Water separates into two different liquids

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Researchers have found new evidence that water can transition from one liquid form to another, more dense liquid.

The research was conducted at the University of Birmingham and Sapienza Università di Roma.

Water separates into two different liquids: A new type of “phase transition” in water was first proposed 30 years ago in a study by researchers at Boston University. However, because the transition was predicted to occur under supercooled conditions, confirming its existence has been a challenge. Because at these low temperatures, water doesn’t really want to be liquid at all, but rather quickly turns to ice. Due to its hidden status, much is still unknown about this liquid-liquid phase transition, in contrast to everyday examples of phase transitions in water between a solid or vapor phase and a liquid phase.

This new evidence Published in natural physics, represents a significant step forward in endorsement the idea of ​​a liquid-liquid phase transition, first proposed in 1992. Francesco Sciortino, now a professor at Sapienza Università di Roma, was a member of the original research team at Boston University and is also a co-author of this paper.

The team used computer simulations to explain what features differentiate the two liquids at the microscopic level. They found that the water molecules in the high-density liquid form arrangements that are considered “topologically complex,” such as a trefoil knot (think of the molecules arranged to resemble a pretzel) or a Hopf link ( think of two links in a steel chain). So the molecules in the high-density liquid are said to be entangled.

In contrast, the molecules in the low-density liquid mostly form simple rings, and hence the molecules in the low-density liquid are unraveled.

This finding has given us a completely new look at a research problem that is now 30 years old and will hopefully be just the beginning.

Andreas Neophytou, Faculty of Chemistry

Andreas Neophytou, a PhD student at the University of Birmingham with Dr. Dwaipayan Chakrabarti, is the lead author of the publication. He says: “This finding has given us a completely new perspective on a research problem that is now 30 years old and will hopefully only be the beginning.”

The researchers used a colloidal water model in their simulation, and then two widely used molecular water models. Colloids are particles that can be thousands of times larger than a single water molecule. Due to their relatively larger size and thus slower movements, colloids are used to observe and understand physical phenomena that also occur on the much smaller atomic and molecular length scales.

dr Chakrabarti, a co-author, says: “This colloidal water model provides a magnifying glass for molecular water and allows us to unravel the mysteries of water in terms of the history of two liquids.”

Professor Sciortino says: “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 that this work will inspire novel theoretical modeling based on topological concepts.”

The team believe the model they have developed will pave the way for new experiments that will validate the theory and extend the concept of ‘entangled’ liquids to other liquids such as silicon.

Pablo Debenedetti, Professor of Chemical and Bioengineering Princeton University in the USA and a world-leading expert in this field of research, remarks: “This beautiful computational work reveals the topological basis underlying the existence of different liquid phases in the same network-forming substance.” He adds: “In this way it enriches and deepens our understanding of a phenomenon that a great deal of experimental and computational evidence increasingly suggests is central to the physics of this most important of all fluids: water.”

Christian Micheletti, Professor at International School for Advanced Studies in Trieste, Italy, whose current research interest is to understand the impact of entanglements, particularly knots and links, on the statics, kinetics, and functionality of biopolymers, notes: “With this single publication, Neophytou et al. achieved several breakthroughs that will impact various scientific fields. First, their elegant and experimentally accessible colloidal model for water opens up completely new perspectives for large-scale studies of liquids. In addition, they provide very strong evidence that phase transitions, which are elusive for traditional analysis of the local structure of liquids, are instead easily captured by tracing the nodes and links in the liquid’s bonding network. The idea of ​​looking for such subtleties in the somewhat abstract space of pathways that travel along transient molecular bonds is very powerful, and I expect it will be widely used to probe complex molecular systems.”

Sciortino adds: “Water reveals its secrets one by one! Dream how nice it would be if we could look down into the liquid and watch the water molecules dance, flicker, and exchange partners and rearrange the hydrogen-bonding network. The realization of the colloidal model for water that we propose can make this dream come true.”

The research was supported by the Royal Society via the International Exchanges Award, which enabled international collaboration between the researchers in the UK and Italy, the EPSRC Center for Doctoral Training in Topological Design and the Institute of Advanced Studies at the University of Birmingham Italian Ministero Istruzione Università Ricerca – Progetti di Rilevante Interest Nazionale.

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