Scientists at the University of Tokyo have studied in detail the structure of water as a liquid and found that its properties depend on the ratio of two phase states.
Everyone knows that the properties of water are different from most liquids: it expands when it freezes (therefore ice is lighter), when it contracts, its viscosity decreases, and so on. These seemingly anomalous properties are explained to us in school by the presence of hydrogen bonds between molecules. However, the details are still poorly understood, although the topic is extremely important for both chemistry and physics. The specific properties of water are also used in medicine and technical disciplines.
The Institute of Industrial Sciences of the University of Tokyo was able to advance in understanding the phenomenon of the structure of water.
Water in a liquid state forms tetrahedral structures of a local nature, which are formed with the help of hydrogen bonds - this has been known for a long time. Japanese scientists have determined that water is not just "disordered water" in which "particles" of "tetrahedral water" float: the system has a state diagram similar to solid phases.
A model has been developed that considers liquid water as a system consisting of two phases. The first is a disordered state with high rotational symmetry. Simply put, this is the absence of any definite pattern in the "directions" of molecules in a liquid. The second phase is not only tetrahedrally ordered, but also thermodynamically in a nonequilibrium state. The interaction of these states is described by the lambda parameter (λ), the physical meaning of which is an assessment of the relative strength of intermolecular interactions of a pair and triple nature. That is, the usual, between two free molecules, and between the molecules that make up a tetrahedral structure. Accordingly, an increase in the parameter λ indicates an increase in the ordering of the system.
This model looks simple, but it predicts well the abnormal behavior of water as a liquid.
One of the leaders of the study, John Russo, explains: "… As λ increases, the tetrahedral shells that form around each molecule become energetically more stable." This compensates for the energy consumption for ordering the structure as a whole. Scientists, by changing λ, have modeled phase-state diagrams, the structure of which can be quite unexpected. So, the figure on the left shows the structure of water of the Si34 type - it is formed under negative pressure. Moreover, its structure is clathrate, that is, in fact, it is an inclusion compound: some of the water molecules are in the cavities of the structure formed by its other molecules.
Image of the structure of water in the phase state Si34 (left) and the phase diagram in coordinates λ / pressure (right) / Institute of Industrial Science, The University of Tokyo.
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The revealed dependence is not linear, the maximum effect on the properties of water occurs at λ = 23.15.
Hajime Tanaka, one of the project leaders, noted the important role of research in physical chemistry.
The relationship of macroscopic parameters such as viscosity to microscopic structures, produced using a relatively simple model, is indeed an important achievement. From a practical point of view, understanding the structure of water should aid the development of efficient fine filters.
Anton Bugaychuk