In 1892, Röntgen, who discovered X-rays and won the first Nobel Prize in Physics, proposed a model for water in which an ice-like structure and an unknown structure are mixed to form water. In 1933, however, after investigating X-ray diffraction data for water, professors at Cambridge University proposed a theoretical model for water in which the structure of ice, comprising H2O molecules placed at the four vertices and center of a regular tetrahedron, is continuously distorted to form water. This model was not universally accepted but was supported by many scientists because corroborative results were obtained in various subsequent spectroscopic analyses and three-dimensional molecular dynamics simulations carried out using computers, the performance of which has been rapidly improving since the 1980sWater, the essence of life No
In 2008, Takashi Tokushima, a research scientist of RIKEN, and his colleagues analyzed the structure of water using high-brilliance soft X-rays from the RIKEN Physical Science III Beamline (BL17SU) and a soft X-ray emission spectrometer with the world's highest resolution equipped with a newly developed special container (a liquid flow cell) with windows made from strong 150-nm-thick thin films with a high transmittance for soft X-rays. When an electron is expelled from the inner orbital of an oxygen atom covalently bonded to hydrogen atoms upon the irradiation of soft X-rays, an electron in the outer orbital moves to the inner orbital to occupy the positively charged vacant position (a hole). At this time, the energy corresponding to the difference between the energy of the electron in the inner and outer orbitals is released as soft X-rays. Therefore, we can determine the state of lone pairs by examining differences in the energy distribution of the released soft X-rays (energy spectrum). This is the principle of the structural analysis of water by soft X-ray emission spectroscopy (Fig. 4). When the soft X-ray emission spectrum of water (liquid) is examined by this method, two peak intensities corresponding to the lone pairs involved in hydrogen bonds are observed (Fig. 5). This finding suggests that two states of water with different patterns of hydrogen bonds simultaneously exist. To examine this finding more precisely, the scientists analyzed the structure of water using various techniques, for example, small-angle X-ray scattering analysis, a method of observing fine structures by analyzing the scattering pattern of X-rays that are scattered with a small angle upon the irradiation of X-rays onto a material, and X-ray Raman scattering to observe molecular orbitals without electrons.
The above analyses revealed a highly unexpected structure for water. Unlike the conventionally accepted model in which the orderly ice structure is continuously and gradually distorted to form the uniform structure of water, water appears to have a relatively large variation in density (dense/thin), namely, ice-like orderly structures (with a low density) are sparsely distributed in a high-density “sea” of H2O molecules that are distorted due to the breaking of hydrogen bonds, resulting in polka-dot-like fine structure (Fig. 5). It was also found that the variation in density becomes small at high temperatures and large at low temperatures. Because of this finding, it is necessary to reappraise the model proposed by Röntgen. Since the results were published in an American scientific journal, there have been calls to modify the new model of water based on molecular dynamics simulations, and a lively debate about the nature of water is still ongoing. The density of ice being lower than that of water and the density of water being greatest at 4°C now appear to be partially explainable by considering the intermolecular distance and the average number of H2O molecules surrounding a H2O molecule (the arrangement number). However, there are still many unclear mechanisms, for example, the rate of change between the two states of water, and how this rate of change is affected when a substance is dissolved in water. Thus, the mysteries of water have not yet been fully clarified.
You may be surprised to hear that the above-mentioned fundamental research on water is indispensable in clarifying the mechanism underlying the dissolution of substances in water and the roles of water in biological organisms and chemical reactions. However, for example, to clarify the functions of biological cells containing water with various ions, it is necessary to fully understand the structure of water and the states of the relevant electrons. Thus, the expectation that SPring-8 will lead the world in fundamental research on water is growing.
