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Abstract Liquid water can be supercooled up to about 50~K below the melting point before undergoing homogeneous ice nucleation. Based on experimental thermodynamic observations and computer simulations it was hypothesized that below this temperature and at pressures of several kbar water undergoes a liquid-liquid phase transition (LLPT) and the transition line ends at a second critical point. However, challenges in experiments and simulations at such deep cooling leave doubts about the nature of the LLPT and the existence of the critical point.Here we use molecular dynamics simulations with a highly accurate and computationally efficient polarizable water model to establish the character of the LLPT and identify the location of the second critical point. Our microsecond-long simulations provide the first direct evidence of a well-defined moving interface between low-density and high-density water at conditions near the phase boundary. This is the ultimate proof of a first-order transition between two liquid phases with distinct free energy basins separated by a barrier, resolving a long-standing debate. These results provide new perspectives on supercooled water under pressure simulated with an accurate and realistic model suitable for studies of water in confined geological and biological environments.
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Supercritical density fluctuations and structural heterogeneity in supercooled water-glycerol microdroplets
Abstract Recent experiments and theoretical studies strongly indicate that water exhibits a liquid-liquid phase transition (LLPT) in the supercooled domain. An open question is how the LLPT of water can affect the properties of aqueous solutions. Here, we study the structural and thermodynamic properties of supercooled glycerol-water microdroplets at dilute conditions (χg = 3.2% glycerol mole fraction). The combination of rapid evaporative cooling with femtosecond X-ray scattering allows us to outrun crystallization and gain access to the deeply supercooled regime down toT = 229.3 K. We find that the density fluctuations of the glycerol-water solution or, equivalently, its isothermal compressibility,κT, increases upon cooling. This is confirmed by molecular dynamics simulations, which indicate that the presence of glycerol shifts the temperature of maximumκTfromT = 230 K in pure water down toT = 223 K in the solution. Our findings elucidate the interplay between the complex behavior of water, including its LLPT, and the properties of aqueous solutions at low temperatures, which can have practical consequences in cryogenic biological applications and cryopreservation techniques.
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- Award ID(s):
- 2223461
- PAR ID:
- 10655445
- Publisher / Repository:
- Nature Communications
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41467-024-54890-y
