The Shape of Water – how cluster formation provides a unifying explanation of water’s anomalous properties and the hydrophobic effect
Abstract CERE seminar
Water, essential to life on Earth, has many anomalous properties, that are not yet fully understood. I have 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 function of temperature.
A significant part of the overall thermodynamic stability comes from configurational entropy of the hydrogen bond network in the clusters, so continuous cluster formation and breakage would be observed as dynamic fluctuations in the water structure, rather than as 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.
My simple unifying 3-cluster theory reproduces anomalous properties of pure water 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.
The theoretical framework and concept put forward for pure water has also been applied to investigate the hydrophobic effect in the form of anomalous aqueous solubilities of a range of hydrophobic and amphiphilic organic compounds. The clusters are significantly more hydrophobic than monomeric water in the COSMO-RS model and including the clusters at their equilibrium concentration leads to an increased solubility of organic compounds at low temperatures.
This in turn gives rise to a temperature where the solubility is at a minimum. The presence of a minimum solubility temperature is quite general across a range of compounds and its value is very well reproduced by the COSMO-RS predictions that include the clusters., My model therefore allows a molecular explanation for temperature dependent water solvation properties.