Scientists Map Elusive Liquid-Liquid Transition Point in Water
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A groundbreaking study published in Nature Physics has revealed the long-sought liquid-liquid critical point of water, a finding with notable implications for various scientific fields. This critical point, where water exists concurrently in two distinct liquid phases—high-density adn low-density—has eluded scientists for decades.
Water’s anomalous properties—unlike most substances, it is denser in its liquid state than its solid state—lead to phenomena like ice floating on water. This unique behaviour,especially in the supercooled regime,has been a subject of intense research. The hypothesized liquid-liquid phase transition (LLPT) below the homogeneous nucleation temperature of -38°C, where water exists in two distinct liquid states, has presented significant challenges to researchers.
Professor Francesco Sciortino of Roma Sapienza University and Professor francesco Paesani of the University of California San Diego, key figures in this research, shed light on the complexities involved. “Water is a unique liquid with properties that have been tried to understand for decades,” said Professor Paesani. He continued, explaining the long-standing hypothesis: Long-standing hypotheses indicate that in extreme conditions, especially at very low temperatures and high pressures, water can exist in two different liquid phases: high-density liquid and low-density liquid.
Professor Sciortino elaborated on the difficulty in experimentally confirming this liquid-liquid critical point: The point at which these two phases become indistinguishable is known as the liquid-liquid critical point. Though, experimental confirmation remains elusive due to the challenges of preventing water from freezing before reaching this condition.
Liquid-Liquid Phase Transition and the “no-Man’s Land”
When pure water is cooled below 0°C, it can remain liquid in a supercooled state, down to approximately -38°C. In 1992, researchers first proposed the possibility of a liquid-liquid phase transition (LLPT) below this temperature. Professor Sciortino, working as a postdoctoral researcher at Boston University at the time, has been involved in this research for over three decades. A major hurdle has been what researchers term “no-man’s land,” a region in the water phase diagram where liquid water typically crystallizes into ice before measurements can be made, preventing observation of the LLPT below -38°C.
This inability to conduct real-time measurements has led to heavy reliance on computer simulations to predict water’s behavior. Previous simulations yielded widely varying predictions for the LLCP’s location, with estimated critical pressures ranging from 36 to 270 MPa and critical temperatures from -123°C to -23°C (150 to 250 K).
The breakthrough came from a collaboration between Professor Sciortino and professor Paesani, focusing on the many-body potential developed by Professor Paesani’s team, MB-Pol.The potential of MB-Pol to accurately investigate the supercooled state of water prompted this examination.
The Role of Neural Networks
“despite its accuracy, MB-POL is computationally more demanding than empirical models. To overcome this limitation, Sigbjørn Bore, the third author of this paper, developed a deep neural network potential (DNN@MB-POL) trained on MB-POL data,” explained Professor Paesani, highlighting the innovative use of neural networks. This approach leverages quantum chemistry calculations from first principles at the combined cluster level, considered a gold standard for molecular interactions.
Using the DNN@MB-POL model, the researchers performed extensive microsecond molecular dynamics simulations under 280 different conditions, varying temperature (188 to 368 K, -85°C to 95°C) and pressure (0.1–131.7 mpa), each simulation using a system of 256 water molecules under periodic boundary conditions.
This is crucial for studying supercooled water as, as temperature decreases, molecular diffusion slows dramatically. This slowdown makes it harder for the system to reach metastable equilibrium, requiring very long simulations to capture the relevant dynamics,
Professor Paesani explained.
Phase Transition Identification and Implications
The simulations revealed direct evidence of two distinct liquid states with different densities and structures. At -85°C (188 K),dramatic density fluctuations where observed on the microsecond timescale,with water spontaneously transitioning between high-density and low-density states around 101.3 MPa. This confirmed the existence of a first-order phase transition between the two liquid forms, with increasing free energy barriers upon cooling—a clear signature of the transition.
Considering systematic deviations from experimental values, the team estimated the actual critical point of water to be around 198 K (-75°C) and 126.7 MPa.Considerably, this critical point lies at lower pressure than many previous predictions, suggesting it may be experimentally accessible. The researchers constructed a complete phase diagram showing the liquid-liquid coexistence curve.
We are very confident in our estimate of the liquid-liquid critical point because it is derived from first-principles quantum chemistry at the combined cluster level: the gold standard for electronic structure calculations,
stated Professor Sciortino.
nanodroplets: A Path to Experimental Validation
This study provides strong computational evidence for the existence of LLPT in water, resolving a scientific puzzle that has persisted for over 30 years. Researchers believe that water nanodroplets—nanometer-sized water drops confined in a limited space or suspended in a medium—could provide a pathway for experimental validation.
For nanodroplets only a few nanometers in diameter, the internal pressure can reach values comparable to the liquid-liquid critical pressure (~1250 atm). This suggests that carefully controlled nanodroplets could provide an experimental route to investigate the LLCP,
noted Professor Paesani.
Professor sciortino added: Neutron and X-ray scattering experiments could be used to detect the structural signatures of the two liquid states in these confined droplets. Specifically, scattering techniques can reveal density fluctuations and characteristics of critical phenomena. Moreover, time-resolved spectroscopy could help capture the dynamics of interconversion between the two liquid phases.
the discovery of LLPT has broad implications across various scientific fields, including improved climate modeling and predictions, a better understanding of oceans on moons and distant planets, enhanced understanding of cellular processes driven by phase separation, and advancements in energy storage and water treatment technologies.
Title: unveiling Water’s Mysteries: Experts Discuss the Breakthrough in Mapping the Liquid-Liquid Transition Point
Introduction:
Imagine a world where water doesn’t just fit the mold—it transcends it. A recent groundbreaking study published in Nature Physics has shattered centuries-old paradigms by identifying the liquid-liquid critical point of water. This discovery challenges our understanding of water’s anomalous properties, unveiling its potential to exist simultaneously in two liquid states—an unprecedented phenomenon with groundbreaking implications across multiple scientific realms.
Question from the World Today News Senior editor:
In light of this transformative discovery, how does the identification of water’s liquid-liquid critical point reshape our understanding of this seemingly ordinary molecule?
expert Answer:
The elucidation of water’s liquid-liquid critical point is revolutionary. Traditionally, we understand water as a single liquid phase, transitioning between liquid, solid, and gas. However, this study reveals that under extreme conditions—specifically, very low temperatures and high pressures—water can simultaneously exist as both a high-density liquid and a low-density liquid. Historically, water’s anomalous behavior, like ice floating due to lower density, prompted scientific curiosity, but the confirmation of this phase transition was elusive, primarily due to the prohibitive conditions needed for experimentation.
Practically, this finding profoundly influences our knowledge, extending beyond theoretical physics into fields like climate science and astrobiology. For instance, the behavior of water under these conditions could drastically alter how we model the ice layers in Earth’s oceans or analyze the potential for habitable environments on icy planetary moons such as Europa. Understanding the phase coexistence curve allows for more accurate predictions and models, enabling us to appreciate water’s role in Earth and beyond.
Question from the world Today News Senior Editor:
What methods did the researchers use to confirm the existence and location of this critical point? And what challenges did they overcome?
expert Answer:
Confirming the existence of the liquid-liquid critical point was a Herculean task due to the technical difficulties of studying supercooled water without it freezing. Researchers utilized molecular dynamics simulations supported by advanced neural networks. Specifically, the study implemented a deep neural network trained using data from the many-body potential (MB-pol) developed by Professor Paesani’s team, which is known for its accuracy in simulating supercooled water states.
These simulations, conducted over 280 conditions varying in temperature and pressure, involved modeling interactions among 256 water molecules. Each simulation adhered to a microsecond timescale to compensate for the considerably slowed molecular motions at lower temperatures. Key to this approach was the development of the DNN@MB-POL model, which balanced accuracy and computational demand, enabling this breakthrough.
One pivotal challenge was overcoming the “no-man’s land” zone where water rapidly crystallizes before the critical point could be observed. By leveraging neural networks and frist-principles quantum chemistry calculations, researchers circumvented this barrier, gaining the insights that suggested the critical point resides at much more accessible conditions than previously theorized.
Question from the world Today News Senior Editor:
Is this discovery purely theoretical, or are there practical steps toward experimental validation?
Expert Answer:
While the study provides a solid computational foundation, experimental validation remains the next frontier. The researchers propose using water nanodroplets as a potential pathway for verification. Given their unique properties, nanodroplets inherently exhibit high internal pressures, circumventing some of the challenges faced in bulk water studies.
Techniques such as neutron and X-ray scattering could be employed to detect structural signatures of the two liquid states within these confined droplets, specifically looking at density fluctuations and other critical phenomena. Time-resolved spectroscopy could further capture the dynamic transitions between the high-density and low-density liquid states. These approaches open the door not just for experimental validation but also for practical applications in advanced material sciences and biochemistry, where understanding phase transitions can enhance technologies in energy storage and drug delivery systems.
Conclusion and Call to Action:
Understanding water’s transition between its liquid states is more than a scientific curiosity—it’s a window into the fundamental nature of the universe and our own planet. This discovery paves the way for breakthroughs across climate modeling, astrobiology, and materials science.
We invite you to share your thoughts: How do you think this discovery will impact future water-related research or technologies? Comment below or share this conversation on social media to inspire further dialog.