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eFluctuationf is a concept to describe variation from eaveragef. Spatial variation in distribution of molecules (static fluctuation) and temporal one (dynamic fluctuation) determine the structure and property of a system, and become driving force of its subsequent development. Prof. Keiko Nishikawa has been involved in structural studies on material systems in which fluctuations become large, and presented a novel concept based on fluctuations for revealing the structures and properties of materials with no regular structure. The efluctuationf can be expressed by a different word edisorderf. She pointed out the importance of recognition of the disorder as is, and established experimental methods to quantitatively display the disorder. She succeeded in revealing the relationships between the fluctuations of various disordered systems and their unique phenomena and properties. She has proposed to actively use the system in which fluctuations become vigorous in their intensity as the chemical efieldf for reaction and separation.
Her major contributions to the progress in structural chemistry based on the fluctuation concept are as follows.
1. Elucidation of mixing schemes of solutions based on concentration fluctuation.
She adopted econcentration fluctuationf as the quantity to describe structure of a solution, instead of a radial distribution function that is a popular function in liquid or solution chemistry. Combining small-angel X-ray scattering (SAXS) intensities with some thermodynamic quantities, she established the experimental method to obtain econcentration fluctuationf of a mixture, which is the quantitative description of the mixing scheme or inhomogeneous distribution of components in the mixture. She applied this method to various systems: for example, aqueous solutions of alcohols that are completely soluble in water but shows different mixing schemes and mixtures that display phase separations. In this manner, she presented the novel structural liquid science for the first time.
2. Study on inhomogeneity of molecular distribution for supercritical fluids.
Supercritical fluids are characterized by large inhomogeneity of molecular distribution. The density fluctuation is a suitable parameter for quantitative and direct description of the inhomogeneity from the viewpoint of mezzoscopic or macroscopic scale. She has comprehensively investigated the density fluctuation of various kinds of supercritical fluids by means of SAXS experiments and thermodynamic calculations, and has found the following facts.
(i) The density fluctuation forms a ridge, when the contour map of their values is drawn in a phase diagram. The eridgef called the Nishikawa Line is the locus of the points where the values of the density fluctuation become the maxima in isothermal change. The Line drawn on a pressure-temperature phase diagram runs in the supercritical region as an extension of the coexistence curve of gas and liquid.
(ii) Nishikawa Line deviates slightly from the critical isochore.
(iii) The Line corresponds to extrema of the various physical quantities that are related to the second derivatives of the Gibbs free energy: for example, heat capacity, isothermal compressibility, partial molar volumes, and sound velocity. In addition, such dynamic properties as thermal conductivity also show maxima.
(iv) The rate of increase in solubility becomes maximum on the Line. Namely, the Line separates the supercritical region into two regions, in which the solubilities are different by an order of magnitude.
(v) The rate constants or equilibrium constants of various chemical reactions in supercritical fluids show singular behaviors at the Line. The singularity means extrema or inflection point of the quantities.
(vi) For most of substances, the density fluctuation and the Line have almost the same functional relationship of reduced density and reduced temperature. However, substances whose main molecular interaction is hydrogen bonding do not follow the same function.
It is concluded that the density fluctuation is one of the most fundamental factors which determine properties of supercritical fluids and Nishikawa Line is the boundary which separates the supercritical region into two: more liquid-like and more gas-like regions.
3. Study on dynamic fluctuations at phase transition.
Thermal responses (exo- or endothermic) are accompanied without fail by any physical and chemical processes as well as biological phenomena. She considered that it is possible to obtain the information on the dynamics of phase transitions and the relaxation processes by thermodynamic measurements if she uses a supersensitive calorimeter that can detect very weak thermal signals and follow the phenomena with adequate apparatus response time to mach the relaxation time of the events. Ordinarily, it had been impossible to detect the details of the change, because a phase transition occurs instantaneously and thermal signals of step-by-step change are very weak. She found that phase changes of many room-temperature ionic liquids are controlled by extremely slow dynamics and their step-by-step phenomena are sometimes detectable. She comprehensively studied phase behaviors of many room-temperature ionic liquids, measuring thermal responses by the supersensitive calorimeter, simultaneously following optical signals, and relaxation times by NMR. These measurements became possible only by construction and improvements of methodologies and apparatuses for these experiments. From these measurements, she detected step-by-step phase changes, such as rhythmic melting and crystallization, intermittent crystallization and reversible phase transitions. They are the first observations in the world. She succeeded in direct observations of the first-order phase transitions, which had been thought to be impossible. She opened the door for elucidation of the dynamics of phase transitions which had been an unsolved problem for long time.
The study of Prof. Keiko Nishikawa is characterized by the originality, in which the methodologies and apparatuses are constructed or improved to observe unique and important phenomena in material science. Using the concept of fluctuation as a probe, she succeeded in elucidating the structures and properties of complex disordered materials. In such a way, she deserves the Award of the chemical Society of Japan.
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