Water holds a special place among liquids for its unusual properties, and remains poorly understood. For example, it expands just upon the freezing to ice, and becomes less viscous under compression, around atmospheric pressure. Rationalizing these oddities is a major challenge for physics and chemistry. Recent research led by The University of Tokyo’s Institute of Industrial Science (IIS) suggests they result from the degree of structural ordering in the fluid.
Water belongs to a class of liquids whose particles form local tetrahedral structures. The tetrahedrality of water is a consequence of hydrogen bonds between molecules, which are constrained to fixed directions. In a study in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), the researchers investigated why the physical properties of water – as expressed by its phase diagram – are so remarkable, even compared with other tetrahedral liquids, such as silicon and carbon.
Tetrahedral liquids are often simulated by an energy potential named the SW model. The liquid is assumed to contain two phases – a disordered state that has high rotational symmetry, and a tetrahedrally ordered state that does not – in thermodynamic equilibrium. Despite its simplicity, the model accurately predicts anomalous liquid behaviors. The two-state property is controlled by the parameter lambda (λ), which describes the relative strength of pairwise and three-body intermolecular interactions. The higher λ is, the degree of tetrahedral order increases.