees Fahrenheit) for a split moment without it turning into ice.

As far as progress has been made in demonstrating these states in the laboratory, scientists are still trying to work out exactly what supercooled liquids look like when deprived of heat.

How water can split into different liquids below freezing

At critical points, competing polar attractions between water molecules rise above the thermodynamic buzz noise of jiggling particles. Molecules must find other comfortable configurations without the elbow room to push into a crystalline form.

With so many factors at play, researchers typically try to simplify what they can and focus on the important variables. Looking at ‘clumps’ of water like larger dissolved particles helps better understand transitions from one arrangement to another.

Computer models based on this perspective pointed to a subtle change between the water pushing apart. This is a form of particles that settle together in a more dense structure.

Interestingly, the shape – or topology – of molecular interactions in this aquatic landscape also looked completely different. Molecules become tangled in intricate networks as they huddle in or as much simpler forms as they push apart.

“In this work, we propose, for the first time, a view of the liquid-liquid phase transition based on network entanglement ideas,” said Francesco Sciortino, a condensed matter physicist at Sapienza Università di Roma.

“I am sure this work will inspire novel theoretical modelling based on topological concepts.”

This strange space of entangled particle networks is ripe for exploring. Though not entirely dissimilar to long chains of covalently-bonded molecules, such knots are transient. They swap out members as the liquid environment shifts.

The nature of the liquid water found in high-pressure, low-temperature environments should be quite unlike anything we’d find sloshing about on Earth’s surface.

Knowing more about the topological behaviour of water other liquids could give insights into the activity of materials in extreme or hard-to-access environments. That could include depths of distant planets.

“Dream how beautiful it would be if we could look inside the liquid and observe the dancing of the water molecules, the way they flicker, and the way they exchange partners, restructuring the hydrogen bond network,” said Sciortino.

“The realisation of the colloidal model for water we propose can make this dream come true.”

This research was published in Nature Physics.

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