Water separates into two different liquids at low temperatures

Fresh evidence that water can change from one form of liquid into another, denser liquid has been uncovered.

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Almost three decades ago, a new kind of phase transition in Water was proposed to occur under supercooled conditions. However, it was challenging to confirm this as Water at these low temperatures does not want to be a liquid; instead, it wants to become ice rapidly.

Unlike the common examples of phase transitions in Water between a solid or vapor phase and a liquid phase, this liquid-liquid phase transition is hidden, thus, there is still much unknown about it.

A new study by University of Birmingham scientists represents a significant step forward in confirming the idea of a liquid-liquid phase transition. Using computer simulations, scientists explained the features that distinguish the two liquids at the microscopic level.

They discovered that the water molecules in the high-density liquid organize themselves into configurations referred to as “topologically complicated,” such as a trefoil knot or a Hopf link (think of two links in a steel chain). Thus, it is argued that the molecules in the high-density liquid are entangled. In contrast, the molecules in the low-density liquid mostly form simple rings; hence, the molecules in the low-density liquid are unentangled.

Andreas Neophytou, a Ph.D. student at the University of Birmingham with Dr. Dwaipayan Chakrabarti, is the paper’s lead author, said, “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.”

Scientists used a colloidal model of Water in their simulation and two widely used molecular models of Water. The size of a colloidal particle can be a thousand times that of a water molecule. Because of their relatively larger size and slower movements, colloids are utilized to monitor and understand physical phenomena that also happen at the much smaller atomic and molecular length scales.

Dr. Dwaipayan Chakrabarti, the lead author of the paper, said, “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.”

Francesco Sciortino, now a professor at Sapienza Università di Roma said, “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.”

According to scientists, their model could lead to new experiments validating the theory and extending the concept of “entangled” liquids to other liquids such as silicon.

Pablo Debenedetti, a professor of chemical and biological engineering at Princeton University in the US and a world-leading expert in this area of research, remarks, “This beautiful computational work uncovers the topological basis underlying the existence of different liquid phases in the same network-forming substance.

“In so doing, it substantially enriches and deepens our understanding of a phenomenon that abundant experimental and computational evidence increasingly suggests is central to the physics of that most important of liquids: water.”

Christian Micheletti, a professor at the International School for Advanced Studies in Trieste, Italy, whose current research interest lies in understanding the impact of entanglement, especially knots and links, on the static, kinetics and functionality of biopolymers, says, “With this single paper, Neophytou et al. made several breakthroughs that will be consequential across diverse scientific areas.

“First, their elegant and experimentally amenable colloidal model for Water opens entirely new perspectives for large-scale studies of liquids. Beyond this, they give very strong evidence that phase transitions that may be elusive to traditional analysis of the local structure of liquids are readily picked up by tracking the knots and links in the bond network of the liquid.”

“The idea of searching for such intricacies in the somewhat abstract space of pathways running along transient molecular bonds is a very powerful one, and I expect it will be widely adopted to study complex molecular systems.”

Sciortino says“Water, one after the other, reveals its secrets. 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. The realization of the colloidal model for Water we propose can make this dream come true.”

Journal Reference:

  1. Andreas Neophytou et al., Topological nature of the liquid-liquid phase transition in tetrahedral liquids, Nature Physics (2022). DOI: 10.1038/s41567-022-01698-6

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