New study proves that water separates into two different liquids at low temperatures

Researchers from the University of Birmingham and Sapienza Università di Roma have found that water can change from one form of liquid into a denser form of liquid, according to a press release published by the University of Birmingham.

This kind of a "phase transition" in water was first proposed by Boston University researchers 30 years ago, in 1992. The existence of this transition, however, has always been challenging to prove because of water's tendency to turn into ice at low temperatures. So these new findings are a significant leap toward confirming the hypothesis of a liquid-liquid phase transition.

The researchers utilized computer simulations to clarify what exact features differentiate the two liquids at the microscopic level. They discovered that the water molecules in the high-density liquid organize themselves into arrangements that can be considered "topologically complicated," such as a trefoil knot or a Hopf link. Thus, it is argued that the molecules in the high-density liquid are entangled. On the contrary, the molecules in the low-density liquid mostly form simple rings. Therefore, the molecules in the low-density liquid are unentangled.

Supporting a 30-year-old research problem

"This insight has provided us with a completely fresh take on what is now a 30-year-old research problem, and will hopefully be just the beginning," said Andreas Neophytou, a Ph.D. student at the University of Birmingham with Dr. Dwaipayan Chakrabarti, a lead author on the paper.

In their simulation, the researchers first employed a colloidal model of water followed by two commonly-used molecular models of water. Called colloids, these particles can be a thousand times bigger than a single water molecule. Colloids are utilized to study and explain physical events because of their relatively larger size and slower movements, which also happen at the much smaller atomic and molecular length scales.

"This colloidal model of water provides a magnifying glass into molecular water, and enables us to unravel the secrets of water concerning the tale of two liquids," said Chakrabarti.

The team anticipates that the model they have created will open the door for novel research that will support the theory and broaden the definition of "entangled" liquids to include other liquids like silicon.

"In this work, we propose, for the first time, a view of the liquid-liquid phase transition based on network entanglement ideas. I am sure this work will inspire novel theoretical modeling based on topological concepts," says Francesco Sciortino, who was a member of the original research group at Boston University in Massachusetts and now is a professor at Sapienza Università di Roma.

The results of the research were published in the journal Nature Physics.

Abstract:

The first-order phase transition between two tetrahedral networks of different density—introduced as a hypothesis to account for the anomalous behaviour of certain thermodynamic properties of deeply supercooled water—has received strong support from a growing body of work in recent years. Here we show that this liquid–liquid phase transition in tetrahedral networks can be described as a transition between an unentangled, low-density liquid and an entangled, high-density liquid, the latter containing an ensemble of topologically complex motifs. We first reveal this distinction in a rationally designed colloidal analogue of water. We show that this colloidal water model displays the well-known water thermodynamic anomalies as well as a liquid–liquid critical point. We then investigate water, employing two widely used molecular models, to demonstrate that there is also a clear topological distinction between its two supercooled liquid networks, thereby establishing the generality of this observation, which might have far-reaching implications for understanding liquid–liquid phase transitions in tetrahedral liquids.