A ferroelectric phase transition underlying the liquid-liquid phase transition in water

What is it about?

Water exhibits unique behaviors, known as thermodynamic anomalies, even in its stable state under Earth's ambient conditions. A possible explanation for the thermodynamic anomalies is that water might switch between two different liquid forms at some temperatures and pressures below water's freezing point, when it is in a metastable liquid state. It is the so-called liquid-liquid phase transition. Its origin has been so-far not clarified. Water is a polar liquid, with each particle having mass and an electric dipole - equal positive and negative charges separated by a given distance - interacting via a specific dipolar potential. Molecules' electric dipoles can align in an ordered pattern, and their sum can create a net, macroscopic, and measurable, electric dipole. This typically is triggered by an external electric field. When instead this order happens on its own at some temperatures and pressures, one talks about a so-called ferroelectric phase transition. Through extensive molecular dynamics simulations analysis and elementary theory, we show not only that the liquid-liquid phase transition is accompanied by a ferroelectric phase transition, but also how the latter can drive the former. Our theory treats water as a polar liquid, and it is grounded on the characteristics of dipolar potential interaction and the disordered arrangement of liquid’s molecules. It permits to obtain a free energy expression that supports phase transitions both ferroelectric and liquid-liquid. This not only characterizes but clarifies the origin of the liquid-liquid phase transition and thermodynamic anomalies in water.

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Photo by Anna Sullivan on Unsplash

Photo by Anna Sullivan on Unsplash

Why is it important?

Our study demonstrates that the dipoles in water, out of focus in previous investigations of liquid-liquid phase transition, can actually play a leading role in this phenomenon and shed light on its origin. The link between liquid-liquid and ferroelectric phase transitions, established in this research, can guide targeted experiments to measure the critical point of the liquid-liquid phase transition, one of the unresolved tasks in this field. Water, with its unique properties among liquids, is essential for life and crucial to Earth’s climate and geological processes. Understanding the origin of its peculiar behavior is thus key to uncovering the fundamental mechanisms behind life and Earth’s processes. Our study explains the peculiarities of water while treating it as a generic polar liquid. Although the expression for free energy that could lead to a phase transition is the same in all polar liquids, its specific coefficients, which determine whether the transition effectively occurs under certain conditions, depend on the microscopic details of the liquid being studied. Our research thus raises significant questions: Could the ferroelectric properties of water, highlighted in this study, influence the natural selection of organisms and Earth's geological evolution? Is water ‘merely’ a polar liquid with the microscopic characteristics suitable to allow a ferroelectric phase transition close to Earth's environmental conditions? And what contributes to shaping its microscopic characteristics?

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