Abstract
Water, essential to life on Earth, has many anomalous properties, that
are not yet fully understood. We used density functional theory, the
COSMO-RS implicit solvent model and statistical thermodynamics to
calculate the dynamic equilibrium of formation of water clusters in
water. Clusters of certain shape are predicted to be thermodynamically
stable at ambient down to supercooled conditions and their presence
almost quantitatively explains water’s anomalous properties as a
function of temperature and pressure. The dodecahedron is the dominant
cluster below ∼235 K and below ∼150 MPa and forms a miscibility gap with
water below a critical point at ∼230 K. The hexagonal torus is the
dominant cluster above ∼235 K and below ∼75 MPa. Both the dodecahedron
and the torus are low-density clusters and their presence explains the
density maximum of pure water at 277 K (+4 °C). The waters in these
clusters are tetrahedrally coordinated, their structures are consistent
with experimental data and their presence explains the observed
tetrahedral fraction as a function of temperature. A significant part of
the overall thermodynamic stability comes from the configurational
entropy of the hydrogen bond network in the clusters, so dynamic cluster
formation and reformation would be observed as fluctuations in the
water structure, rather than stable clusters. At pressures above ∼100
MPa, many of water’s anomalous properties diminish or vanish, which is
also the pressure above which high-density water clusters become more
stable and prevalent. Our simple unifying 3-cluster theory reproduces
water’s anomalous properties from supercooled to ambient temperatures
and from ambient pressure up to at least 400 MPa, including density
anomalies, two-liquid behavior, compressibility, and heat capacity.
Original language | English |
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Article number | 122169 |
Journal | Journal of Molecular Liquids |
Volume | 383 |
Number of pages | 10 |
ISSN | 0167-7322 |
DOIs | |
Publication status | Published - 2023 |